1 @node String and Array Utilities, Character Set Handling, Character Handling, Top
2 @c %MENU% Utilities for copying and comparing strings and arrays
3 @chapter String and Array Utilities
5 Operations on strings (null-terminated byte sequences) are an important part of
6 many programs. @Theglibc{} provides an extensive set of string
7 utility functions, including functions for copying, concatenating,
8 comparing, and searching strings. Many of these functions can also
9 operate on arbitrary regions of storage; for example, the @code{memcpy}
10 function can be used to copy the contents of any kind of array.
12 It's fairly common for beginning C programmers to ``reinvent the wheel''
13 by duplicating this functionality in their own code, but it pays to
14 become familiar with the library functions and to make use of them,
15 since this offers benefits in maintenance, efficiency, and portability.
17 For instance, you could easily compare one string to another in two
18 lines of C code, but if you use the built-in @code{strcmp} function,
19 you're less likely to make a mistake. And, since these library
20 functions are typically highly optimized, your program may run faster
24 * Representation of Strings:: Introduction to basic concepts.
25 * String/Array Conventions:: Whether to use a string function or an
26 arbitrary array function.
27 * String Length:: Determining the length of a string.
28 * Copying Strings and Arrays:: Functions to copy strings and arrays.
29 * Concatenating Strings:: Functions to concatenate strings while copying.
30 * Truncating Strings:: Functions to truncate strings while copying.
31 * String/Array Comparison:: Functions for byte-wise and character-wise
33 * Collation Functions:: Functions for collating strings.
34 * Search Functions:: Searching for a specific element or substring.
35 * Finding Tokens in a String:: Splitting a string into tokens by looking
37 * Erasing Sensitive Data:: Clearing memory which contains sensitive
38 data, after it's no longer needed.
39 * Shuffling Bytes:: Or how to flash-cook a string.
40 * Obfuscating Data:: Reversibly obscuring data from casual view.
41 * Encode Binary Data:: Encoding and Decoding of Binary Data.
42 * Argz and Envz Vectors:: Null-separated string vectors.
45 @node Representation of Strings
46 @section Representation of Strings
47 @cindex string, representation of
49 This section is a quick summary of string concepts for beginning C
50 programmers. It describes how strings are represented in C
51 and some common pitfalls. If you are already familiar with this
52 material, you can skip this section.
55 A @dfn{string} is a null-terminated array of bytes of type @code{char},
56 including the terminating null byte. String-valued
57 variables are usually declared to be pointers of type @code{char *}.
58 Such variables do not include space for the contents of a string; that has
59 to be stored somewhere else---in an array variable, a string constant,
60 or dynamically allocated memory (@pxref{Memory Allocation}). It's up to
61 you to store the address of the chosen memory space into the pointer
62 variable. Alternatively you can store a @dfn{null pointer} in the
63 pointer variable. The null pointer does not point anywhere, so
64 attempting to reference the string it points to gets an error.
66 @cindex multibyte character
67 @cindex multibyte string
69 A @dfn{multibyte character} is a sequence of one or more bytes that
70 represents a single character using the locale's encoding scheme; a
71 null byte always represents the null character. A @dfn{multibyte
72 string} is a string that consists entirely of multibyte
73 characters. In contrast, a @dfn{wide string} is a null-terminated
74 sequence of @code{wchar_t} objects. A wide-string variable is usually
75 declared to be a pointer of type @code{wchar_t *}, by analogy with
76 string variables and @code{char *}. @xref{Extended Char Intro}.
79 @cindex null wide character
80 By convention, the @dfn{null byte}, @code{'\0'},
81 marks the end of a string and the @dfn{null wide character},
82 @code{L'\0'}, marks the end of a wide string. For example, in
83 testing to see whether the @code{char *} variable @var{p} points to a
84 null byte marking the end of a string, you can write
85 @code{!*@var{p}} or @code{*@var{p} == '\0'}.
87 A null byte is quite different conceptually from a null pointer,
88 although both are represented by the integer constant @code{0}.
90 @cindex string literal
91 A @dfn{string literal} appears in C program source as a multibyte
92 string between double-quote characters (@samp{"}). If the
93 initial double-quote character is immediately preceded by a capital
94 @samp{L} (ell) character (as in @code{L"foo"}), it is a wide string
95 literal. String literals can also contribute to @dfn{string
96 concatenation}: @code{"a" "b"} is the same as @code{"ab"}.
97 For wide strings one can use either
98 @code{L"a" L"b"} or @code{L"a" "b"}. Modification of string literals is
99 not allowed by the GNU C compiler, because literals are placed in
102 Arrays that are declared @code{const} cannot be modified
103 either. It's generally good style to declare non-modifiable string
104 pointers to be of type @code{const char *}, since this often allows the
105 C compiler to detect accidental modifications as well as providing some
106 amount of documentation about what your program intends to do with the
109 The amount of memory allocated for a byte array may extend past the null byte
110 that marks the end of the string that the array contains. In this
111 document, the term @dfn{allocated size} is always used to refer to the
112 total amount of memory allocated for an array, while the term
113 @dfn{length} refers to the number of bytes up to (but not including)
114 the terminating null byte. Wide strings are similar, except their
115 sizes and lengths count wide characters, not bytes.
116 @cindex length of string
117 @cindex allocation size of string
118 @cindex size of string
119 @cindex string length
120 @cindex string allocation
122 A notorious source of program bugs is trying to put more bytes into a
123 string than fit in its allocated size. When writing code that extends
124 strings or moves bytes into a pre-allocated array, you should be
125 very careful to keep track of the length of the string and make explicit
126 checks for overflowing the array. Many of the library functions
127 @emph{do not} do this for you! Remember also that you need to allocate
128 an extra byte to hold the null byte that marks the end of the
131 @cindex single-byte string
132 @cindex multibyte string
133 Originally strings were sequences of bytes where each byte represented a
134 single character. This is still true today if the strings are encoded
135 using a single-byte character encoding. Things are different if the
136 strings are encoded using a multibyte encoding (for more information on
137 encodings see @ref{Extended Char Intro}). There is no difference in
138 the programming interface for these two kind of strings; the programmer
139 has to be aware of this and interpret the byte sequences accordingly.
141 But since there is no separate interface taking care of these
142 differences the byte-based string functions are sometimes hard to use.
143 Since the count parameters of these functions specify bytes a call to
144 @code{memcpy} could cut a multibyte character in the middle and put an
145 incomplete (and therefore unusable) byte sequence in the target buffer.
148 To avoid these problems later versions of the @w{ISO C} standard
149 introduce a second set of functions which are operating on @dfn{wide
150 characters} (@pxref{Extended Char Intro}). These functions don't have
151 the problems the single-byte versions have since every wide character is
152 a legal, interpretable value. This does not mean that cutting wide
153 strings at arbitrary points is without problems. It normally
154 is for alphabet-based languages (except for non-normalized text) but
155 languages based on syllables still have the problem that more than one
156 wide character is necessary to complete a logical unit. This is a
157 higher level problem which the @w{C library} functions are not designed
158 to solve. But it is at least good that no invalid byte sequences can be
159 created. Also, the higher level functions can also much more easily operate
160 on wide characters than on multibyte characters so that a common strategy
161 is to use wide characters internally whenever text is more than simply
164 The remaining of this chapter will discuss the functions for handling
165 wide strings in parallel with the discussion of
166 strings since there is almost always an exact equivalent
169 @node String/Array Conventions
170 @section String and Array Conventions
172 This chapter describes both functions that work on arbitrary arrays or
173 blocks of memory, and functions that are specific to strings and wide
176 Functions that operate on arbitrary blocks of memory have names
177 beginning with @samp{mem} and @samp{wmem} (such as @code{memcpy} and
178 @code{wmemcpy}) and invariably take an argument which specifies the size
179 (in bytes and wide characters respectively) of the block of memory to
180 operate on. The array arguments and return values for these functions
181 have type @code{void *} or @code{wchar_t *}. As a matter of style, the
182 elements of the arrays used with the @samp{mem} functions are referred
183 to as ``bytes''. You can pass any kind of pointer to these functions,
184 and the @code{sizeof} operator is useful in computing the value for the
185 size argument. Parameters to the @samp{wmem} functions must be of type
186 @code{wchar_t *}. These functions are not really usable with anything
187 but arrays of this type.
189 In contrast, functions that operate specifically on strings and wide
190 strings have names beginning with @samp{str} and @samp{wcs}
191 respectively (such as @code{strcpy} and @code{wcscpy}) and look for a
192 terminating null byte or null wide character instead of requiring an explicit
193 size argument to be passed. (Some of these functions accept a specified
194 maximum length, but they also check for premature termination.)
195 The array arguments and return values for these
196 functions have type @code{char *} and @code{wchar_t *} respectively, and
197 the array elements are referred to as ``bytes'' and ``wide
200 In many cases, there are both @samp{mem} and @samp{str}/@samp{wcs}
201 versions of a function. The one that is more appropriate to use depends
202 on the exact situation. When your program is manipulating arbitrary
203 arrays or blocks of storage, then you should always use the @samp{mem}
204 functions. On the other hand, when you are manipulating
205 strings it is usually more convenient to use the @samp{str}/@samp{wcs}
206 functions, unless you already know the length of the string in advance.
207 The @samp{wmem} functions should be used for wide character arrays with
211 @cindex parameter promotion
212 Some of the memory and string functions take single characters as
213 arguments. Since a value of type @code{char} is automatically promoted
214 into a value of type @code{int} when used as a parameter, the functions
215 are declared with @code{int} as the type of the parameter in question.
216 In case of the wide character functions the situation is similar: the
217 parameter type for a single wide character is @code{wint_t} and not
218 @code{wchar_t}. This would for many implementations not be necessary
219 since @code{wchar_t} is large enough to not be automatically
220 promoted, but since the @w{ISO C} standard does not require such a
221 choice of types the @code{wint_t} type is used.
224 @section String Length
226 You can get the length of a string using the @code{strlen} function.
227 This function is declared in the header file @file{string.h}.
230 @deftypefun size_t strlen (const char *@var{s})
231 @standards{ISO, string.h}
232 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
233 The @code{strlen} function returns the length of the
234 string @var{s} in bytes. (In other words, it returns the offset of the
235 terminating null byte within the array.)
239 strlen ("hello, world")
243 When applied to an array, the @code{strlen} function returns
244 the length of the string stored there, not its allocated size. You can
245 get the allocated size of the array that holds a string using
246 the @code{sizeof} operator:
249 char string[32] = "hello, world";
256 But beware, this will not work unless @var{string} is the
257 array itself, not a pointer to it. For example:
260 char string[32] = "hello, world";
265 @result{} 4 /* @r{(on a machine with 4 byte pointers)} */
268 This is an easy mistake to make when you are working with functions that
269 take string arguments; those arguments are always pointers, not arrays.
271 It must also be noted that for multibyte encoded strings the return
272 value does not have to correspond to the number of characters in the
273 string. To get this value the string can be converted to wide
274 characters and @code{wcslen} can be used or something like the following
278 /* @r{The input is in @code{string}.}
279 @r{The length is expected in @code{n}.} */
282 char *scopy = string;
283 /* In initial state. */
284 memset (&t, '\0', sizeof (t));
285 /* Determine number of characters. */
286 n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
290 This is cumbersome to do so if the number of characters (as opposed to
291 bytes) is needed often it is better to work with wide characters.
294 The wide character equivalent is declared in @file{wchar.h}.
296 @deftypefun size_t wcslen (const wchar_t *@var{ws})
297 @standards{ISO, wchar.h}
298 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
299 The @code{wcslen} function is the wide character equivalent to
300 @code{strlen}. The return value is the number of wide characters in the
301 wide string pointed to by @var{ws} (this is also the offset of
302 the terminating null wide character of @var{ws}).
304 Since there are no multi wide character sequences making up one wide
305 character the return value is not only the offset in the array, it is
306 also the number of wide characters.
308 This function was introduced in @w{Amendment 1} to @w{ISO C90}.
311 @deftypefun size_t strnlen (const char *@var{s}, size_t @var{maxlen})
312 @standards{GNU, string.h}
313 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
314 If the array @var{s} of size @var{maxlen} contains a null byte,
315 the @code{strnlen} function returns the length of the string @var{s} in
317 returns @var{maxlen}. Therefore this function is equivalent to
318 @code{(strlen (@var{s}) < @var{maxlen} ? strlen (@var{s}) : @var{maxlen})}
320 is more efficient and works even if @var{s} is not null-terminated so
321 long as @var{maxlen} does not exceed the size of @var{s}'s array.
324 char string[32] = "hello, world";
331 This function is a GNU extension and is declared in @file{string.h}.
334 @deftypefun size_t wcsnlen (const wchar_t *@var{ws}, size_t @var{maxlen})
335 @standards{GNU, wchar.h}
336 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
337 @code{wcsnlen} is the wide character equivalent to @code{strnlen}. The
338 @var{maxlen} parameter specifies the maximum number of wide characters.
340 This function is a GNU extension and is declared in @file{wchar.h}.
343 @node Copying Strings and Arrays
344 @section Copying Strings and Arrays
346 You can use the functions described in this section to copy the contents
347 of strings, wide strings, and arrays. The @samp{str} and @samp{mem}
348 functions are declared in @file{string.h} while the @samp{w} functions
349 are declared in @file{wchar.h}.
352 @cindex copying strings and arrays
353 @cindex string copy functions
354 @cindex array copy functions
355 @cindex concatenating strings
356 @cindex string concatenation functions
358 A helpful way to remember the ordering of the arguments to the functions
359 in this section is that it corresponds to an assignment expression, with
360 the destination array specified to the left of the source array. Most
361 of these functions return the address of the destination array; a few
362 return the address of the destination's terminating null, or of just
363 past the destination.
365 Most of these functions do not work properly if the source and
366 destination arrays overlap. For example, if the beginning of the
367 destination array overlaps the end of the source array, the original
368 contents of that part of the source array may get overwritten before it
369 is copied. Even worse, in the case of the string functions, the null
370 byte marking the end of the string may be lost, and the copy
371 function might get stuck in a loop trashing all the memory allocated to
374 All functions that have problems copying between overlapping arrays are
375 explicitly identified in this manual. In addition to functions in this
376 section, there are a few others like @code{sprintf} (@pxref{Formatted
377 Output Functions}) and @code{scanf} (@pxref{Formatted Input
380 @deftypefun {void *} memcpy (void *restrict @var{to}, const void *restrict @var{from}, size_t @var{size})
381 @standards{ISO, string.h}
382 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
383 The @code{memcpy} function copies @var{size} bytes from the object
384 beginning at @var{from} into the object beginning at @var{to}. The
385 behavior of this function is undefined if the two arrays @var{to} and
386 @var{from} overlap; use @code{memmove} instead if overlapping is possible.
388 The value returned by @code{memcpy} is the value of @var{to}.
390 Here is an example of how you might use @code{memcpy} to copy the
391 contents of an array:
394 struct foo *oldarray, *newarray;
397 memcpy (new, old, arraysize * sizeof (struct foo));
401 @deftypefun {wchar_t *} wmemcpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom}, size_t @var{size})
402 @standards{ISO, wchar.h}
403 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
404 The @code{wmemcpy} function copies @var{size} wide characters from the object
405 beginning at @var{wfrom} into the object beginning at @var{wto}. The
406 behavior of this function is undefined if the two arrays @var{wto} and
407 @var{wfrom} overlap; use @code{wmemmove} instead if overlapping is possible.
409 The following is a possible implementation of @code{wmemcpy} but there
410 are more optimizations possible.
414 wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
417 return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
421 The value returned by @code{wmemcpy} is the value of @var{wto}.
423 This function was introduced in @w{Amendment 1} to @w{ISO C90}.
426 @deftypefun {void *} mempcpy (void *restrict @var{to}, const void *restrict @var{from}, size_t @var{size})
427 @standards{GNU, string.h}
428 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
429 The @code{mempcpy} function is nearly identical to the @code{memcpy}
430 function. It copies @var{size} bytes from the object beginning at
431 @code{from} into the object pointed to by @var{to}. But instead of
432 returning the value of @var{to} it returns a pointer to the byte
433 following the last written byte in the object beginning at @var{to}.
434 I.e., the value is @code{((void *) ((char *) @var{to} + @var{size}))}.
436 This function is useful in situations where a number of objects shall be
437 copied to consecutive memory positions.
441 combine (void *o1, size_t s1, void *o2, size_t s2)
443 void *result = malloc (s1 + s2);
445 mempcpy (mempcpy (result, o1, s1), o2, s2);
450 This function is a GNU extension.
453 @deftypefun {wchar_t *} wmempcpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom}, size_t @var{size})
454 @standards{GNU, wchar.h}
455 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
456 The @code{wmempcpy} function is nearly identical to the @code{wmemcpy}
457 function. It copies @var{size} wide characters from the object
458 beginning at @code{wfrom} into the object pointed to by @var{wto}. But
459 instead of returning the value of @var{wto} it returns a pointer to the
460 wide character following the last written wide character in the object
461 beginning at @var{wto}. I.e., the value is @code{@var{wto} + @var{size}}.
463 This function is useful in situations where a number of objects shall be
464 copied to consecutive memory positions.
466 The following is a possible implementation of @code{wmemcpy} but there
467 are more optimizations possible.
471 wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
474 return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
478 This function is a GNU extension.
481 @deftypefun {void *} memmove (void *@var{to}, const void *@var{from}, size_t @var{size})
482 @standards{ISO, string.h}
483 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
484 @code{memmove} copies the @var{size} bytes at @var{from} into the
485 @var{size} bytes at @var{to}, even if those two blocks of space
486 overlap. In the case of overlap, @code{memmove} is careful to copy the
487 original values of the bytes in the block at @var{from}, including those
488 bytes which also belong to the block at @var{to}.
490 The value returned by @code{memmove} is the value of @var{to}.
493 @deftypefun {wchar_t *} wmemmove (wchar_t *@var{wto}, const wchar_t *@var{wfrom}, size_t @var{size})
494 @standards{ISO, wchar.h}
495 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
496 @code{wmemmove} copies the @var{size} wide characters at @var{wfrom}
497 into the @var{size} wide characters at @var{wto}, even if those two
498 blocks of space overlap. In the case of overlap, @code{wmemmove} is
499 careful to copy the original values of the wide characters in the block
500 at @var{wfrom}, including those wide characters which also belong to the
503 The following is a possible implementation of @code{wmemcpy} but there
504 are more optimizations possible.
508 wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
511 return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
515 The value returned by @code{wmemmove} is the value of @var{wto}.
517 This function is a GNU extension.
520 @deftypefun {void *} memccpy (void *restrict @var{to}, const void *restrict @var{from}, int @var{c}, size_t @var{size})
521 @standards{SVID, string.h}
522 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
523 This function copies no more than @var{size} bytes from @var{from} to
524 @var{to}, stopping if a byte matching @var{c} is found. The return
525 value is a pointer into @var{to} one byte past where @var{c} was copied,
526 or a null pointer if no byte matching @var{c} appeared in the first
527 @var{size} bytes of @var{from}.
530 @deftypefun {void *} memset (void *@var{block}, int @var{c}, size_t @var{size})
531 @standards{ISO, string.h}
532 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
533 This function copies the value of @var{c} (converted to an
534 @code{unsigned char}) into each of the first @var{size} bytes of the
535 object beginning at @var{block}. It returns the value of @var{block}.
538 @deftypefun {wchar_t *} wmemset (wchar_t *@var{block}, wchar_t @var{wc}, size_t @var{size})
539 @standards{ISO, wchar.h}
540 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
541 This function copies the value of @var{wc} into each of the first
542 @var{size} wide characters of the object beginning at @var{block}. It
543 returns the value of @var{block}.
546 @deftypefun {char *} strcpy (char *restrict @var{to}, const char *restrict @var{from})
547 @standards{ISO, string.h}
548 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
549 This copies bytes from the string @var{from} (up to and including
550 the terminating null byte) into the string @var{to}. Like
551 @code{memcpy}, this function has undefined results if the strings
552 overlap. The return value is the value of @var{to}.
555 @deftypefun {wchar_t *} wcscpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom})
556 @standards{ISO, wchar.h}
557 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
558 This copies wide characters from the wide string @var{wfrom} (up to and
559 including the terminating null wide character) into the string
560 @var{wto}. Like @code{wmemcpy}, this function has undefined results if
561 the strings overlap. The return value is the value of @var{wto}.
564 @deftypefun {char *} strdup (const char *@var{s})
565 @standards{SVID, string.h}
566 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
567 This function copies the string @var{s} into a newly
568 allocated string. The string is allocated using @code{malloc}; see
569 @ref{Unconstrained Allocation}. If @code{malloc} cannot allocate space
570 for the new string, @code{strdup} returns a null pointer. Otherwise it
571 returns a pointer to the new string.
574 @deftypefun {wchar_t *} wcsdup (const wchar_t *@var{ws})
575 @standards{GNU, wchar.h}
576 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
577 This function copies the wide string @var{ws}
578 into a newly allocated string. The string is allocated using
579 @code{malloc}; see @ref{Unconstrained Allocation}. If @code{malloc}
580 cannot allocate space for the new string, @code{wcsdup} returns a null
581 pointer. Otherwise it returns a pointer to the new wide string.
583 This function is a GNU extension.
586 @deftypefun {char *} stpcpy (char *restrict @var{to}, const char *restrict @var{from})
587 @standards{Unknown origin, string.h}
588 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
589 This function is like @code{strcpy}, except that it returns a pointer to
590 the end of the string @var{to} (that is, the address of the terminating
591 null byte @code{to + strlen (from)}) rather than the beginning.
593 For example, this program uses @code{stpcpy} to concatenate @samp{foo}
594 and @samp{bar} to produce @samp{foobar}, which it then prints.
597 @include stpcpy.c.texi
600 This function is part of POSIX.1-2008 and later editions, but was
601 available in @theglibc{} and other systems as an extension long before
604 Its behavior is undefined if the strings overlap. The function is
605 declared in @file{string.h}.
608 @deftypefun {wchar_t *} wcpcpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom})
609 @standards{GNU, wchar.h}
610 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
611 This function is like @code{wcscpy}, except that it returns a pointer to
612 the end of the string @var{wto} (that is, the address of the terminating
613 null wide character @code{wto + wcslen (wfrom)}) rather than the beginning.
615 This function is not part of ISO or POSIX but was found useful while
616 developing @theglibc{} itself.
618 The behavior of @code{wcpcpy} is undefined if the strings overlap.
620 @code{wcpcpy} is a GNU extension and is declared in @file{wchar.h}.
623 @deftypefn {Macro} {char *} strdupa (const char *@var{s})
624 @standards{GNU, string.h}
625 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
626 This macro is similar to @code{strdup} but allocates the new string
627 using @code{alloca} instead of @code{malloc} (@pxref{Variable Size
628 Automatic}). This means of course the returned string has the same
629 limitations as any block of memory allocated using @code{alloca}.
631 For obvious reasons @code{strdupa} is implemented only as a macro;
632 you cannot get the address of this function. Despite this limitation
633 it is a useful function. The following code shows a situation where
634 using @code{malloc} would be a lot more expensive.
637 @include strdupa.c.texi
640 Please note that calling @code{strtok} using @var{path} directly is
641 invalid. It is also not allowed to call @code{strdupa} in the argument
642 list of @code{strtok} since @code{strdupa} uses @code{alloca}
643 (@pxref{Variable Size Automatic}) can interfere with the parameter
646 This function is only available if GNU CC is used.
649 @deftypefun void bcopy (const void *@var{from}, void *@var{to}, size_t @var{size})
650 @standards{BSD, string.h}
651 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
652 This is a partially obsolete alternative for @code{memmove}, derived from
653 BSD. Note that it is not quite equivalent to @code{memmove}, because the
654 arguments are not in the same order and there is no return value.
657 @deftypefun void bzero (void *@var{block}, size_t @var{size})
658 @standards{BSD, string.h}
659 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
660 This is a partially obsolete alternative for @code{memset}, derived from
661 BSD. Note that it is not as general as @code{memset}, because the only
662 value it can store is zero.
665 @node Concatenating Strings
666 @section Concatenating Strings
669 @cindex concatenating strings
670 @cindex string concatenation functions
672 The functions described in this section concatenate the contents of a
673 string or wide string to another. They follow the string-copying
674 functions in their conventions. @xref{Copying Strings and Arrays}.
675 @samp{strcat} is declared in the header file @file{string.h} while
676 @samp{wcscat} is declared in @file{wchar.h}.
678 As noted below, these functions are problematic as their callers may
679 have performance issues.
681 @deftypefun {char *} strcat (char *restrict @var{to}, const char *restrict @var{from})
682 @standards{ISO, string.h}
683 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
684 The @code{strcat} function is similar to @code{strcpy}, except that the
685 bytes from @var{from} are concatenated or appended to the end of
686 @var{to}, instead of overwriting it. That is, the first byte from
687 @var{from} overwrites the null byte marking the end of @var{to}.
689 An equivalent definition for @code{strcat} would be:
693 strcat (char *restrict to, const char *restrict from)
695 strcpy (to + strlen (to), from);
700 This function has undefined results if the strings overlap.
702 As noted below, this function has significant performance issues.
705 @deftypefun {wchar_t *} wcscat (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom})
706 @standards{ISO, wchar.h}
707 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
708 The @code{wcscat} function is similar to @code{wcscpy}, except that the
709 wide characters from @var{wfrom} are concatenated or appended to the end of
710 @var{wto}, instead of overwriting it. That is, the first wide character from
711 @var{wfrom} overwrites the null wide character marking the end of @var{wto}.
713 An equivalent definition for @code{wcscat} would be:
717 wcscat (wchar_t *wto, const wchar_t *wfrom)
719 wcscpy (wto + wcslen (wto), wfrom);
724 This function has undefined results if the strings overlap.
726 As noted below, this function has significant performance issues.
729 Programmers using the @code{strcat} or @code{wcscat} functions (or the
730 @code{strlcat}, @code{strncat} and @code{wcsncat} functions defined in
731 a later section, for that matter)
732 can easily be recognized as lazy and reckless. In almost all situations
733 the lengths of the participating strings are known (it better should be
734 since how can one otherwise ensure the allocated size of the buffer is
735 sufficient?) Or at least, one could know them if one keeps track of the
736 results of the various function calls. But then it is very inefficient
737 to use @code{strcat}/@code{wcscat}. A lot of time is wasted finding the
738 end of the destination string so that the actual copying can start.
739 This is a common example:
743 /* @r{This function concatenates arbitrarily many strings. The last}
744 @r{parameter must be @code{NULL}.} */
746 concat (const char *str, @dots{})
754 /* @r{Determine how much space we need.} */
755 for (const char *s = str; s != NULL; s = va_arg (ap, const char *))
760 char *result = malloc (total);
765 /* @r{Copy the strings.} */
766 for (s = str; s != NULL; s = va_arg (ap2, const char *))
776 This looks quite simple, especially the second loop where the strings
777 are actually copied. But these innocent lines hide a major performance
778 penalty. Just imagine that ten strings of 100 bytes each have to be
779 concatenated. For the second string we search the already stored 100
780 bytes for the end of the string so that we can append the next string.
781 For all strings in total the comparisons necessary to find the end of
782 the intermediate results sums up to 5500! If we combine the copying
783 with the search for the allocation we can write this function more
788 concat (const char *str, @dots{})
790 size_t allocated = 100;
791 char *result = malloc (allocated);
796 size_t resultlen = 0;
801 for (const char *s = str; s != NULL; s = va_arg (ap, const char *))
803 size_t len = strlen (s);
805 /* @r{Resize the allocated memory if necessary.} */
806 if (resultlen + len + 1 > allocated)
809 newp = reallocarray (result, allocated, 2);
819 memcpy (result + resultlen, s, len);
823 /* @r{Terminate the result string.} */
824 result[resultlen++] = '\0';
826 /* @r{Resize memory to the optimal size.} */
827 newp = realloc (result, resultlen);
838 With a bit more knowledge about the input strings one could fine-tune
839 the memory allocation. The difference we are pointing to here is that
840 we don't use @code{strcat} anymore. We always keep track of the length
841 of the current intermediate result so we can save ourselves the search for the
842 end of the string and use @code{mempcpy}. Please note that we also
843 don't use @code{stpcpy} which might seem more natural since we are handling
844 strings. But this is not necessary since we already know the
845 length of the string and therefore can use the faster memory copying
846 function. The example would work for wide characters the same way.
848 Whenever a programmer feels the need to use @code{strcat} she or he
849 should think twice and look through the program to see whether the code cannot
850 be rewritten to take advantage of already calculated results.
851 The related functions @code{strlcat}, @code{strncat},
852 @code{wcscat} and @code{wcsncat}
853 are almost always unnecessary, too.
854 Again: it is almost always unnecessary to use functions like @code{strcat}.
856 @node Truncating Strings
857 @section Truncating Strings while Copying
858 @cindex truncating strings
859 @cindex string truncation
861 The functions described in this section copy or concatenate the
862 possibly-truncated contents of a string or array to another, and
863 similarly for wide strings. They follow the string-copying functions
864 in their header conventions. @xref{Copying Strings and Arrays}. The
865 @samp{str} functions are declared in the header file @file{string.h}
866 and the @samp{wc} functions are declared in the file @file{wchar.h}.
868 As noted below, these functions are problematic as their callers may
869 have truncation-related bugs and performance issues.
871 @deftypefun {char *} strncpy (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
872 @standards{C90, string.h}
873 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
874 This function is similar to @code{strcpy} but always copies exactly
875 @var{size} bytes into @var{to}.
877 If @var{from} does not contain a null byte in its first @var{size}
878 bytes, @code{strncpy} copies just the first @var{size} bytes. In this
879 case no null terminator is written into @var{to}.
881 Otherwise @var{from} must be a string with length less than
882 @var{size}. In this case @code{strncpy} copies all of @var{from},
883 followed by enough null bytes to add up to @var{size} bytes in all.
885 The behavior of @code{strncpy} is undefined if the strings overlap.
887 This function was designed for now-rarely-used arrays consisting of
888 non-null bytes followed by zero or more null bytes. It needs to set
889 all @var{size} bytes of the destination, even when @var{size} is much
890 greater than the length of @var{from}. As noted below, this function
891 is generally a poor choice for processing strings.
894 @deftypefun {wchar_t *} wcsncpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom}, size_t @var{size})
895 @standards{ISO, wchar.h}
896 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
897 This function is similar to @code{wcscpy} but always copies exactly
898 @var{size} wide characters into @var{wto}.
900 If @var{wfrom} does not contain a null wide character in its first
901 @var{size} wide characters, then @code{wcsncpy} copies just the first
902 @var{size} wide characters. In this case no null terminator is
903 written into @var{wto}.
905 Otherwise @var{wfrom} must be a wide string with length less than
906 @var{size}. In this case @code{wcsncpy} copies all of @var{wfrom},
907 followed by enough null wide characters to add up to @var{size} wide
910 The behavior of @code{wcsncpy} is undefined if the strings overlap.
912 This function is the wide-character counterpart of @code{strncpy} and
913 suffers from most of the problems that @code{strncpy} does. For
914 example, as noted below, this function is generally a poor choice for
918 @deftypefun {char *} strndup (const char *@var{s}, size_t @var{size})
919 @standards{GNU, string.h}
920 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
921 This function is similar to @code{strdup} but always copies at most
922 @var{size} bytes into the newly allocated string.
924 If the length of @var{s} is more than @var{size}, then @code{strndup}
925 copies just the first @var{size} bytes and adds a closing null byte.
926 Otherwise all bytes are copied and the string is terminated.
928 This function differs from @code{strncpy} in that it always terminates
929 the destination string.
931 As noted below, this function is generally a poor choice for
934 @code{strndup} is a GNU extension.
937 @deftypefn {Macro} {char *} strndupa (const char *@var{s}, size_t @var{size})
938 @standards{GNU, string.h}
939 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
940 This function is similar to @code{strndup} but like @code{strdupa} it
941 allocates the new string using @code{alloca} @pxref{Variable Size
942 Automatic}. The same advantages and limitations of @code{strdupa} are
943 valid for @code{strndupa}, too.
945 This function is implemented only as a macro, just like @code{strdupa}.
946 Just as @code{strdupa} this macro also must not be used inside the
947 parameter list in a function call.
949 As noted below, this function is generally a poor choice for
952 @code{strndupa} is only available if GNU CC is used.
955 @deftypefun {char *} stpncpy (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
956 @standards{GNU, string.h}
957 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
958 This function is similar to @code{stpcpy} but copies always exactly
959 @var{size} bytes into @var{to}.
961 If the length of @var{from} is more than @var{size}, then @code{stpncpy}
962 copies just the first @var{size} bytes and returns a pointer to the
963 byte directly following the one which was copied last. Note that in
964 this case there is no null terminator written into @var{to}.
966 If the length of @var{from} is less than @var{size}, then @code{stpncpy}
967 copies all of @var{from}, followed by enough null bytes to add up
968 to @var{size} bytes in all. This behavior is rarely useful, but it
969 is implemented to be useful in contexts where this behavior of the
970 @code{strncpy} is used. @code{stpncpy} returns a pointer to the
971 @emph{first} written null byte.
973 This function is not part of ISO or POSIX but was found useful while
974 developing @theglibc{} itself.
976 Its behavior is undefined if the strings overlap. The function is
977 declared in @file{string.h}.
979 As noted below, this function is generally a poor choice for
983 @deftypefun {wchar_t *} wcpncpy (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom}, size_t @var{size})
984 @standards{GNU, wchar.h}
985 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
986 This function is similar to @code{wcpcpy} but copies always exactly
987 @var{wsize} wide characters into @var{wto}.
989 If the length of @var{wfrom} is more than @var{size}, then
990 @code{wcpncpy} copies just the first @var{size} wide characters and
991 returns a pointer to the wide character directly following the last
992 non-null wide character which was copied last. Note that in this case
993 there is no null terminator written into @var{wto}.
995 If the length of @var{wfrom} is less than @var{size}, then @code{wcpncpy}
996 copies all of @var{wfrom}, followed by enough null wide characters to add up
997 to @var{size} wide characters in all. This behavior is rarely useful, but it
998 is implemented to be useful in contexts where this behavior of the
999 @code{wcsncpy} is used. @code{wcpncpy} returns a pointer to the
1000 @emph{first} written null wide character.
1002 This function is not part of ISO or POSIX but was found useful while
1003 developing @theglibc{} itself.
1005 Its behavior is undefined if the strings overlap.
1007 As noted below, this function is generally a poor choice for
1010 @code{wcpncpy} is a GNU extension.
1013 @deftypefun {char *} strncat (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
1014 @standards{ISO, string.h}
1015 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1016 This function is like @code{strcat} except that not more than @var{size}
1017 bytes from @var{from} are appended to the end of @var{to}, and
1018 @var{from} need not be null-terminated. A single null byte is also
1019 always appended to @var{to}, so the total
1020 allocated size of @var{to} must be at least @code{@var{size} + 1} bytes
1021 longer than its initial length.
1023 The @code{strncat} function could be implemented like this:
1028 strncat (char *to, const char *from, size_t size)
1030 size_t len = strlen (to);
1031 memcpy (to + len, from, strnlen (from, size));
1032 to[len + strnlen (from, size)] = '\0';
1038 The behavior of @code{strncat} is undefined if the strings overlap.
1040 As a companion to @code{strncpy}, @code{strncat} was designed for
1041 now-rarely-used arrays consisting of non-null bytes followed by zero
1042 or more null bytes. However, As noted below, this function is generally a poor
1043 choice for processing strings. Also, this function has significant
1044 performance issues. @xref{Concatenating Strings}.
1047 @deftypefun {wchar_t *} wcsncat (wchar_t *restrict @var{wto}, const wchar_t *restrict @var{wfrom}, size_t @var{size})
1048 @standards{ISO, wchar.h}
1049 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1050 This function is like @code{wcscat} except that not more than @var{size}
1051 wide characters from @var{from} are appended to the end of @var{to},
1052 and @var{from} need not be null-terminated. A single null wide
1053 character is also always appended to @var{to}, so the total allocated
1054 size of @var{to} must be at least @code{wcsnlen (@var{wfrom},
1055 @var{size}) + 1} wide characters longer than its initial length.
1057 The @code{wcsncat} function could be implemented like this:
1062 wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
1065 size_t len = wcslen (wto);
1066 memcpy (wto + len, wfrom, wcsnlen (wfrom, size) * sizeof (wchar_t));
1067 wto[len + wcsnlen (wfrom, size)] = L'\0';
1073 The behavior of @code{wcsncat} is undefined if the strings overlap.
1075 As noted below, this function is generally a poor choice for
1076 processing strings. Also, this function has significant performance
1077 issues. @xref{Concatenating Strings}.
1080 @deftypefun size_t strlcpy (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
1081 @standards{BSD, string.h}
1082 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1083 This function copies the string @var{from} to the destination array
1084 @var{to}, limiting the result's size (including the null terminator)
1085 to @var{size}. The caller should ensure that @var{size} includes room
1086 for the result's terminating null byte.
1088 If @var{size} is greater than the length of the string @var{from},
1089 this function copies the non-null bytes of the string
1090 @var{from} to the destination array @var{to},
1091 and terminates the copy with a null byte. Like other
1092 string functions such as @code{strcpy}, but unlike @code{strncpy}, any
1093 remaining bytes in the destination array remain unchanged.
1095 If @var{size} is nonzero and less than or equal to the the length of the string
1096 @var{from}, this function copies only the first @samp{@var{size} - 1}
1097 bytes to the destination array @var{to}, and writes a terminating null
1098 byte to the last byte of the array.
1100 This function returns the length of the string @var{from}. This means
1101 that truncation occurs if and only if the returned value is greater
1102 than or equal to @var{size}.
1104 The behavior is undefined if @var{to} or @var{from} is a null pointer,
1105 or if the destination array's size is less than @var{size}, or if the
1106 string @var{from} overlaps the first @var{size} bytes of the
1109 As noted below, this function is generally a poor choice for
1110 processing strings. Also, this function has a performance issue,
1111 as its time cost is proportional to the length of @var{from}
1112 even when @var{size} is small.
1114 This function is derived from OpenBSD 2.4.
1117 @deftypefun size_t wcslcpy (wchar_t *restrict @var{to}, const wchar_t *restrict @var{from}, size_t @var{size})
1118 @standards{BSD, string.h}
1119 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1120 This function is a variant of @code{strlcpy} for wide strings.
1121 The @var{size} argument counts the length of the destination buffer in
1122 wide characters (and not bytes).
1124 This function is derived from BSD.
1127 @deftypefun size_t strlcat (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
1128 @standards{BSD, string.h}
1129 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1130 This function appends the string @var{from} to the
1131 string @var{to}, limiting the result's total size (including the null
1132 terminator) to @var{size}. The caller should ensure that @var{size}
1133 includes room for the result's terminating null byte.
1135 This function copies as much as possible of the string @var{from} into
1136 the array at @var{to} of @var{size} bytes, starting at the terminating
1137 null byte of the original string @var{to}. In effect, this appends
1138 the string @var{from} to the string @var{to}. Although the resulting
1139 string will contain a null terminator, it can be truncated (not all
1140 bytes in @var{from} may be copied).
1142 This function returns the sum of the original length of @var{to} and
1143 the length of @var{from}. This means that truncation occurs if and
1144 only if the returned value is greater than or equal to @var{size}.
1146 The behavior is undefined if @var{to} or @var{from} is a null pointer,
1147 or if the destination array's size is less than @var{size}, or if the
1148 destination array does not contain a null byte in its first @var{size}
1149 bytes, or if the string @var{from} overlaps the first @var{size} bytes
1150 of the destination array.
1152 As noted below, this function is generally a poor choice for
1153 processing strings. Also, this function has significant performance
1154 issues. @xref{Concatenating Strings}.
1156 This function is derived from OpenBSD 2.4.
1159 @deftypefun size_t wcslcat (wchar_t *restrict @var{to}, const wchar_t *restrict @var{from}, size_t @var{size})
1160 @standards{BSD, string.h}
1161 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1162 This function is a variant of @code{strlcat} for wide strings.
1163 The @var{size} argument counts the length of the destination buffer in
1164 wide characters (and not bytes).
1166 This function is derived from BSD.
1169 Because these functions can abruptly truncate strings or wide strings,
1170 they are generally poor choices for processing them. When copying or
1171 concatening multibyte strings, they can truncate within a multibyte
1172 character so that the result is not a valid multibyte string. When
1173 combining or concatenating multibyte or wide strings, they may
1174 truncate the output after a combining character, resulting in a
1175 corrupted grapheme. They can cause bugs even when processing
1176 single-byte strings: for example, when calculating an ASCII-only user
1177 name, a truncated name can identify the wrong user.
1179 Although some buffer overruns can be prevented by manually replacing
1180 calls to copying functions with calls to truncation functions, there
1181 are often easier and safer automatic techniques, such as fortification
1182 (@pxref{Source Fortification}) and AddressSanitizer
1183 (@pxref{Instrumentation Options,, Program Instrumentation Options, gcc, Using GCC}).
1184 Because truncation functions can mask
1185 application bugs that would otherwise be caught by the automatic
1186 techniques, these functions should be used only when the application's
1187 underlying logic requires truncation.
1189 @strong{Note:} GNU programs should not truncate strings or wide
1190 strings to fit arbitrary size limits. @xref{Semantics, , Writing
1191 Robust Programs, standards, The GNU Coding Standards}. Instead of
1192 string-truncation functions, it is usually better to use dynamic
1193 memory allocation (@pxref{Unconstrained Allocation}) and functions
1194 such as @code{strdup} or @code{asprintf} to construct strings.
1196 @node String/Array Comparison
1197 @section String/Array Comparison
1198 @cindex comparing strings and arrays
1199 @cindex string comparison functions
1200 @cindex array comparison functions
1201 @cindex predicates on strings
1202 @cindex predicates on arrays
1204 You can use the functions in this section to perform comparisons on the
1205 contents of strings and arrays. As well as checking for equality, these
1206 functions can also be used as the ordering functions for sorting
1207 operations. @xref{Searching and Sorting}, for an example of this.
1209 Unlike most comparison operations in C, the string comparison functions
1210 return a nonzero value if the strings are @emph{not} equivalent rather
1211 than if they are. The sign of the value indicates the relative ordering
1212 of the first part of the strings that are not equivalent: a
1213 negative value indicates that the first string is ``less'' than the
1214 second, while a positive value indicates that the first string is
1217 The most common use of these functions is to check only for equality.
1218 This is canonically done with an expression like @w{@samp{! strcmp (s1, s2)}}.
1220 All of these functions are declared in the header file @file{string.h}.
1223 @deftypefun int memcmp (const void *@var{a1}, const void *@var{a2}, size_t @var{size})
1224 @standards{ISO, string.h}
1225 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1226 The function @code{memcmp} compares the @var{size} bytes of memory
1227 beginning at @var{a1} against the @var{size} bytes of memory beginning
1228 at @var{a2}. The value returned has the same sign as the difference
1229 between the first differing pair of bytes (interpreted as @code{unsigned
1230 char} objects, then promoted to @code{int}).
1232 If the contents of the two blocks are equal, @code{memcmp} returns
1236 @deftypefun int wmemcmp (const wchar_t *@var{a1}, const wchar_t *@var{a2}, size_t @var{size})
1237 @standards{ISO, wchar.h}
1238 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1239 The function @code{wmemcmp} compares the @var{size} wide characters
1240 beginning at @var{a1} against the @var{size} wide characters beginning
1241 at @var{a2}. The value returned is smaller than or larger than zero
1242 depending on whether the first differing wide character is @var{a1} is
1243 smaller or larger than the corresponding wide character in @var{a2}.
1245 If the contents of the two blocks are equal, @code{wmemcmp} returns
1249 On arbitrary arrays, the @code{memcmp} function is mostly useful for
1250 testing equality. It usually isn't meaningful to do byte-wise ordering
1251 comparisons on arrays of things other than bytes. For example, a
1252 byte-wise comparison on the bytes that make up floating-point numbers
1253 isn't likely to tell you anything about the relationship between the
1254 values of the floating-point numbers.
1256 @code{wmemcmp} is really only useful to compare arrays of type
1257 @code{wchar_t} since the function looks at @code{sizeof (wchar_t)} bytes
1258 at a time and this number of bytes is system dependent.
1260 You should also be careful about using @code{memcmp} to compare objects
1261 that can contain ``holes'', such as the padding inserted into structure
1262 objects to enforce alignment requirements, extra space at the end of
1263 unions, and extra bytes at the ends of strings whose length is less
1264 than their allocated size. The contents of these ``holes'' are
1265 indeterminate and may cause strange behavior when performing byte-wise
1266 comparisons. For more predictable results, perform an explicit
1267 component-wise comparison.
1269 For example, given a structure type definition like:
1285 you are better off writing a specialized comparison function to compare
1286 @code{struct foo} objects instead of comparing them with @code{memcmp}.
1288 @deftypefun int strcmp (const char *@var{s1}, const char *@var{s2})
1289 @standards{ISO, string.h}
1290 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1291 The @code{strcmp} function compares the string @var{s1} against
1292 @var{s2}, returning a value that has the same sign as the difference
1293 between the first differing pair of bytes (interpreted as
1294 @code{unsigned char} objects, then promoted to @code{int}).
1296 If the two strings are equal, @code{strcmp} returns @code{0}.
1298 A consequence of the ordering used by @code{strcmp} is that if @var{s1}
1299 is an initial substring of @var{s2}, then @var{s1} is considered to be
1300 ``less than'' @var{s2}.
1302 @code{strcmp} does not take sorting conventions of the language the
1303 strings are written in into account. To get that one has to use
1307 @deftypefun int wcscmp (const wchar_t *@var{ws1}, const wchar_t *@var{ws2})
1308 @standards{ISO, wchar.h}
1309 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1311 The @code{wcscmp} function compares the wide string @var{ws1}
1312 against @var{ws2}. The value returned is smaller than or larger than zero
1313 depending on whether the first differing wide character is @var{ws1} is
1314 smaller or larger than the corresponding wide character in @var{ws2}.
1316 If the two strings are equal, @code{wcscmp} returns @code{0}.
1318 A consequence of the ordering used by @code{wcscmp} is that if @var{ws1}
1319 is an initial substring of @var{ws2}, then @var{ws1} is considered to be
1320 ``less than'' @var{ws2}.
1322 @code{wcscmp} does not take sorting conventions of the language the
1323 strings are written in into account. To get that one has to use
1327 @deftypefun int strcasecmp (const char *@var{s1}, const char *@var{s2})
1328 @standards{BSD, string.h}
1329 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1330 @c Although this calls tolower multiple times, it's a macro, and
1331 @c strcasecmp is optimized so that the locale pointer is read only once.
1332 @c There are some asm implementations too, for which the single-read
1333 @c from locale TLS pointers also applies.
1334 This function is like @code{strcmp}, except that differences in case are
1335 ignored, and its arguments must be multibyte strings.
1336 How uppercase and lowercase characters are related is
1337 determined by the currently selected locale. In the standard @code{"C"}
1338 locale the characters @"A and @"a do not match but in a locale which
1339 regards these characters as parts of the alphabet they do match.
1342 @code{strcasecmp} is derived from BSD.
1345 @deftypefun int wcscasecmp (const wchar_t *@var{ws1}, const wchar_t *@var{ws2})
1346 @standards{GNU, wchar.h}
1347 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1348 @c Since towlower is not a macro, the locale object may be read multiple
1350 This function is like @code{wcscmp}, except that differences in case are
1351 ignored. How uppercase and lowercase characters are related is
1352 determined by the currently selected locale. In the standard @code{"C"}
1353 locale the characters @"A and @"a do not match but in a locale which
1354 regards these characters as parts of the alphabet they do match.
1357 @code{wcscasecmp} is a GNU extension.
1360 @deftypefun int strncmp (const char *@var{s1}, const char *@var{s2}, size_t @var{size})
1361 @standards{ISO, string.h}
1362 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1363 This function is the similar to @code{strcmp}, except that no more than
1364 @var{size} bytes are compared. In other words, if the two
1365 strings are the same in their first @var{size} bytes, the
1366 return value is zero.
1369 @deftypefun int wcsncmp (const wchar_t *@var{ws1}, const wchar_t *@var{ws2}, size_t @var{size})
1370 @standards{ISO, wchar.h}
1371 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1372 This function is similar to @code{wcscmp}, except that no more than
1373 @var{size} wide characters are compared. In other words, if the two
1374 strings are the same in their first @var{size} wide characters, the
1375 return value is zero.
1378 @deftypefun int strncasecmp (const char *@var{s1}, const char *@var{s2}, size_t @var{n})
1379 @standards{BSD, string.h}
1380 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1381 This function is like @code{strncmp}, except that differences in case
1382 are ignored, and the compared parts of the arguments should consist of
1383 valid multibyte characters.
1384 Like @code{strcasecmp}, it is locale dependent how
1385 uppercase and lowercase characters are related.
1388 @code{strncasecmp} is a GNU extension.
1391 @deftypefun int wcsncasecmp (const wchar_t *@var{ws1}, const wchar_t *@var{s2}, size_t @var{n})
1392 @standards{GNU, wchar.h}
1393 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1394 This function is like @code{wcsncmp}, except that differences in case
1395 are ignored. Like @code{wcscasecmp}, it is locale dependent how
1396 uppercase and lowercase characters are related.
1399 @code{wcsncasecmp} is a GNU extension.
1402 Here are some examples showing the use of @code{strcmp} and
1403 @code{strncmp} (equivalent examples can be constructed for the wide
1404 character functions). These examples assume the use of the ASCII
1405 character set. (If some other character set---say, EBCDIC---is used
1406 instead, then the glyphs are associated with different numeric codes,
1407 and the return values and ordering may differ.)
1410 strcmp ("hello", "hello")
1411 @result{} 0 /* @r{These two strings are the same.} */
1412 strcmp ("hello", "Hello")
1413 @result{} 32 /* @r{Comparisons are case-sensitive.} */
1414 strcmp ("hello", "world")
1415 @result{} -15 /* @r{The byte @code{'h'} comes before @code{'w'}.} */
1416 strcmp ("hello", "hello, world")
1417 @result{} -44 /* @r{Comparing a null byte against a comma.} */
1418 strncmp ("hello", "hello, world", 5)
1419 @result{} 0 /* @r{The initial 5 bytes are the same.} */
1420 strncmp ("hello, world", "hello, stupid world!!!", 5)
1421 @result{} 0 /* @r{The initial 5 bytes are the same.} */
1424 @deftypefun int strverscmp (const char *@var{s1}, const char *@var{s2})
1425 @standards{GNU, string.h}
1426 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1427 @c Calls isdigit multiple times, locale may change in between.
1428 The @code{strverscmp} function compares the string @var{s1} against
1429 @var{s2}, considering them as holding indices/version numbers. The
1430 return value follows the same conventions as found in the
1431 @code{strcmp} function. In fact, if @var{s1} and @var{s2} contain no
1432 digits, @code{strverscmp} behaves like @code{strcmp}
1433 (in the sense that the sign of the result is the same).
1435 The comparison algorithm which the @code{strverscmp} function implements
1436 differs slightly from other version-comparison algorithms. The
1437 implementation is based on a finite-state machine, whose behavior is
1442 The input strings are each split into sequences of non-digits and
1443 digits. These sequences can be empty at the beginning and end of the
1444 string. Digits are determined by the @code{isdigit} function and are
1445 thus subject to the current locale.
1448 Comparison starts with a (possibly empty) non-digit sequence. The first
1449 non-equal sequences of non-digits or digits determines the outcome of
1453 Corresponding non-digit sequences in both strings are compared
1454 lexicographically if their lengths are equal. If the lengths differ,
1455 the shorter non-digit sequence is extended with the input string
1456 character immediately following it (which may be the null terminator),
1457 the other sequence is truncated to be of the same (extended) length, and
1458 these two sequences are compared lexicographically. In the last case,
1459 the sequence comparison determines the result of the function because
1460 the extension character (or some character before it) is necessarily
1461 different from the character at the same offset in the other input
1465 For two sequences of digits, the number of leading zeros is counted (which
1466 can be zero). If the count differs, the string with more leading zeros
1467 in the digit sequence is considered smaller than the other string.
1470 If the two sequences of digits have no leading zeros, they are compared
1471 as integers, that is, the string with the longer digit sequence is
1472 deemed larger, and if both sequences are of equal length, they are
1473 compared lexicographically.
1476 If both digit sequences start with a zero and have an equal number of
1477 leading zeros, they are compared lexicographically if their lengths are
1478 the same. If the lengths differ, the shorter sequence is extended with
1479 the following character in its input string, and the other sequence is
1480 truncated to the same length, and both sequences are compared
1481 lexicographically (similar to the non-digit sequence case above).
1484 The treatment of leading zeros and the tie-breaking extension characters
1485 (which in effect propagate across non-digit/digit sequence boundaries)
1486 differs from other version-comparison algorithms.
1489 strverscmp ("no digit", "no digit")
1490 @result{} 0 /* @r{same behavior as strcmp.} */
1491 strverscmp ("item#99", "item#100")
1492 @result{} <0 /* @r{same prefix, but 99 < 100.} */
1493 strverscmp ("alpha1", "alpha001")
1494 @result{} >0 /* @r{different number of leading zeros (0 and 2).} */
1495 strverscmp ("part1_f012", "part1_f01")
1496 @result{} >0 /* @r{lexicographical comparison with leading zeros.} */
1497 strverscmp ("foo.009", "foo.0")
1498 @result{} <0 /* @r{different number of leading zeros (2 and 1).} */
1501 @code{strverscmp} is a GNU extension.
1504 @deftypefun int bcmp (const void *@var{a1}, const void *@var{a2}, size_t @var{size})
1505 @standards{BSD, string.h}
1506 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1507 This is an obsolete alias for @code{memcmp}, derived from BSD.
1510 @node Collation Functions
1511 @section Collation Functions
1513 @cindex collating strings
1514 @cindex string collation functions
1516 In some locales, the conventions for lexicographic ordering differ from
1517 the strict numeric ordering of character codes. For example, in Spanish
1518 most glyphs with diacritical marks such as accents are not considered
1519 distinct letters for the purposes of collation. On the other hand, in
1520 Czech the two-character sequence @samp{ch} is treated as a single letter
1521 that is collated between @samp{h} and @samp{i}.
1523 You can use the functions @code{strcoll} and @code{strxfrm} (declared in
1524 the headers file @file{string.h}) and @code{wcscoll} and @code{wcsxfrm}
1525 (declared in the headers file @file{wchar}) to compare strings using a
1526 collation ordering appropriate for the current locale. The locale used
1527 by these functions in particular can be specified by setting the locale
1528 for the @code{LC_COLLATE} category; see @ref{Locales}.
1532 In the standard C locale, the collation sequence for @code{strcoll} is
1533 the same as that for @code{strcmp}. Similarly, @code{wcscoll} and
1534 @code{wcscmp} are the same in this situation.
1536 Effectively, the way these functions work is by applying a mapping to
1537 transform the characters in a multibyte string to a byte
1538 sequence that represents
1539 the string's position in the collating sequence of the current locale.
1540 Comparing two such byte sequences in a simple fashion is equivalent to
1541 comparing the strings with the locale's collating sequence.
1543 The functions @code{strcoll} and @code{wcscoll} perform this translation
1544 implicitly, in order to do one comparison. By contrast, @code{strxfrm}
1545 and @code{wcsxfrm} perform the mapping explicitly. If you are making
1546 multiple comparisons using the same string or set of strings, it is
1547 likely to be more efficient to use @code{strxfrm} or @code{wcsxfrm} to
1548 transform all the strings just once, and subsequently compare the
1549 transformed strings with @code{strcmp} or @code{wcscmp}.
1551 @deftypefun int strcoll (const char *@var{s1}, const char *@var{s2})
1552 @standards{ISO, string.h}
1553 @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
1554 @c Calls strcoll_l with the current locale, which dereferences only the
1555 @c LC_COLLATE data pointer.
1556 The @code{strcoll} function is similar to @code{strcmp} but uses the
1557 collating sequence of the current locale for collation (the
1558 @code{LC_COLLATE} locale). The arguments are multibyte strings.
1561 @deftypefun int wcscoll (const wchar_t *@var{ws1}, const wchar_t *@var{ws2})
1562 @standards{ISO, wchar.h}
1563 @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
1564 @c Same as strcoll, but calling wcscoll_l.
1565 The @code{wcscoll} function is similar to @code{wcscmp} but uses the
1566 collating sequence of the current locale for collation (the
1567 @code{LC_COLLATE} locale).
1570 Here is an example of sorting an array of strings, using @code{strcoll}
1571 to compare them. The actual sort algorithm is not written here; it
1572 comes from @code{qsort} (@pxref{Array Sort Function}). The job of the
1573 code shown here is to say how to compare the strings while sorting them.
1574 (Later on in this section, we will show a way to do this more
1575 efficiently using @code{strxfrm}.)
1578 /* @r{This is the comparison function used with @code{qsort}.} */
1581 compare_elements (const void *v1, const void *v2)
1583 char * const *p1 = v1;
1584 char * const *p2 = v2;
1586 return strcoll (*p1, *p2);
1589 /* @r{This is the entry point---the function to sort}
1590 @r{strings using the locale's collating sequence.} */
1593 sort_strings (char **array, int nstrings)
1595 /* @r{Sort @code{temp_array} by comparing the strings.} */
1596 qsort (array, nstrings,
1597 sizeof (char *), compare_elements);
1601 @cindex converting string to collation order
1602 @deftypefun size_t strxfrm (char *restrict @var{to}, const char *restrict @var{from}, size_t @var{size})
1603 @standards{ISO, string.h}
1604 @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
1605 The function @code{strxfrm} transforms the multibyte string
1606 @var{from} using the
1607 collation transformation determined by the locale currently selected for
1608 collation, and stores the transformed string in the array @var{to}. Up
1609 to @var{size} bytes (including a terminating null byte) are
1612 The behavior is undefined if the strings @var{to} and @var{from}
1613 overlap; see @ref{Copying Strings and Arrays}.
1615 The return value is the length of the entire transformed string. This
1616 value is not affected by the value of @var{size}, but if it is greater
1617 or equal than @var{size}, it means that the transformed string did not
1618 entirely fit in the array @var{to}. In this case, only as much of the
1619 string as actually fits was stored. To get the whole transformed
1620 string, call @code{strxfrm} again with a bigger output array.
1622 The transformed string may be longer than the original string, and it
1623 may also be shorter.
1625 If @var{size} is zero, no bytes are stored in @var{to}. In this
1626 case, @code{strxfrm} simply returns the number of bytes that would
1627 be the length of the transformed string. This is useful for determining
1628 what size the allocated array should be. It does not matter what
1629 @var{to} is if @var{size} is zero; @var{to} may even be a null pointer.
1632 @deftypefun size_t wcsxfrm (wchar_t *restrict @var{wto}, const wchar_t *@var{wfrom}, size_t @var{size})
1633 @standards{ISO, wchar.h}
1634 @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
1635 The function @code{wcsxfrm} transforms wide string @var{wfrom}
1636 using the collation transformation determined by the locale currently
1637 selected for collation, and stores the transformed string in the array
1638 @var{wto}. Up to @var{size} wide characters (including a terminating null
1639 wide character) are stored.
1641 The behavior is undefined if the strings @var{wto} and @var{wfrom}
1642 overlap; see @ref{Copying Strings and Arrays}.
1644 The return value is the length of the entire transformed wide
1645 string. This value is not affected by the value of @var{size}, but if
1646 it is greater or equal than @var{size}, it means that the transformed
1647 wide string did not entirely fit in the array @var{wto}. In
1648 this case, only as much of the wide string as actually fits
1649 was stored. To get the whole transformed wide string, call
1650 @code{wcsxfrm} again with a bigger output array.
1652 The transformed wide string may be longer than the original
1653 wide string, and it may also be shorter.
1655 If @var{size} is zero, no wide characters are stored in @var{to}. In this
1656 case, @code{wcsxfrm} simply returns the number of wide characters that
1657 would be the length of the transformed wide string. This is
1658 useful for determining what size the allocated array should be (remember
1659 to multiply with @code{sizeof (wchar_t)}). It does not matter what
1660 @var{wto} is if @var{size} is zero; @var{wto} may even be a null pointer.
1663 Here is an example of how you can use @code{strxfrm} when
1664 you plan to do many comparisons. It does the same thing as the previous
1665 example, but much faster, because it has to transform each string only
1666 once, no matter how many times it is compared with other strings. Even
1667 the time needed to allocate and free storage is much less than the time
1668 we save, when there are many strings.
1671 struct sorter @{ char *input; char *transformed; @};
1673 /* @r{This is the comparison function used with @code{qsort}}
1674 @r{to sort an array of @code{struct sorter}.} */
1677 compare_elements (const void *v1, const void *v2)
1679 const struct sorter *p1 = v1;
1680 const struct sorter *p2 = v2;
1682 return strcmp (p1->transformed, p2->transformed);
1685 /* @r{This is the entry point---the function to sort}
1686 @r{strings using the locale's collating sequence.} */
1689 sort_strings_fast (char **array, int nstrings)
1691 struct sorter temp_array[nstrings];
1694 /* @r{Set up @code{temp_array}. Each element contains}
1695 @r{one input string and its transformed string.} */
1696 for (i = 0; i < nstrings; i++)
1698 size_t length = strlen (array[i]) * 2;
1700 size_t transformed_length;
1702 temp_array[i].input = array[i];
1704 /* @r{First try a buffer perhaps big enough.} */
1705 transformed = (char *) xmalloc (length);
1707 /* @r{Transform @code{array[i]}.} */
1708 transformed_length = strxfrm (transformed, array[i], length);
1710 /* @r{If the buffer was not large enough, resize it}
1711 @r{and try again.} */
1712 if (transformed_length >= length)
1714 /* @r{Allocate the needed space. +1 for terminating}
1715 @r{@code{'\0'} byte.} */
1716 transformed = xrealloc (transformed,
1717 transformed_length + 1);
1719 /* @r{The return value is not interesting because we know}
1720 @r{how long the transformed string is.} */
1721 (void) strxfrm (transformed, array[i],
1722 transformed_length + 1);
1725 temp_array[i].transformed = transformed;
1728 /* @r{Sort @code{temp_array} by comparing transformed strings.} */
1729 qsort (temp_array, nstrings,
1730 sizeof (struct sorter), compare_elements);
1732 /* @r{Put the elements back in the permanent array}
1733 @r{in their sorted order.} */
1734 for (i = 0; i < nstrings; i++)
1735 array[i] = temp_array[i].input;
1737 /* @r{Free the strings we allocated.} */
1738 for (i = 0; i < nstrings; i++)
1739 free (temp_array[i].transformed);
1743 The interesting part of this code for the wide character version would
1748 sort_strings_fast (wchar_t **array, int nstrings)
1751 /* @r{Transform @code{array[i]}.} */
1752 transformed_length = wcsxfrm (transformed, array[i], length);
1754 /* @r{If the buffer was not large enough, resize it}
1755 @r{and try again.} */
1756 if (transformed_length >= length)
1758 /* @r{Allocate the needed space. +1 for terminating}
1759 @r{@code{L'\0'} wide character.} */
1760 transformed = xreallocarray (transformed,
1761 transformed_length + 1,
1762 sizeof *transformed);
1764 /* @r{The return value is not interesting because we know}
1765 @r{how long the transformed string is.} */
1766 (void) wcsxfrm (transformed, array[i],
1767 transformed_length + 1);
1773 Note the additional multiplication with @code{sizeof (wchar_t)} in the
1774 @code{realloc} call.
1776 @strong{Compatibility Note:} The string collation functions are a new
1777 feature of @w{ISO C90}. Older C dialects have no equivalent feature.
1778 The wide character versions were introduced in @w{Amendment 1} to @w{ISO
1781 @node Search Functions
1782 @section Search Functions
1784 This section describes library functions which perform various kinds
1785 of searching operations on strings and arrays. These functions are
1786 declared in the header file @file{string.h}.
1788 @cindex search functions (for strings)
1789 @cindex string search functions
1791 @deftypefun {void *} memchr (const void *@var{block}, int @var{c}, size_t @var{size})
1792 @standards{ISO, string.h}
1793 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1794 This function finds the first occurrence of the byte @var{c} (converted
1795 to an @code{unsigned char}) in the initial @var{size} bytes of the
1796 object beginning at @var{block}. The return value is a pointer to the
1797 located byte, or a null pointer if no match was found.
1800 @deftypefun {wchar_t *} wmemchr (const wchar_t *@var{block}, wchar_t @var{wc}, size_t @var{size})
1801 @standards{ISO, wchar.h}
1802 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1803 This function finds the first occurrence of the wide character @var{wc}
1804 in the initial @var{size} wide characters of the object beginning at
1805 @var{block}. The return value is a pointer to the located wide
1806 character, or a null pointer if no match was found.
1809 @deftypefun {void *} rawmemchr (const void *@var{block}, int @var{c})
1810 @standards{GNU, string.h}
1811 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1812 Often the @code{memchr} function is used with the knowledge that the
1813 byte @var{c} is available in the memory block specified by the
1814 parameters. But this means that the @var{size} parameter is not really
1815 needed and that the tests performed with it at runtime (to check whether
1816 the end of the block is reached) are not needed.
1818 The @code{rawmemchr} function exists for just this situation which is
1819 surprisingly frequent. The interface is similar to @code{memchr} except
1820 that the @var{size} parameter is missing. The function will look beyond
1821 the end of the block pointed to by @var{block} in case the programmer
1822 made an error in assuming that the byte @var{c} is present in the block.
1823 In this case the result is unspecified. Otherwise the return value is a
1824 pointer to the located byte.
1826 When looking for the end of a string, use @code{strchr}.
1828 This function is a GNU extension.
1831 @deftypefun {void *} memrchr (const void *@var{block}, int @var{c}, size_t @var{size})
1832 @standards{GNU, string.h}
1833 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1834 The function @code{memrchr} is like @code{memchr}, except that it searches
1835 backwards from the end of the block defined by @var{block} and @var{size}
1836 (instead of forwards from the front).
1838 This function is a GNU extension.
1841 @deftypefun {char *} strchr (const char *@var{string}, int @var{c})
1842 @standards{ISO, string.h}
1843 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1844 The @code{strchr} function finds the first occurrence of the byte
1845 @var{c} (converted to a @code{char}) in the string
1846 beginning at @var{string}. The return value is a pointer to the located
1847 byte, or a null pointer if no match was found.
1851 strchr ("hello, world", 'l')
1852 @result{} "llo, world"
1853 strchr ("hello, world", '?')
1857 The terminating null byte is considered to be part of the string,
1858 so you can use this function get a pointer to the end of a string by
1859 specifying zero as the value of the @var{c} argument.
1861 When @code{strchr} returns a null pointer, it does not let you know
1862 the position of the terminating null byte it has found. If you
1863 need that information, it is better (but less portable) to use
1864 @code{strchrnul} than to search for it a second time.
1867 @deftypefun {wchar_t *} wcschr (const wchar_t *@var{wstring}, wchar_t @var{wc})
1868 @standards{ISO, wchar.h}
1869 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1870 The @code{wcschr} function finds the first occurrence of the wide
1871 character @var{wc} in the wide string
1872 beginning at @var{wstring}. The return value is a pointer to the
1873 located wide character, or a null pointer if no match was found.
1875 The terminating null wide character is considered to be part of the wide
1876 string, so you can use this function get a pointer to the end
1877 of a wide string by specifying a null wide character as the
1878 value of the @var{wc} argument. It would be better (but less portable)
1879 to use @code{wcschrnul} in this case, though.
1882 @deftypefun {char *} strchrnul (const char *@var{string}, int @var{c})
1883 @standards{GNU, string.h}
1884 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1885 @code{strchrnul} is the same as @code{strchr} except that if it does
1886 not find the byte, it returns a pointer to string's terminating
1887 null byte rather than a null pointer.
1889 This function is a GNU extension.
1892 @deftypefun {wchar_t *} wcschrnul (const wchar_t *@var{wstring}, wchar_t @var{wc})
1893 @standards{GNU, wchar.h}
1894 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1895 @code{wcschrnul} is the same as @code{wcschr} except that if it does not
1896 find the wide character, it returns a pointer to the wide string's
1897 terminating null wide character rather than a null pointer.
1899 This function is a GNU extension.
1902 One useful, but unusual, use of the @code{strchr}
1903 function is when one wants to have a pointer pointing to the null byte
1904 terminating a string. This is often written in this way:
1911 This is almost optimal but the addition operation duplicated a bit of
1912 the work already done in the @code{strlen} function. A better solution
1916 s = strchr (s, '\0');
1919 There is no restriction on the second parameter of @code{strchr} so it
1920 could very well also be zero. Those readers thinking very
1921 hard about this might now point out that the @code{strchr} function is
1922 more expensive than the @code{strlen} function since we have two abort
1923 criteria. This is right. But in @theglibc{} the implementation of
1924 @code{strchr} is optimized in a special way so that @code{strchr}
1927 @deftypefun {char *} strrchr (const char *@var{string}, int @var{c})
1928 @standards{ISO, string.h}
1929 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1930 The function @code{strrchr} is like @code{strchr}, except that it searches
1931 backwards from the end of the string @var{string} (instead of forwards
1936 strrchr ("hello, world", 'l')
1941 @deftypefun {wchar_t *} wcsrchr (const wchar_t *@var{wstring}, wchar_t @var{wc})
1942 @standards{ISO, wchar.h}
1943 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1944 The function @code{wcsrchr} is like @code{wcschr}, except that it searches
1945 backwards from the end of the string @var{wstring} (instead of forwards
1949 @deftypefun {char *} strstr (const char *@var{haystack}, const char *@var{needle})
1950 @standards{ISO, string.h}
1951 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1952 This is like @code{strchr}, except that it searches @var{haystack} for a
1953 substring @var{needle} rather than just a single byte. It
1954 returns a pointer into the string @var{haystack} that is the first
1955 byte of the substring, or a null pointer if no match was found. If
1956 @var{needle} is an empty string, the function returns @var{haystack}.
1960 strstr ("hello, world", "l")
1961 @result{} "llo, world"
1962 strstr ("hello, world", "wo")
1967 @deftypefun {wchar_t *} wcsstr (const wchar_t *@var{haystack}, const wchar_t *@var{needle})
1968 @standards{ISO, wchar.h}
1969 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1970 This is like @code{wcschr}, except that it searches @var{haystack} for a
1971 substring @var{needle} rather than just a single wide character. It
1972 returns a pointer into the string @var{haystack} that is the first wide
1973 character of the substring, or a null pointer if no match was found. If
1974 @var{needle} is an empty string, the function returns @var{haystack}.
1977 @deftypefun {wchar_t *} wcswcs (const wchar_t *@var{haystack}, const wchar_t *@var{needle})
1978 @standards{XPG, wchar.h}
1979 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1980 @code{wcswcs} is a deprecated alias for @code{wcsstr}. This is the
1981 name originally used in the X/Open Portability Guide before the
1982 @w{Amendment 1} to @w{ISO C90} was published.
1986 @deftypefun {char *} strcasestr (const char *@var{haystack}, const char *@var{needle})
1987 @standards{GNU, string.h}
1988 @safety{@prelim{}@mtsafe{@mtslocale{}}@assafe{}@acsafe{}}
1989 @c There may be multiple calls of strncasecmp, each accessing the locale
1990 @c object independently.
1991 This is like @code{strstr}, except that it ignores case in searching for
1992 the substring. Like @code{strcasecmp}, it is locale dependent how
1993 uppercase and lowercase characters are related, and arguments are
1999 strcasestr ("hello, world", "L")
2000 @result{} "llo, world"
2001 strcasestr ("hello, World", "wo")
2007 @deftypefun {void *} memmem (const void *@var{haystack}, size_t @var{haystack-len},@*const void *@var{needle}, size_t @var{needle-len})
2008 @standards{GNU, string.h}
2009 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2010 This is like @code{strstr}, but @var{needle} and @var{haystack} are byte
2011 arrays rather than strings. @var{needle-len} is the
2012 length of @var{needle} and @var{haystack-len} is the length of
2015 This function is a GNU extension.
2018 @deftypefun size_t strspn (const char *@var{string}, const char *@var{skipset})
2019 @standards{ISO, string.h}
2020 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2021 The @code{strspn} (``string span'') function returns the length of the
2022 initial substring of @var{string} that consists entirely of bytes that
2023 are members of the set specified by the string @var{skipset}. The order
2024 of the bytes in @var{skipset} is not important.
2028 strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
2032 In a multibyte string, characters consisting of
2033 more than one byte are not treated as single entities. Each byte is treated
2034 separately. The function is not locale-dependent.
2037 @deftypefun size_t wcsspn (const wchar_t *@var{wstring}, const wchar_t *@var{skipset})
2038 @standards{ISO, wchar.h}
2039 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2040 The @code{wcsspn} (``wide character string span'') function returns the
2041 length of the initial substring of @var{wstring} that consists entirely
2042 of wide characters that are members of the set specified by the string
2043 @var{skipset}. The order of the wide characters in @var{skipset} is not
2047 @deftypefun size_t strcspn (const char *@var{string}, const char *@var{stopset})
2048 @standards{ISO, string.h}
2049 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2050 The @code{strcspn} (``string complement span'') function returns the length
2051 of the initial substring of @var{string} that consists entirely of bytes
2052 that are @emph{not} members of the set specified by the string @var{stopset}.
2053 (In other words, it returns the offset of the first byte in @var{string}
2054 that is a member of the set @var{stopset}.)
2058 strcspn ("hello, world", " \t\n,.;!?")
2062 In a multibyte string, characters consisting of
2063 more than one byte are not treated as a single entities. Each byte is treated
2064 separately. The function is not locale-dependent.
2067 @deftypefun size_t wcscspn (const wchar_t *@var{wstring}, const wchar_t *@var{stopset})
2068 @standards{ISO, wchar.h}
2069 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2070 The @code{wcscspn} (``wide character string complement span'') function
2071 returns the length of the initial substring of @var{wstring} that
2072 consists entirely of wide characters that are @emph{not} members of the
2073 set specified by the string @var{stopset}. (In other words, it returns
2074 the offset of the first wide character in @var{string} that is a member of
2075 the set @var{stopset}.)
2078 @deftypefun {char *} strpbrk (const char *@var{string}, const char *@var{stopset})
2079 @standards{ISO, string.h}
2080 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2081 The @code{strpbrk} (``string pointer break'') function is related to
2082 @code{strcspn}, except that it returns a pointer to the first byte
2083 in @var{string} that is a member of the set @var{stopset} instead of the
2084 length of the initial substring. It returns a null pointer if no such
2085 byte from @var{stopset} is found.
2087 @c @group Invalid outside the example.
2091 strpbrk ("hello, world", " \t\n,.;!?")
2096 In a multibyte string, characters consisting of
2097 more than one byte are not treated as single entities. Each byte is treated
2098 separately. The function is not locale-dependent.
2101 @deftypefun {wchar_t *} wcspbrk (const wchar_t *@var{wstring}, const wchar_t *@var{stopset})
2102 @standards{ISO, wchar.h}
2103 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2104 The @code{wcspbrk} (``wide character string pointer break'') function is
2105 related to @code{wcscspn}, except that it returns a pointer to the first
2106 wide character in @var{wstring} that is a member of the set
2107 @var{stopset} instead of the length of the initial substring. It
2108 returns a null pointer if no such wide character from @var{stopset} is found.
2112 @subsection Compatibility String Search Functions
2114 @deftypefun {char *} index (const char *@var{string}, int @var{c})
2115 @standards{BSD, string.h}
2116 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2117 @code{index} is another name for @code{strchr}; they are exactly the same.
2118 New code should always use @code{strchr} since this name is defined in
2119 @w{ISO C} while @code{index} is a BSD invention which never was available
2120 on @w{System V} derived systems.
2123 @deftypefun {char *} rindex (const char *@var{string}, int @var{c})
2124 @standards{BSD, string.h}
2125 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2126 @code{rindex} is another name for @code{strrchr}; they are exactly the same.
2127 New code should always use @code{strrchr} since this name is defined in
2128 @w{ISO C} while @code{rindex} is a BSD invention which never was available
2129 on @w{System V} derived systems.
2132 @node Finding Tokens in a String
2133 @section Finding Tokens in a String
2135 @cindex tokenizing strings
2136 @cindex breaking a string into tokens
2137 @cindex parsing tokens from a string
2138 It's fairly common for programs to have a need to do some simple kinds
2139 of lexical analysis and parsing, such as splitting a command string up
2140 into tokens. You can do this with the @code{strtok} function, declared
2141 in the header file @file{string.h}.
2144 @deftypefun {char *} strtok (char *restrict @var{newstring}, const char *restrict @var{delimiters})
2145 @standards{ISO, string.h}
2146 @safety{@prelim{}@mtunsafe{@mtasurace{:strtok}}@asunsafe{}@acsafe{}}
2147 A string can be split into tokens by making a series of calls to the
2148 function @code{strtok}.
2150 The string to be split up is passed as the @var{newstring} argument on
2151 the first call only. The @code{strtok} function uses this to set up
2152 some internal state information. Subsequent calls to get additional
2153 tokens from the same string are indicated by passing a null pointer as
2154 the @var{newstring} argument. Calling @code{strtok} with another
2155 non-null @var{newstring} argument reinitializes the state information.
2156 It is guaranteed that no other library function ever calls @code{strtok}
2157 behind your back (which would mess up this internal state information).
2159 The @var{delimiters} argument is a string that specifies a set of delimiters
2160 that may surround the token being extracted. All the initial bytes
2161 that are members of this set are discarded. The first byte that is
2162 @emph{not} a member of this set of delimiters marks the beginning of the
2163 next token. The end of the token is found by looking for the next
2164 byte that is a member of the delimiter set. This byte in the
2165 original string @var{newstring} is overwritten by a null byte, and the
2166 pointer to the beginning of the token in @var{newstring} is returned.
2168 On the next call to @code{strtok}, the searching begins at the next
2169 byte beyond the one that marked the end of the previous token.
2170 Note that the set of delimiters @var{delimiters} do not have to be the
2171 same on every call in a series of calls to @code{strtok}.
2173 If the end of the string @var{newstring} is reached, or if the remainder of
2174 string consists only of delimiter bytes, @code{strtok} returns
2177 In a multibyte string, characters consisting of
2178 more than one byte are not treated as single entities. Each byte is treated
2179 separately. The function is not locale-dependent.
2182 @deftypefun {wchar_t *} wcstok (wchar_t *@var{newstring}, const wchar_t *@var{delimiters}, wchar_t **@var{save_ptr})
2183 @standards{ISO, wchar.h}
2184 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2185 A string can be split into tokens by making a series of calls to the
2186 function @code{wcstok}.
2188 The string to be split up is passed as the @var{newstring} argument on
2189 the first call only. The @code{wcstok} function uses this to set up
2190 some internal state information. Subsequent calls to get additional
2191 tokens from the same wide string are indicated by passing a
2192 null pointer as the @var{newstring} argument, which causes the pointer
2193 previously stored in @var{save_ptr} to be used instead.
2195 The @var{delimiters} argument is a wide string that specifies
2196 a set of delimiters that may surround the token being extracted. All
2197 the initial wide characters that are members of this set are discarded.
2198 The first wide character that is @emph{not} a member of this set of
2199 delimiters marks the beginning of the next token. The end of the token
2200 is found by looking for the next wide character that is a member of the
2201 delimiter set. This wide character in the original wide
2202 string @var{newstring} is overwritten by a null wide character, the
2203 pointer past the overwritten wide character is saved in @var{save_ptr},
2204 and the pointer to the beginning of the token in @var{newstring} is
2207 On the next call to @code{wcstok}, the searching begins at the next
2208 wide character beyond the one that marked the end of the previous token.
2209 Note that the set of delimiters @var{delimiters} do not have to be the
2210 same on every call in a series of calls to @code{wcstok}.
2212 If the end of the wide string @var{newstring} is reached, or
2213 if the remainder of string consists only of delimiter wide characters,
2214 @code{wcstok} returns a null pointer.
2217 @strong{Warning:} Since @code{strtok} and @code{wcstok} alter the string
2218 they is parsing, you should always copy the string to a temporary buffer
2219 before parsing it with @code{strtok}/@code{wcstok} (@pxref{Copying Strings
2220 and Arrays}). If you allow @code{strtok} or @code{wcstok} to modify
2221 a string that came from another part of your program, you are asking for
2222 trouble; that string might be used for other purposes after
2223 @code{strtok} or @code{wcstok} has modified it, and it would not have
2226 The string that you are operating on might even be a constant. Then
2227 when @code{strtok} or @code{wcstok} tries to modify it, your program
2228 will get a fatal signal for writing in read-only memory. @xref{Program
2229 Error Signals}. Even if the operation of @code{strtok} or @code{wcstok}
2230 would not require a modification of the string (e.g., if there is
2231 exactly one token) the string can (and in the @glibcadj{} case will) be
2234 This is a special case of a general principle: if a part of a program
2235 does not have as its purpose the modification of a certain data
2236 structure, then it is error-prone to modify the data structure
2239 The function @code{strtok} is not reentrant, whereas @code{wcstok} is.
2240 @xref{Nonreentrancy}, for a discussion of where and why reentrancy is
2243 Here is a simple example showing the use of @code{strtok}.
2245 @comment Yes, this example has been tested.
2252 const char string[] = "words separated by spaces -- and, punctuation!";
2253 const char delimiters[] = " .,;:!-";
2258 cp = strdupa (string); /* Make writable copy. */
2259 token = strtok (cp, delimiters); /* token => "words" */
2260 token = strtok (NULL, delimiters); /* token => "separated" */
2261 token = strtok (NULL, delimiters); /* token => "by" */
2262 token = strtok (NULL, delimiters); /* token => "spaces" */
2263 token = strtok (NULL, delimiters); /* token => "and" */
2264 token = strtok (NULL, delimiters); /* token => "punctuation" */
2265 token = strtok (NULL, delimiters); /* token => NULL */
2268 @Theglibc{} contains two more functions for tokenizing a string
2269 which overcome the limitation of non-reentrancy. They are not
2270 available available for wide strings.
2272 @deftypefun {char *} strtok_r (char *@var{newstring}, const char *@var{delimiters}, char **@var{save_ptr})
2273 @standards{POSIX, string.h}
2274 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2275 Just like @code{strtok}, this function splits the string into several
2276 tokens which can be accessed by successive calls to @code{strtok_r}.
2277 The difference is that, as in @code{wcstok}, the information about the
2278 next token is stored in the space pointed to by the third argument,
2279 @var{save_ptr}, which is a pointer to a string pointer. Calling
2280 @code{strtok_r} with a null pointer for @var{newstring} and leaving
2281 @var{save_ptr} between the calls unchanged does the job without
2282 hindering reentrancy.
2284 This function is defined in POSIX.1 and can be found on many systems
2285 which support multi-threading.
2288 @deftypefun {char *} strsep (char **@var{string_ptr}, const char *@var{delimiter})
2289 @standards{BSD, string.h}
2290 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2291 This function has a similar functionality as @code{strtok_r} with the
2292 @var{newstring} argument replaced by the @var{save_ptr} argument. The
2293 initialization of the moving pointer has to be done by the user.
2294 Successive calls to @code{strsep} move the pointer along the tokens
2295 separated by @var{delimiter}, returning the address of the next token
2296 and updating @var{string_ptr} to point to the beginning of the next
2299 One difference between @code{strsep} and @code{strtok_r} is that if the
2300 input string contains more than one byte from @var{delimiter} in a
2301 row @code{strsep} returns an empty string for each pair of bytes
2302 from @var{delimiter}. This means that a program normally should test
2303 for @code{strsep} returning an empty string before processing it.
2305 This function was introduced in 4.3BSD and therefore is widely available.
2308 Here is how the above example looks like when @code{strsep} is used.
2310 @comment Yes, this example has been tested.
2317 const char string[] = "words separated by spaces -- and, punctuation!";
2318 const char delimiters[] = " .,;:!-";
2324 running = strdupa (string);
2325 token = strsep (&running, delimiters); /* token => "words" */
2326 token = strsep (&running, delimiters); /* token => "separated" */
2327 token = strsep (&running, delimiters); /* token => "by" */
2328 token = strsep (&running, delimiters); /* token => "spaces" */
2329 token = strsep (&running, delimiters); /* token => "" */
2330 token = strsep (&running, delimiters); /* token => "" */
2331 token = strsep (&running, delimiters); /* token => "" */
2332 token = strsep (&running, delimiters); /* token => "and" */
2333 token = strsep (&running, delimiters); /* token => "" */
2334 token = strsep (&running, delimiters); /* token => "punctuation" */
2335 token = strsep (&running, delimiters); /* token => "" */
2336 token = strsep (&running, delimiters); /* token => NULL */
2339 @deftypefun {char *} basename (const char *@var{filename})
2340 @standards{GNU, string.h}
2341 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2342 The GNU version of the @code{basename} function returns the last
2343 component of the path in @var{filename}. This function is the preferred
2344 usage, since it does not modify the argument, @var{filename}, and
2345 respects trailing slashes. The prototype for @code{basename} can be
2346 found in @file{string.h}. Note, this function is overridden by the XPG
2347 version, if @file{libgen.h} is included.
2349 Example of using GNU @code{basename}:
2355 main (int argc, char *argv[])
2357 char *prog = basename (argv[0]);
2361 fprintf (stderr, "Usage %s <arg>\n", prog);
2369 @strong{Portability Note:} This function may produce different results
2370 on different systems.
2374 @deftypefun {char *} basename (char *@var{path})
2375 @standards{XPG, libgen.h}
2376 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2377 This is the standard XPG defined @code{basename}. It is similar in
2378 spirit to the GNU version, but may modify the @var{path} by removing
2379 trailing '/' bytes. If the @var{path} is made up entirely of '/'
2380 bytes, then "/" will be returned. Also, if @var{path} is
2381 @code{NULL} or an empty string, then "." is returned. The prototype for
2382 the XPG version can be found in @file{libgen.h}.
2384 Example of using XPG @code{basename}:
2390 main (int argc, char *argv[])
2393 char *path = strdupa (argv[0]);
2395 prog = basename (path);
2399 fprintf (stderr, "Usage %s <arg>\n", prog);
2409 @deftypefun {char *} dirname (char *@var{path})
2410 @standards{XPG, libgen.h}
2411 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2412 The @code{dirname} function is the compliment to the XPG version of
2413 @code{basename}. It returns the parent directory of the file specified
2414 by @var{path}. If @var{path} is @code{NULL}, an empty string, or
2415 contains no '/' bytes, then "." is returned. The prototype for this
2416 function can be found in @file{libgen.h}.
2419 @node Erasing Sensitive Data
2420 @section Erasing Sensitive Data
2422 Sensitive data, such as cryptographic keys, should be erased from
2423 memory after use, to reduce the risk that a bug will expose it to the
2424 outside world. However, compiler optimizations may determine that an
2425 erasure operation is ``unnecessary,'' and remove it from the generated
2426 code, because no @emph{correct} program could access the variable or
2427 heap object containing the sensitive data after it's deallocated.
2428 Since erasure is a precaution against bugs, this optimization is
2431 The function @code{explicit_bzero} erases a block of memory, and
2432 guarantees that the compiler will not remove the erasure as
2439 extern void encrypt (const char *key, const char *in,
2440 char *out, size_t n);
2441 extern void genkey (const char *phrase, char *key);
2443 void encrypt_with_phrase (const char *phrase, const char *in,
2444 char *out, size_t n)
2447 genkey (phrase, key);
2448 encrypt (key, in, out, n);
2449 explicit_bzero (key, 16);
2455 In this example, if @code{memset}, @code{bzero}, or a hand-written
2456 loop had been used, the compiler might remove them as ``unnecessary.''
2458 @strong{Warning:} @code{explicit_bzero} does not guarantee that
2459 sensitive data is @emph{completely} erased from the computer's memory.
2460 There may be copies in temporary storage areas, such as registers and
2461 ``scratch'' stack space; since these are invisible to the source code,
2462 a library function cannot erase them.
2464 Also, @code{explicit_bzero} only operates on RAM. If a sensitive data
2465 object never needs to have its address taken other than to call
2466 @code{explicit_bzero}, it might be stored entirely in CPU registers
2467 @emph{until} the call to @code{explicit_bzero}. Then it will be
2468 copied into RAM, the copy will be erased, and the original will remain
2469 intact. Data in RAM is more likely to be exposed by a bug than data
2470 in registers, so this creates a brief window where the data is at
2471 greater risk of exposure than it would have been if the program didn't
2472 try to erase it at all.
2474 Declaring sensitive variables as @code{volatile} will make both the
2475 above problems @emph{worse}; a @code{volatile} variable will be stored
2476 in memory for its entire lifetime, and the compiler will make
2477 @emph{more} copies of it than it would otherwise have. Attempting to
2478 erase a normal variable ``by hand'' through a
2479 @code{volatile}-qualified pointer doesn't work at all---because the
2480 variable itself is not @code{volatile}, some compilers will ignore the
2481 qualification on the pointer and remove the erasure anyway.
2483 Having said all that, in most situations, using @code{explicit_bzero}
2484 is better than not using it. At present, the only way to do a more
2485 thorough job is to write the entire sensitive operation in assembly
2486 language. We anticipate that future compilers will recognize calls to
2487 @code{explicit_bzero} and take appropriate steps to erase all the
2488 copies of the affected data, wherever they may be.
2490 @deftypefun void explicit_bzero (void *@var{block}, size_t @var{len})
2491 @standards{BSD, string.h}
2492 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2494 @code{explicit_bzero} writes zero into @var{len} bytes of memory
2495 beginning at @var{block}, just as @code{bzero} would. The zeroes are
2496 always written, even if the compiler could determine that this is
2497 ``unnecessary'' because no correct program could read them back.
2499 @strong{Note:} The @emph{only} optimization that @code{explicit_bzero}
2500 disables is removal of ``unnecessary'' writes to memory. The compiler
2501 can perform all the other optimizations that it could for a call to
2502 @code{memset}. For instance, it may replace the function call with
2503 inline memory writes, and it may assume that @var{block} cannot be a
2506 @strong{Portability Note:} This function first appeared in OpenBSD 5.5
2507 and has not been standardized. Other systems may provide the same
2508 functionality under a different name, such as @code{explicit_memset},
2509 @code{memset_s}, or @code{SecureZeroMemory}.
2511 @Theglibc{} declares this function in @file{string.h}, but on other
2512 systems it may be in @file{strings.h} instead.
2516 @node Shuffling Bytes
2517 @section Shuffling Bytes
2519 The function below addresses the perennial programming quandary: ``How do
2520 I take good data in string form and painlessly turn it into garbage?''
2521 This is not a difficult thing to code for oneself, but the authors of
2522 @theglibc{} wish to make it as convenient as possible.
2524 To @emph{erase} data, use @code{explicit_bzero} (@pxref{Erasing
2525 Sensitive Data}); to obfuscate it reversibly, use @code{memfrob}
2526 (@pxref{Obfuscating Data}).
2528 @deftypefun {char *} strfry (char *@var{string})
2529 @standards{GNU, string.h}
2530 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2531 @c Calls initstate_r, time, getpid, strlen, and random_r.
2533 @code{strfry} performs an in-place shuffle on @var{string}. Each
2534 character is swapped to a position selected at random, within the
2535 portion of the string starting with the character's original position.
2536 (This is the Fisher-Yates algorithm for unbiased shuffling.)
2538 Calling @code{strfry} will not disturb any of the random number
2539 generators that have global state (@pxref{Pseudo-Random Numbers}).
2541 The return value of @code{strfry} is always @var{string}.
2543 @strong{Portability Note:} This function is unique to @theglibc{}.
2544 It is declared in @file{string.h}.
2548 @node Obfuscating Data
2549 @section Obfuscating Data
2552 The @code{memfrob} function reversibly obfuscates an array of binary
2553 data. This is not true encryption; the obfuscated data still bears a
2554 clear relationship to the original, and no secret key is required to
2555 undo the obfuscation. It is analogous to the ``Rot13'' cipher used on
2556 Usenet for obscuring offensive jokes, spoilers for works of fiction,
2557 and so on, but it can be applied to arbitrary binary data.
2559 Programs that need true encryption---a transformation that completely
2560 obscures the original and cannot be reversed without knowledge of a
2561 secret key---should use a dedicated cryptography library, such as
2562 @uref{https://www.gnu.org/software/libgcrypt/,,libgcrypt}.
2564 Programs that need to @emph{destroy} data should use
2565 @code{explicit_bzero} (@pxref{Erasing Sensitive Data}), or possibly
2566 @code{strfry} (@pxref{Shuffling Bytes}).
2568 @deftypefun {void *} memfrob (void *@var{mem}, size_t @var{length})
2569 @standards{GNU, string.h}
2570 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2572 The function @code{memfrob} obfuscates @var{length} bytes of data
2573 beginning at @var{mem}, in place. Each byte is bitwise xor-ed with
2574 the binary pattern 00101010 (hexadecimal 0x2A). The return value is
2577 @code{memfrob} a second time on the same data returns it to
2580 @strong{Portability Note:} This function is unique to @theglibc{}.
2581 It is declared in @file{string.h}.
2584 @node Encode Binary Data
2585 @section Encode Binary Data
2587 To store or transfer binary data in environments which only support text
2588 one has to encode the binary data by mapping the input bytes to
2589 bytes in the range allowed for storing or transferring. SVID
2590 systems (and nowadays XPG compliant systems) provide minimal support for
2593 @deftypefun {char *} l64a (long int @var{n})
2594 @standards{XPG, stdlib.h}
2595 @safety{@prelim{}@mtunsafe{@mtasurace{:l64a}}@asunsafe{}@acsafe{}}
2596 This function encodes a 32-bit input value using bytes from the
2597 basic character set. It returns a pointer to a 7 byte buffer which
2598 contains an encoded version of @var{n}. To encode a series of bytes the
2599 user must copy the returned string to a destination buffer. It returns
2600 the empty string if @var{n} is zero, which is somewhat bizarre but
2601 mandated by the standard.@*
2602 @strong{Warning:} Since a static buffer is used this function should not
2603 be used in multi-threaded programs. There is no thread-safe alternative
2604 to this function in the C library.@*
2605 @strong{Compatibility Note:} The XPG standard states that the return
2606 value of @code{l64a} is undefined if @var{n} is negative. In the GNU
2607 implementation, @code{l64a} treats its argument as unsigned, so it will
2608 return a sensible encoding for any nonzero @var{n}; however, portable
2609 programs should not rely on this.
2611 To encode a large buffer @code{l64a} must be called in a loop, once for
2612 each 32-bit word of the buffer. For example, one could do something
2617 encode (const void *buf, size_t len)
2619 /* @r{We know in advance how long the buffer has to be.} */
2620 unsigned char *in = (unsigned char *) buf;
2621 char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
2624 /* @r{Encode the length.} */
2625 /* @r{Using `htonl' is necessary so that the data can be}
2626 @r{decoded even on machines with different byte order.}
2627 @r{`l64a' can return a string shorter than 6 bytes, so }
2628 @r{we pad it with encoding of 0 (}'.'@r{) at the end by }
2631 p = stpcpy (cp, l64a (htonl (len)));
2632 cp = mempcpy (p, "......", 6 - (p - cp));
2636 unsigned long int n = *in++;
2637 n = (n << 8) | *in++;
2638 n = (n << 8) | *in++;
2639 n = (n << 8) | *in++;
2641 p = stpcpy (cp, l64a (htonl (n)));
2642 cp = mempcpy (p, "......", 6 - (p - cp));
2646 unsigned long int n = *in++;
2649 n = (n << 8) | *in++;
2653 cp = stpcpy (cp, l64a (htonl (n)));
2660 It is strange that the library does not provide the complete
2661 functionality needed but so be it.
2665 To decode data produced with @code{l64a} the following function should be
2668 @deftypefun {long int} a64l (const char *@var{string})
2669 @standards{XPG, stdlib.h}
2670 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2671 The parameter @var{string} should contain a string which was produced by
2672 a call to @code{l64a}. The function processes at least 6 bytes of
2673 this string, and decodes the bytes it finds according to the table
2674 below. It stops decoding when it finds a byte not in the table,
2675 rather like @code{atoi}; if you have a buffer which has been broken into
2676 lines, you must be careful to skip over the end-of-line bytes.
2678 The decoded number is returned as a @code{long int} value.
2681 The @code{l64a} and @code{a64l} functions use a base 64 encoding, in
2682 which each byte of an encoded string represents six bits of an
2683 input word. These symbols are used for the base 64 digits:
2685 @multitable {xxxxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx}
2686 @item @tab 0 @tab 1 @tab 2 @tab 3 @tab 4 @tab 5 @tab 6 @tab 7
2687 @item 0 @tab @code{.} @tab @code{/} @tab @code{0} @tab @code{1}
2688 @tab @code{2} @tab @code{3} @tab @code{4} @tab @code{5}
2689 @item 8 @tab @code{6} @tab @code{7} @tab @code{8} @tab @code{9}
2690 @tab @code{A} @tab @code{B} @tab @code{C} @tab @code{D}
2691 @item 16 @tab @code{E} @tab @code{F} @tab @code{G} @tab @code{H}
2692 @tab @code{I} @tab @code{J} @tab @code{K} @tab @code{L}
2693 @item 24 @tab @code{M} @tab @code{N} @tab @code{O} @tab @code{P}
2694 @tab @code{Q} @tab @code{R} @tab @code{S} @tab @code{T}
2695 @item 32 @tab @code{U} @tab @code{V} @tab @code{W} @tab @code{X}
2696 @tab @code{Y} @tab @code{Z} @tab @code{a} @tab @code{b}
2697 @item 40 @tab @code{c} @tab @code{d} @tab @code{e} @tab @code{f}
2698 @tab @code{g} @tab @code{h} @tab @code{i} @tab @code{j}
2699 @item 48 @tab @code{k} @tab @code{l} @tab @code{m} @tab @code{n}
2700 @tab @code{o} @tab @code{p} @tab @code{q} @tab @code{r}
2701 @item 56 @tab @code{s} @tab @code{t} @tab @code{u} @tab @code{v}
2702 @tab @code{w} @tab @code{x} @tab @code{y} @tab @code{z}
2705 This encoding scheme is not standard. There are some other encoding
2706 methods which are much more widely used (UU encoding, MIME encoding).
2707 Generally, it is better to use one of these encodings.
2709 @node Argz and Envz Vectors
2710 @section Argz and Envz Vectors
2712 @cindex argz vectors (string vectors)
2713 @cindex string vectors, null-byte separated
2714 @cindex argument vectors, null-byte separated
2715 @dfn{argz vectors} are vectors of strings in a contiguous block of
2716 memory, each element separated from its neighbors by null bytes
2719 @cindex envz vectors (environment vectors)
2720 @cindex environment vectors, null-byte separated
2721 @dfn{Envz vectors} are an extension of argz vectors where each element is a
2722 name-value pair, separated by a @code{'='} byte (as in a Unix
2726 * Argz Functions:: Operations on argz vectors.
2727 * Envz Functions:: Additional operations on environment vectors.
2730 @node Argz Functions, Envz Functions, , Argz and Envz Vectors
2731 @subsection Argz Functions
2733 Each argz vector is represented by a pointer to the first element, of
2734 type @code{char *}, and a size, of type @code{size_t}, both of which can
2735 be initialized to @code{0} to represent an empty argz vector. All argz
2736 functions accept either a pointer and a size argument, or pointers to
2737 them, if they will be modified.
2739 The argz functions use @code{malloc}/@code{realloc} to allocate/grow
2740 argz vectors, and so any argz vector created using these functions may
2741 be freed by using @code{free}; conversely, any argz function that may
2742 grow a string expects that string to have been allocated using
2743 @code{malloc} (those argz functions that only examine their arguments or
2744 modify them in place will work on any sort of memory).
2745 @xref{Unconstrained Allocation}.
2747 All argz functions that do memory allocation have a return type of
2748 @code{error_t}, and return @code{0} for success, and @code{ENOMEM} if an
2749 allocation error occurs.
2752 These functions are declared in the standard include file @file{argz.h}.
2754 @deftypefun {error_t} argz_create (char *const @var{argv}[], char **@var{argz}, size_t *@var{argz_len})
2755 @standards{GNU, argz.h}
2756 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2757 The @code{argz_create} function converts the Unix-style argument vector
2758 @var{argv} (a vector of pointers to normal C strings, terminated by
2759 @code{(char *)0}; @pxref{Program Arguments}) into an argz vector with
2760 the same elements, which is returned in @var{argz} and @var{argz_len}.
2763 @deftypefun {error_t} argz_create_sep (const char *@var{string}, int @var{sep}, char **@var{argz}, size_t *@var{argz_len})
2764 @standards{GNU, argz.h}
2765 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2766 The @code{argz_create_sep} function converts the string
2767 @var{string} into an argz vector (returned in @var{argz} and
2768 @var{argz_len}) by splitting it into elements at every occurrence of the
2772 @deftypefun {size_t} argz_count (const char *@var{argz}, size_t @var{argz_len})
2773 @standards{GNU, argz.h}
2774 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2775 Returns the number of elements in the argz vector @var{argz} and
2779 @deftypefun {void} argz_extract (const char *@var{argz}, size_t @var{argz_len}, char **@var{argv})
2780 @standards{GNU, argz.h}
2781 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2782 The @code{argz_extract} function converts the argz vector @var{argz} and
2783 @var{argz_len} into a Unix-style argument vector stored in @var{argv},
2784 by putting pointers to every element in @var{argz} into successive
2785 positions in @var{argv}, followed by a terminator of @code{0}.
2786 @var{Argv} must be pre-allocated with enough space to hold all the
2787 elements in @var{argz} plus the terminating @code{(char *)0}
2788 (@code{(argz_count (@var{argz}, @var{argz_len}) + 1) * sizeof (char *)}
2789 bytes should be enough). Note that the string pointers stored into
2790 @var{argv} point into @var{argz}---they are not copies---and so
2791 @var{argz} must be copied if it will be changed while @var{argv} is
2792 still active. This function is useful for passing the elements in
2793 @var{argz} to an exec function (@pxref{Executing a File}).
2796 @deftypefun {void} argz_stringify (char *@var{argz}, size_t @var{len}, int @var{sep})
2797 @standards{GNU, argz.h}
2798 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2799 The @code{argz_stringify} converts @var{argz} into a normal string with
2800 the elements separated by the byte @var{sep}, by replacing each
2801 @code{'\0'} inside @var{argz} (except the last one, which terminates the
2802 string) with @var{sep}. This is handy for printing @var{argz} in a
2806 @deftypefun {error_t} argz_add (char **@var{argz}, size_t *@var{argz_len}, const char *@var{str})
2807 @standards{GNU, argz.h}
2808 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2809 @c Calls strlen and argz_append.
2810 The @code{argz_add} function adds the string @var{str} to the end of the
2811 argz vector @code{*@var{argz}}, and updates @code{*@var{argz}} and
2812 @code{*@var{argz_len}} accordingly.
2815 @deftypefun {error_t} argz_add_sep (char **@var{argz}, size_t *@var{argz_len}, const char *@var{str}, int @var{delim})
2816 @standards{GNU, argz.h}
2817 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2818 The @code{argz_add_sep} function is similar to @code{argz_add}, but
2819 @var{str} is split into separate elements in the result at occurrences of
2820 the byte @var{delim}. This is useful, for instance, for
2821 adding the components of a Unix search path to an argz vector, by using
2822 a value of @code{':'} for @var{delim}.
2825 @deftypefun {error_t} argz_append (char **@var{argz}, size_t *@var{argz_len}, const char *@var{buf}, size_t @var{buf_len})
2826 @standards{GNU, argz.h}
2827 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2828 The @code{argz_append} function appends @var{buf_len} bytes starting at
2829 @var{buf} to the argz vector @code{*@var{argz}}, reallocating
2830 @code{*@var{argz}} to accommodate it, and adding @var{buf_len} to
2831 @code{*@var{argz_len}}.
2834 @deftypefun {void} argz_delete (char **@var{argz}, size_t *@var{argz_len}, char *@var{entry})
2835 @standards{GNU, argz.h}
2836 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2837 @c Calls free if no argument is left.
2838 If @var{entry} points to the beginning of one of the elements in the
2839 argz vector @code{*@var{argz}}, the @code{argz_delete} function will
2840 remove this entry and reallocate @code{*@var{argz}}, modifying
2841 @code{*@var{argz}} and @code{*@var{argz_len}} accordingly. Note that as
2842 destructive argz functions usually reallocate their argz argument,
2843 pointers into argz vectors such as @var{entry} will then become invalid.
2846 @deftypefun {error_t} argz_insert (char **@var{argz}, size_t *@var{argz_len}, char *@var{before}, const char *@var{entry})
2847 @standards{GNU, argz.h}
2848 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2849 @c Calls argz_add or realloc and memmove.
2850 The @code{argz_insert} function inserts the string @var{entry} into the
2851 argz vector @code{*@var{argz}} at a point just before the existing
2852 element pointed to by @var{before}, reallocating @code{*@var{argz}} and
2853 updating @code{*@var{argz}} and @code{*@var{argz_len}}. If @var{before}
2854 is @code{0}, @var{entry} is added to the end instead (as if by
2855 @code{argz_add}). Since the first element is in fact the same as
2856 @code{*@var{argz}}, passing in @code{*@var{argz}} as the value of
2857 @var{before} will result in @var{entry} being inserted at the beginning.
2860 @deftypefun {char *} argz_next (const char *@var{argz}, size_t @var{argz_len}, const char *@var{entry})
2861 @standards{GNU, argz.h}
2862 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2863 The @code{argz_next} function provides a convenient way of iterating
2864 over the elements in the argz vector @var{argz}. It returns a pointer
2865 to the next element in @var{argz} after the element @var{entry}, or
2866 @code{0} if there are no elements following @var{entry}. If @var{entry}
2867 is @code{0}, the first element of @var{argz} is returned.
2869 This behavior suggests two styles of iteration:
2873 while ((entry = argz_next (@var{argz}, @var{argz_len}, entry)))
2877 (the double parentheses are necessary to make some C compilers shut up
2878 about what they consider a questionable @code{while}-test) and:
2882 for (entry = @var{argz};
2884 entry = argz_next (@var{argz}, @var{argz_len}, entry))
2888 Note that the latter depends on @var{argz} having a value of @code{0} if
2889 it is empty (rather than a pointer to an empty block of memory); this
2890 invariant is maintained for argz vectors created by the functions here.
2893 @deftypefun error_t argz_replace (@w{char **@var{argz}, size_t *@var{argz_len}}, @w{const char *@var{str}, const char *@var{with}}, @w{unsigned *@var{replace_count}})
2894 @standards{GNU, argz.h}
2895 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2896 Replace any occurrences of the string @var{str} in @var{argz} with
2897 @var{with}, reallocating @var{argz} as necessary. If
2898 @var{replace_count} is non-zero, @code{*@var{replace_count}} will be
2899 incremented by the number of replacements performed.
2902 @node Envz Functions, , Argz Functions, Argz and Envz Vectors
2903 @subsection Envz Functions
2905 Envz vectors are just argz vectors with additional constraints on the form
2906 of each element; as such, argz functions can also be used on them, where it
2909 Each element in an envz vector is a name-value pair, separated by a @code{'='}
2910 byte; if multiple @code{'='} bytes are present in an element, those
2911 after the first are considered part of the value, and treated like all other
2912 non-@code{'\0'} bytes.
2914 If @emph{no} @code{'='} bytes are present in an element, that element is
2915 considered the name of a ``null'' entry, as distinct from an entry with an
2916 empty value: @code{envz_get} will return @code{0} if given the name of null
2917 entry, whereas an entry with an empty value would result in a value of
2918 @code{""}; @code{envz_entry} will still find such entries, however. Null
2919 entries can be removed with the @code{envz_strip} function.
2921 As with argz functions, envz functions that may allocate memory (and thus
2922 fail) have a return type of @code{error_t}, and return either @code{0} or
2926 These functions are declared in the standard include file @file{envz.h}.
2928 @deftypefun {char *} envz_entry (const char *@var{envz}, size_t @var{envz_len}, const char *@var{name})
2929 @standards{GNU, envz.h}
2930 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2931 The @code{envz_entry} function finds the entry in @var{envz} with the name
2932 @var{name}, and returns a pointer to the whole entry---that is, the argz
2933 element which begins with @var{name} followed by a @code{'='} byte. If
2934 there is no entry with that name, @code{0} is returned.
2937 @deftypefun {char *} envz_get (const char *@var{envz}, size_t @var{envz_len}, const char *@var{name})
2938 @standards{GNU, envz.h}
2939 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2940 The @code{envz_get} function finds the entry in @var{envz} with the name
2941 @var{name} (like @code{envz_entry}), and returns a pointer to the value
2942 portion of that entry (following the @code{'='}). If there is no entry with
2943 that name (or only a null entry), @code{0} is returned.
2946 @deftypefun {error_t} envz_add (char **@var{envz}, size_t *@var{envz_len}, const char *@var{name}, const char *@var{value})
2947 @standards{GNU, envz.h}
2948 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2949 @c Calls envz_remove, which calls enz_entry and argz_delete, and then
2950 @c argz_add or equivalent code that reallocs and appends name=value.
2951 The @code{envz_add} function adds an entry to @code{*@var{envz}}
2952 (updating @code{*@var{envz}} and @code{*@var{envz_len}}) with the name
2953 @var{name}, and value @var{value}. If an entry with the same name
2954 already exists in @var{envz}, it is removed first. If @var{value} is
2955 @code{0}, then the new entry will be the special null type of entry
2959 @deftypefun {error_t} envz_merge (char **@var{envz}, size_t *@var{envz_len}, const char *@var{envz2}, size_t @var{envz2_len}, int @var{override})
2960 @standards{GNU, envz.h}
2961 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2962 The @code{envz_merge} function adds each entry in @var{envz2} to @var{envz},
2963 as if with @code{envz_add}, updating @code{*@var{envz}} and
2964 @code{*@var{envz_len}}. If @var{override} is true, then values in @var{envz2}
2965 will supersede those with the same name in @var{envz}, otherwise not.
2967 Null entries are treated just like other entries in this respect, so a null
2968 entry in @var{envz} can prevent an entry of the same name in @var{envz2} from
2969 being added to @var{envz}, if @var{override} is false.
2972 @deftypefun {void} envz_strip (char **@var{envz}, size_t *@var{envz_len})
2973 @standards{GNU, envz.h}
2974 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
2975 The @code{envz_strip} function removes any null entries from @var{envz},
2976 updating @code{*@var{envz}} and @code{*@var{envz_len}}.
2979 @deftypefun {void} envz_remove (char **@var{envz}, size_t *@var{envz_len}, const char *@var{name})
2980 @standards{GNU, envz.h}
2981 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
2982 The @code{envz_remove} function removes an entry named @var{name} from
2983 @var{envz}, updating @code{*@var{envz}} and @code{*@var{envz_len}}.
2986 @c FIXME this are undocumented:
2987 @c strcasecmp_l @safety{@mtsafe{}@assafe{}@acsafe{}} see strcasecmp