1 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
2 .\" All rights reserved.
4 .\" This code is derived from software contributed to Berkeley by
5 .\" Chris Torek and the American National Standards Committee X3,
6 .\" on Information Processing Systems.
8 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
9 .\" Redistribution and use in source and binary forms, with or without
10 .\" modification, are permitted provided that the following conditions
12 .\" 1. Redistributions of source code must retain the above copyright
13 .\" notice, this list of conditions and the following disclaimer.
14 .\" 2. Redistributions in binary form must reproduce the above copyright
15 .\" notice, this list of conditions and the following disclaimer in the
16 .\" documentation and/or other materials provided with the distribution.
17 .\" 3. All advertising materials mentioning features or use of this software
18 .\" must display the following acknowledgement:
19 .\" This product includes software developed by the University of
20 .\" California, Berkeley and its contributors.
21 .\" 4. Neither the name of the University nor the names of its contributors
22 .\" may be used to endorse or promote products derived from this software
23 .\" without specific prior written permission.
25 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
26 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
27 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
28 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
29 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
30 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
31 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
32 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
33 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
34 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
38 .\" @(#)scanf.3 6.14 (Berkeley) 1/8/93
40 .\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
41 .\" modified to resemble the GNU libio setup used in the Linux libc
42 .\" used in versions 4.x (x>4) and 5 Helmut.Geyer@iwr.uni-heidelberg.de
43 .\" Modified, aeb, 970121
44 .\" 2005-07-14, mtk, added description of %n$ form; various text
45 .\" incorporated from the GNU C library documentation ((C) The
46 .\" Free Software Foundation); other parts substantially rewritten.
49 .\" Add ERRORS section.
50 .\" Document the 'a' and 'm' modifiers for dynamic string allocation.
52 .TH SCANF 3 2021-03-22 "GNU" "Linux Programmer's Manual"
54 scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf \- input format conversion
59 .BI "int scanf(const char *restrict " format ", ...);"
60 .BI "int fscanf(FILE *restrict " stream ,
61 .BI " const char *restrict " format ", ...);"
62 .BI "int sscanf(const char *restrict " str ,
63 .BI " const char *restrict " format ", ...);"
65 .B #include <stdarg.h>
67 .BI "int vscanf(const char *restrict " format ", va_list " ap );
68 .BI "int vfscanf(FILE *restrict " stream ,
69 .BI " const char *restrict " format ", va_list " ap );
70 .BI "int vsscanf(const char *restrict " str ,
71 .BI " const char *restrict " format ", va_list " ap );
75 Feature Test Macro Requirements for glibc (see
76 .BR feature_test_macros (7)):
83 _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
88 family of functions scans input according to
91 This format may contain
92 .IR "conversion specifications" ;
93 the results from such conversions, if any,
94 are stored in the locations pointed to by the
100 argument must be of a type that is appropriate for the value returned
101 by the corresponding conversion specification.
103 If the number of conversion specifications in
105 exceeds the number of
107 arguments, the results are undefined.
110 arguments exceeds the number of conversion specifications, then the excess
112 arguments are evaluated, but are otherwise ignored.
116 function reads input from the standard input stream
119 reads input from the stream pointer
123 reads its input from the character string pointed to by
128 function is analogous to
130 and reads input from the stream pointer
132 using a variable argument list of pointers (see
136 function scans a variable argument list from the standard input and the
138 function scans it from a string; these are analogous to the
142 functions respectively.
146 string consists of a sequence of
148 which describe how to process the sequence of input characters.
149 If processing of a directive fails, no further input is read, and
152 A "failure" can be either of the following:
153 .IR "input failure" ,
154 meaning that input characters were unavailable, or
155 .IR "matching failure" ,
156 meaning that the input was inappropriate (see below).
158 A directive is one of the following:
161 A sequence of white-space characters (space, tab, newline, etc.; see
163 This directive matches any amount of white space,
164 including none, in the input.
167 An ordinary character (i.e., one other than white space or \(aq%\(aq).
168 This character must exactly match the next character of input.
171 A conversion specification,
172 which commences with a \(aq%\(aq (percent) character.
173 A sequence of characters from the input is converted according to
174 this specification, and the result is placed in the corresponding
177 If the next item of input does not match the conversion specification,
178 the conversion fails\(emthis is a
179 .IR "matching failure" .
182 .I conversion specification
185 begins with either the character \(aq%\(aq or the character sequence
186 "\fB%\fP\fIn\fP\fB$\fP"
187 (see below for the distinction) followed by:
190 An optional \(aq*\(aq assignment-suppression character:
192 reads input as directed by the conversion specification,
193 but discards the input.
196 argument is required, and this specification is not
197 included in the count of successful assignments returned by
201 For decimal conversions, an optional quote character (\(aq).
202 This specifies that the input number may include thousands'
203 separators as defined by the
205 category of the current locale.
208 The quote character may precede or follow the \(aq*\(aq
209 assignment-suppression character.
212 An optional \(aqm\(aq character.
213 This is used with string conversions
217 and relieves the caller of the
218 need to allocate a corresponding buffer to hold the input: instead,
220 allocates a buffer of sufficient size,
221 and assigns the address of this buffer to the corresponding
223 argument, which should be a pointer to a
225 variable (this variable does not need to be initialized before the call).
226 The caller should subsequently
228 this buffer when it is no longer required.
231 An optional decimal integer which specifies the
232 .IR "maximum field width" .
233 Reading of characters stops either when this maximum is reached or
234 when a nonmatching character is found, whichever happens first.
235 Most conversions discard initial white space characters (the exceptions
237 and these discarded characters don't count toward the maximum field width.
238 String input conversions store a terminating null byte (\(aq\e0\(aq)
239 to mark the end of the input;
240 the maximum field width does not include this terminator.
244 .IR "type modifier character" .
247 type modifier is used with integer conversions such as
249 to specify that the corresponding
253 rather than a pointer to an
258 .I "conversion specifier"
259 that specifies the type of input conversion to be performed.
261 The conversion specifications in
263 are of two forms, either beginning with \(aq%\(aq or beginning with
264 "\fB%\fP\fIn\fP\fB$\fP".
265 The two forms should not be mixed in the same
267 string, except that a string containing
268 "\fB%\fP\fIn\fP\fB$\fP"
269 specifications can include
276 specifications, then these correspond in order with successive
280 "\fB%\fP\fIn\fP\fB$\fP"
281 form (which is specified in POSIX.1-2001, but not C99),
283 is a decimal integer that specifies that the converted input should
284 be placed in the location referred to by the
291 .I "type modifier characters"
292 can appear in a conversion specification:
295 Indicates that the conversion will be one of
296 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
297 and the next pointer is a pointer to a
307 but the next pointer is a pointer to a
310 .IR "unsigned char" .
315 but the next pointer is a pointer to an
319 This modifier was introduced in C99.
322 Indicates either that the conversion will be one of
323 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, \fBX\fP, or \fBn\fP
324 and the next pointer is a pointer to a
330 or that the conversion will be one of
331 \fBe\fP, \fBf\fP, or \fBg\fP
332 and the next pointer is a pointer to
338 characters is equivalent to
344 the corresponding parameter is considered
345 as a pointer to a wide character or wide-character string respectively.
346 .\" This use of l was introduced in Amendment 1 to ISO C90.
349 Indicates that the conversion will be either
350 \fBe\fP, \fBf\fP, or \fBg\fP
351 and the next pointer is a pointer to
353 or the conversion will be
354 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, or \fBx\fP
355 and the next pointer is a pointer to
357 .\" MTK, Jul 05: The following is no longer true for modern
358 .\" ANSI C (i.e., C99):
359 .\" (Note that long long is not an
361 .\" type. Any program using this will not be portable to all
367 This specifier does not exist in ANSI C.
372 but the next pointer is a pointer to a
374 This modifier was introduced in C99.
379 but the next pointer is a pointer to a
381 This modifier was introduced in C99.
384 .I "conversion specifiers"
388 Matches a literal \(aq%\(aq.
391 in the format string matches a
392 single input \(aq%\(aq character.
393 No conversion is done (but initial white space characters are discarded),
394 and assignment does not occur.
397 Matches an optionally signed decimal integer;
398 the next pointer must be a pointer to
404 .\" this exists only for backward compatibility.
405 .\" (Note: thus only in libc4
406 .\" In libc5 and glibc the
408 .\" is silently ignored, causing old programs to fail mysteriously.)
411 Matches an optionally signed integer; the next pointer must be a pointer to
413 The integer is read in base 16 if it begins with
417 in base 8 if it begins with
419 and in base 10 otherwise.
420 Only characters that correspond to the base are used.
423 Matches an unsigned octal integer; the next pointer must be a pointer to
427 Matches an unsigned decimal integer; the next pointer must be a
432 Matches an unsigned hexadecimal integer
433 (that may optionally begin with a prefix of
437 which is discarded); the next pointer must
446 Matches an optionally signed floating-point number; the next pointer must
467 Matches a sequence of non-white-space characters;
468 the next pointer must be a pointer to the initial element of a
469 character array that is long enough to hold the input sequence and
470 the terminating null byte (\(aq\e0\(aq), which is added automatically.
471 The input string stops at white space or at the maximum field
472 width, whichever occurs first.
475 Matches a sequence of characters whose length is specified by the
476 .I maximum field width
477 (default 1); the next pointer must be a pointer to
479 and there must be enough room for all the characters
480 (no terminating null byte is added).
481 The usual skip of leading white space is suppressed.
482 To skip white space first, use an explicit space in the format.
485 Matches a nonempty sequence of characters from the specified set of
486 accepted characters; the next pointer must be a pointer to
488 and there must be enough room for all the characters in the string, plus a
489 terminating null byte.
490 The usual skip of leading white space is suppressed.
491 The string is to be made up of characters in (or not in) a particular set;
492 the set is defined by the characters between the open bracket
494 character and a close bracket
499 those characters if the first character after the open bracket is a
502 To include a close bracket in the set, make it the first character after
503 the open bracket or the circumflex; any other position will end the set.
506 is also special; when placed between two other characters, it adds all
507 intervening characters to the set.
508 To include a hyphen, make it the last
509 character before the final close bracket.
513 the set "everything except close bracket, zero through nine, and hyphen".
514 The string ends with the appearance of a character not in the (or, with a
515 circumflex, in) set or when the field width runs out.
518 Matches a pointer value (as printed by
522 the next pointer must be a pointer to a pointer to
526 Nothing is expected; instead, the number of characters consumed thus far
527 from the input is stored through the next pointer, which must be a pointer
530 or variant whose size matches the (optionally)
531 supplied integer length modifier.
534 a conversion and does
536 increase the count returned by the function.
537 The assignment can be suppressed with the
539 assignment-suppression character, but the effect on the
540 return value is undefined.
543 conversions should not be used.
545 On success, these functions return the number of input items
546 successfully matched and assigned;
547 this can be fewer than provided for,
548 or even zero, in the event of an early matching failure.
552 is returned if the end of input is reached before either the first
553 successful conversion or a matching failure occurs.
555 is also returned if a read error occurs,
556 in which case the error indicator for the stream (see
560 is set to indicate the error.
564 The file descriptor underlying
566 is marked nonblocking, and the read operation would block.
569 The file descriptor underlying
571 is invalid, or not open for reading.
574 Input byte sequence does not form a valid character.
577 The read operation was interrupted by a signal; see
581 Not enough arguments; or
589 The result of an integer conversion would exceed the size
590 that can be stored in the corresponding integer type.
592 For an explanation of the terms used in this section, see
600 Interface Attribute Value
608 T} Thread safety MT-Safe locale
619 conform to C89 and C99 and POSIX.1-2001.
620 These standards do not specify the
626 specifier is the 4.4BSD notation for
632 in integer conversions is the GNU notation.
634 The Linux version of these functions is based on the
643 for a more concise description.
645 .SS The 'a' assignment-allocation modifier
646 Originally, the GNU C library supported dynamic allocation for string inputs
647 (as a nonstandard extension) via the
650 (This feature is present at least as far back as glibc 2.0.)
651 Thus, one could write the following to have
653 allocate a buffer for an input string,
654 with a pointer to that buffer being returned in
660 The use of the letter
662 for this purpose was problematic, since
664 is also specified by the ISO C standard as a synonym for
666 (floating-point input).
667 POSIX.1-2008 instead specifies the
669 modifier for assignment allocation (as documented in DESCRIPTION, above).
673 modifier is not available if the program is compiled with
676 .IR "gcc \-D_ISOC99_SOURCE"
679 is also specified), in which case the
681 is interpreted as a specifier for floating-point numbers (see above).
685 modifier was added to glibc starting with version 2.7,
686 and new programs should use that modifier instead of
689 As well as being standardized by POSIX, the
691 modifier has the following further advantages over
695 It may also be applied to
697 conversion specifiers (e.g.,
700 It avoids ambiguity with respect to the
702 floating-point conversion specifier (and is unaffected by
706 All functions are fully C89 conformant, but provide the
707 additional specifiers
711 as well as an additional behavior of the
716 The latter may be considered to be a bug, as it changes the
717 behavior of specifiers defined in C89.
719 Some combinations of the type modifiers and conversion
720 specifiers defined by ANSI C do not make sense
723 While they may have a well-defined behavior on Linux, this need not
724 to be so on other architectures.
725 Therefore it usually is better to use
726 modifiers that are not defined by ANSI C at all, that is, use
731 \fBd\fP, \fBi\fP, \fBo\fP, \fBu\fP, \fBx\fP, and \fBX\fP
737 is not the same as on 4.4BSD,
738 as it may be used in float conversions equivalently to
741 To use the dynamic allocation conversion specifier, specify
743 as a length modifier (thus
746 \fB%m[\fP\fIrange\fP\fB]\fP).
749 the returned string, as in the following example:
757 n = scanf("%m[a\-z]", &p);
759 printf("read: %s\en", p);
761 } else if (errno != 0) {
764 fprintf(stderr, "No matching characters\en");
769 As shown in the above example, it is necessary to call
773 call successfully read a string.