1 .\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
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13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
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18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" References consulted:
26 .\" Linux libc source code
27 .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
29 .\" Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faith@cs.unc.edu)
30 .\" Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aeb@cwi.nl)
31 .\" Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aeb@cwi.nl)
32 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
33 .\" Improve discussion of strncpy().
35 .TH STRCPY 3 2019-03-06 "GNU" "Linux Programmer's Manual"
37 strcpy, strncpy \- copy a string
40 .B #include <string.h>
42 .BI "char *strcpy(char *" dest ", const char *" src );
44 .BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
49 function copies the string pointed to by
51 including the terminating null byte (\(aq\e0\(aq),
52 to the buffer pointed to by
54 The strings may not overlap, and the destination string
56 must be large enough to receive the copy.
57 .IR "Beware of buffer overruns!"
62 function is similar, except that at most
68 If there is no null byte
75 will not be null-terminated.
82 writes additional null bytes to
84 to ensure that a total of
88 A simple implementation of
95 strncpy(char *dest, const char *src, size_t n)
99 for (i = 0; i < n && src[i] != \(aq\e0\(aq; i++)
102 dest[i] = \(aq\e0\(aq;
113 functions return a pointer to
114 the destination string
117 For an explanation of the terms used in this section, see
123 Interface Attribute Value
127 T} Thread safety MT-Safe
130 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
132 Some programmers consider
134 to be inefficient and error prone.
135 If the programmer knows (i.e., includes code to test!)
145 One valid (and intended) use of
147 is to copy a C string to a fixed-length buffer
148 while ensuring both that the buffer is not overflowed
149 and that unused bytes in the destination buffer are zeroed out
150 (perhaps to prevent information leaks if the buffer is to be
151 written to media or transmitted to another process via an
152 interprocess communication technique).
154 If there is no terminating null byte in the first
159 produces an unterminated string in
165 you can force termination using something like the following:
170 strncpy(buf, str, buflen \- 1);
171 buf[buflen \- 1]= \(aq\e0\(aq;
176 (Of course, the above technique ignores the fact that, if
180 bytes, information is lost in the copying to
184 Some systems (the BSDs, Solaris, and others) provide the following function:
186 size_t strlcpy(char *dest, const char *src, size_t size);
188 .\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
189 .\" "strlcpy and strlcat - consistent, safe, string copy and concatenation"
190 .\" 1999 USENIX Annual Technical Conference
191 This function is similar to
193 but it copies at most
197 always adds a terminating null byte,
198 and does not pad the destination with (further) null bytes.
199 This function fixes some of the problems of
203 but the caller must still handle the possibility of data loss if
206 The return value of the function is the length of
208 which allows truncation to be easily detected:
209 if the return value is greater than or equal to
212 If loss of data matters, the caller
214 either check the arguments before the call,
215 or test the function return value.
217 is not present in glibc and is not standardized by POSIX,
218 .\" https://lwn.net/Articles/506530/
219 but is available on Linux via the
223 If the destination string of a
225 is not large enough, then anything might happen.
226 Overflowing fixed-length string buffers is a favorite cracker technique
227 for taking complete control of the machine.
228 Any time a program reads or copies data into a buffer,
229 the program first needs to check that there's enough space.
230 This may be unnecessary if you can show that overflow is impossible,
231 but be careful: programs can get changed over time,
232 in ways that may make the impossible possible.