1 .\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
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8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
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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14 .\" manual page may be incorrect or out-of-date.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 2015-08-08 "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\\0\(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\\0\(aq; i++)
102 dest[i] = \(aq\\0\(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 target 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:
169 strncpy(buf, str, buflen \- 1);
171 buf[buflen \- 1]= \(aq\\0\(aq;
175 (Of course, the above technique ignores the fact that, if
179 bytes, information is lost in the copying to
183 Some systems (the BSDs, Solaris, and others) provide the following function:
185 size_t strlcpy(char *dest, const char *src, size_t size);
187 .\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
188 .\" "strlcpy and strlcat - consistent, safe, string copy and concatenation"
189 .\" 1999 USENIX Annual Technical Conference
190 This function is similar to
192 but it copies at most
196 always adds a terminating null byte,
197 and does not pad the target with (further) null bytes.
198 This function fixes some of the problems of
202 but the caller must still handle the possibility of data loss if
205 The return value of the function is the length of
207 which allows truncation to be easily detected:
208 if the return value is greater than or equal to
211 If loss of data matters, the caller
213 either check the arguments before the call,
214 or test the function return value.
216 is not present in glibc and is not standardized by POSIX,
217 .\" https://lwn.net/Articles/506530/
218 but is available on Linux via the
222 If the destination string of a
224 is not large enough, then anything might happen.
225 Overflowing fixed-length string buffers is a favorite cracker technique
226 for taking complete control of the machine.
227 Any time a program reads or copies data into a buffer,
228 the program first needs to check that there's enough space.
229 This may be unnecessary if you can show that overflow is impossible,
230 but be careful: programs can get changed over time,
231 in ways that may make the impossible possible.