man3/strcpy.3

   1 .\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" References consulted:
   6 .\"     Linux libc source code
   7 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
   8 .\"     386BSD man pages
   9 .\" Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faith@cs.unc.edu)
  10 .\" Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aeb@cwi.nl)
  11 .\" Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aeb@cwi.nl)
  12 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
  13 .\"     Improve discussion of strncpy().
  14 .\"
  15 .TH STRCPY 3  2021-03-22 "GNU" "Linux Programmer's Manual"
  16 .SH NAME
  17 strcpy, strncpy \- copy a string
  18 .SH LIBRARY
  19 Standard C library
  20 .RI ( libc ", " -lc )
  21 .SH SYNOPSIS
  22 .nf
  23 .B #include <string.h>
  24 .PP
  25 .BI "char *strcpy(char *restrict " dest ", const char *" src );
  26 .BI "char *strncpy(char *restrict " dest ", const char *restrict " src \
  27 ", size_t " n );
  28 .fi
  29 .SH DESCRIPTION
  30 The
  31 .BR strcpy ()
  32 function copies the string pointed to by
  33 .IR src ,
  34 including the terminating null byte (\(aq\e0\(aq),
  35 to the buffer pointed to by
  36 .IR dest .
  37 The strings may not overlap, and the destination string
  38 .I dest
  39 must be large enough to receive the copy.
  40 .IR "Beware of buffer overruns!"
  41 (See BUGS.)
  42 .PP
  43 The
  44 .BR strncpy ()
  45 function is similar, except that at most
  46 .I n
  47 bytes of
  48 .I src
  49 are copied.
  50 .BR Warning :
  51 If there is no null byte
  52 among the first
  53 .I n
  54 bytes of
  55 .IR src ,
  56 the string placed in
  57 .I dest
  58 will not be null-terminated.
  59 .PP
  60 If the length of
  61 .I src
  62 is less than
  63 .IR n ,
  64 .BR strncpy ()
  65 writes additional null bytes to
  66 .I dest
  67 to ensure that a total of
  68 .I n
  69 bytes are written.
  70 .PP
  71 A simple implementation of
  72 .BR strncpy ()
  73 might be:
  74 .PP
  75 .in +4n
  76 .EX
  77 char *
  78 strncpy(char *dest, const char *src, size_t n)
  79 {
  80     size_t i;
  81
  82     for (i = 0; i < n && src[i] != \(aq\e0\(aq; i++)
  83         dest[i] = src[i];
  84     for ( ; i < n; i++)
  85         dest[i] = \(aq\e0\(aq;
  86
  87     return dest;
  88 }
  89 .EE
  90 .in
  91 .SH RETURN VALUE
  92 The
  93 .BR strcpy ()
  94 and
  95 .BR strncpy ()
  96 functions return a pointer to
  97 the destination string
  98 .IR dest .
  99 .SH ATTRIBUTES
 100 For an explanation of the terms used in this section, see
 101 .BR attributes (7).
 102 .ad l
 103 .nh
 104 .TS
 105 allbox;
 106 lbx lb lb
 107 l l l.
 108 Interface       Attribute       Value
 109 T{
 110 .BR strcpy (),
 111 .BR strncpy ()
 112 T}      Thread safety   MT-Safe
 113 .TE
 114 .hy
 115 .ad
 116 .sp 1
 117 .SH CONFORMING TO
 118 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
 119 .SH NOTES
 120 Some programmers consider
 121 .BR strncpy ()
 122 to be inefficient and error prone.
 123 If the programmer knows (i.e., includes code to test!)
 124 that the size of
 125 .I dest
 126 is greater than
 127 the length of
 128 .IR src ,
 129 then
 130 .BR strcpy ()
 131 can be used.
 132 .PP
 133 One valid (and intended) use of
 134 .BR strncpy ()
 135 is to copy a C string to a fixed-length buffer
 136 while ensuring both that the buffer is not overflowed
 137 and that unused bytes in the destination buffer are zeroed out
 138 (perhaps to prevent information leaks if the buffer is to be
 139 written to media or transmitted to another process via an
 140 interprocess communication technique).
 141 .PP
 142 If there is no terminating null byte in the first
 143 .I n
 144 bytes of
 145 .IR src ,
 146 .BR strncpy ()
 147 produces an unterminated string in
 148 .IR dest .
 149 If
 150 .I buf
 151 has length
 152 .IR buflen ,
 153 you can force termination using something like the following:
 154 .PP
 155 .in +4n
 156 .EX
 157 if (buflen > 0) {
 158     strncpy(buf, str, buflen \- 1);
 159     buf[buflen \- 1]= \(aq\e0\(aq;
 160 }
 161 .EE
 162 .in
 163 .PP
 164 (Of course, the above technique ignores the fact that, if
 165 .I src
 166 contains more than
 167 .I "buflen\ \-\ 1"
 168 bytes, information is lost in the copying to
 169 .IR dest .)
 170 .\"
 171 .SS strlcpy()
 172 Some systems (the BSDs, Solaris, and others) provide the following function:
 173 .PP
 174     size_t strlcpy(char *dest, const char *src, size_t size);
 175 .PP
 176 .\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
 177 .\"     "strlcpy and strlcat - consistent, safe, string copy and concatenation"
 178 .\"     1999 USENIX Annual Technical Conference
 179 This function is similar to
 180 .BR strncpy (),
 181 but it copies at most
 182 .I size\-1
 183 bytes to
 184 .IR dest ,
 185 always adds a terminating null byte,
 186 and does not pad the destination with (further) null bytes.
 187 This function fixes some of the problems of
 188 .BR strcpy ()
 189 and
 190 .BR strncpy (),
 191 but the caller must still handle the possibility of data loss if
 192 .I size
 193 is too small.
 194 The return value of the function is the length of
 195 .IR src ,
 196 which allows truncation to be easily detected:
 197 if the return value is greater than or equal to
 198 .IR size ,
 199 truncation occurred.
 200 If loss of data matters, the caller
 201 .I must
 202 either check the arguments before the call,
 203 or test the function return value.
 204 .BR strlcpy ()
 205 is not present in glibc and is not standardized by POSIX,
 206 .\" https://lwn.net/Articles/506530/
 207 but is available on Linux via the
 208 .IR libbsd
 209 library.
 210 .SH BUGS
 211 If the destination string of a
 212 .BR strcpy ()
 213 is not large enough, then anything might happen.
 214 Overflowing fixed-length string buffers is a favorite cracker technique
 215 for taking complete control of the machine.
 216 Any time a program reads or copies data into a buffer,
 217 the program first needs to check that there's enough space.
 218 This may be unnecessary if you can show that overflow is impossible,
 219 but be careful: programs can get changed over time,
 220 in ways that may make the impossible possible.
 221 .SH SEE ALSO
 222 .BR bcopy (3),
 223 .BR memccpy (3),
 224 .BR memcpy (3),
 225 .BR memmove (3),
 226 .BR stpcpy (3),
 227 .BR stpncpy (3),
 228 .BR strdup (3),
 229 .BR string (3),
 230 .BR wcscpy (3),
 231 .BR wcsncpy (3)