man3/strcpy.3

   1 .\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  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().
  34 .\"
  35 .TH STRCPY 3  2019-03-06 "GNU" "Linux Programmer's Manual"
  36 .SH NAME
  37 strcpy, strncpy \- copy a string
  38 .SH SYNOPSIS
  39 .nf
  40 .B #include <string.h>
  41 .PP
  42 .BI "char *strcpy(char *" dest ", const char *" src );
  43 .PP
  44 .BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
  45 .fi
  46 .SH DESCRIPTION
  47 The
  48 .BR strcpy ()
  49 function copies the string pointed to by
  50 .IR src ,
  51 including the terminating null byte (\(aq\e0\(aq),
  52 to the buffer pointed to by
  53 .IR dest .
  54 The strings may not overlap, and the destination string
  55 .I dest
  56 must be large enough to receive the copy.
  57 .IR "Beware of buffer overruns!"
  58 (See BUGS.)
  59 .PP
  60 The
  61 .BR strncpy ()
  62 function is similar, except that at most
  63 .I n
  64 bytes of
  65 .I src
  66 are copied.
  67 .BR Warning :
  68 If there is no null byte
  69 among the first
  70 .I n
  71 bytes of
  72 .IR src ,
  73 the string placed in
  74 .I dest
  75 will not be null-terminated.
  76 .PP
  77 If the length of
  78 .I src
  79 is less than
  80 .IR n ,
  81 .BR strncpy ()
  82 writes additional null bytes to
  83 .I dest
  84 to ensure that a total of
  85 .I n
  86 bytes are written.
  87 .PP
  88 A simple implementation of
  89 .BR strncpy ()
  90 might be:
  91 .PP
  92 .in +4n
  93 .EX
  94 char *
  95 strncpy(char *dest, const char *src, size_t n)
  96 {
  97     size_t i;
  98
  99     for (i = 0; i < n && src[i] != \(aq\e0\(aq; i++)
 100         dest[i] = src[i];
 101     for ( ; i < n; i++)
 102         dest[i] = \(aq\e0\(aq;
 103
 104     return dest;
 105 }
 106 .EE
 107 .in
 108 .SH RETURN VALUE
 109 The
 110 .BR strcpy ()
 111 and
 112 .BR strncpy ()
 113 functions return a pointer to
 114 the destination string
 115 .IR dest .
 116 .SH ATTRIBUTES
 117 For an explanation of the terms used in this section, see
 118 .BR attributes (7).
 119 .TS
 120 allbox;
 121 lbw19 lb lb
 122 l l l.
 123 Interface       Attribute       Value
 124 T{
 125 .BR strcpy (),
 126 .BR strncpy ()
 127 T}      Thread safety   MT-Safe
 128 .TE
 129 .SH CONFORMING TO
 130 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
 131 .SH NOTES
 132 Some programmers consider
 133 .BR strncpy ()
 134 to be inefficient and error prone.
 135 If the programmer knows (i.e., includes code to test!)
 136 that the size of
 137 .I dest
 138 is greater than
 139 the length of
 140 .IR src ,
 141 then
 142 .BR strcpy ()
 143 can be used.
 144 .PP
 145 One valid (and intended) use of
 146 .BR strncpy ()
 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).
 153 .PP
 154 If there is no terminating null byte in the first
 155 .I n
 156 bytes of
 157 .IR src ,
 158 .BR strncpy ()
 159 produces an unterminated string in
 160 .IR dest .
 161 If
 162 .I buf
 163 has length
 164 .IR buflen ,
 165 you can force termination using something like the following:
 166 .PP
 167 .in +4n
 168 .EX
 169 if (buflen > 0) {
 170     strncpy(buf, str, buflen \- 1);
 171     buf[buflen \- 1]= \(aq\e0\(aq;
 172 }
 173 .EE
 174 .in
 175 .PP
 176 (Of course, the above technique ignores the fact that, if
 177 .I src
 178 contains more than
 179 .I "buflen\ \-\ 1"
 180 bytes, information is lost in the copying to
 181 .IR dest .)
 182 .\"
 183 .SS strlcpy()
 184 Some systems (the BSDs, Solaris, and others) provide the following function:
 185 .PP
 186     size_t strlcpy(char *dest, const char *src, size_t size);
 187 .PP
 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
 192 .BR strncpy (),
 193 but it copies at most
 194 .I size\-1
 195 bytes to
 196 .IR dest ,
 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
 200 .BR strcpy ()
 201 and
 202 .BR strncpy (),
 203 but the caller must still handle the possibility of data loss if
 204 .I size
 205 is too small.
 206 The return value of the function is the length of
 207 .IR src ,
 208 which allows truncation to be easily detected:
 209 if the return value is greater than or equal to
 210 .IR size ,
 211 truncation occurred.
 212 If loss of data matters, the caller
 213 .I must
 214 either check the arguments before the call,
 215 or test the function return value.
 216 .BR strlcpy ()
 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
 220 .IR libbsd
 221 library.
 222 .SH BUGS
 223 If the destination string of a
 224 .BR strcpy ()
 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.
 233 .SH SEE ALSO
 234 .BR bcopy (3),
 235 .BR memccpy (3),
 236 .BR memcpy (3),
 237 .BR memmove (3),
 238 .BR stpcpy (3),
 239 .BR stpncpy (3),
 240 .BR strdup (3),
 241 .BR string (3),
 242 .BR wcscpy (3),
 243 .BR wcsncpy (3)