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  2012-07-19 "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 .sp
  42 .BI "char *strcpy(char *" dest ", const char *" src );
  43 .sp
  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 \fIsrc\fP,
  50 including the terminating null byte (\(aq\\0\(aq),
  51 to the buffer pointed to by \fIdest\fP.
  52 The strings may not overlap, and the destination string
  53 \fIdest\fP must be large enough to receive the copy.
  54 .IR "Beware of buffer overruns!"
  55 (See BUGS.)
  56 .PP
  57 The
  58 .BR strncpy ()
  59 function is similar, except that at most
  60 \fIn\fP bytes of \fIsrc\fP are copied.
  61 .BR Warning :
  62 If there is no null byte
  63 among the first \fIn\fP bytes of \fIsrc\fP,
  64 the string placed in \fIdest\fP will not be null-terminated.
  65 .PP
  66 If the length of
  67 .I src
  68 is less than
  69 .IR n ,
  70 .BR strncpy ()
  71 writes additional null bytes to
  72 .I dest
  73 to ensure that a total of
  74 .I n
  75 bytes are written.
  76 .PP
  77 A simple implementation of
  78 .BR strncpy ()
  79 might be:
  80 .in +4n
  81 .nf
  82
  83 char *
  84 strncpy(char *dest, const char *src, size_t n)
  85 {
  86     size_t i;
  87
  88     for (i = 0; i < n && src[i] != \(aq\\0\(aq; i++)
  89         dest[i] = src[i];
  90     for ( ; i < n; i++)
  91         dest[i] = \(aq\\0\(aq;
  92
  93     return dest;
  94 }
  95 .fi
  96 .in
  97 .SH RETURN VALUE
  98 The
  99 .BR strcpy ()
 100 and
 101 .BR strncpy ()
 102 functions return a pointer to
 103 the destination string \fIdest\fP.
 104 .SH CONFORMING TO
 105 SVr4, 4.3BSD, C89, C99.
 106 .SH NOTES
 107 Some programmers consider
 108 .BR strncpy ()
 109 to be inefficient and error prone.
 110 If the programmer knows (i.e., includes code to test!)
 111 that the size of \fIdest\fP is greater than
 112 the length of \fIsrc\fP, then
 113 .BR strcpy ()
 114 can be used.
 115
 116 One valid (and intended) use of
 117 .BR strncpy ()
 118 is to copy a C string to a fixed-length buffer
 119 while ensuring both that the buffer is not overflowed
 120 and that unused bytes in the target buffer are zeroed out
 121 (perhaps to prevent information leaks if the buffer is to be
 122 written to media or transmitted to another process via an
 123 interprocess communication technique).
 124
 125 If there is no terminating null byte in the first \fIn\fP
 126 bytes of \fIsrc\fP,
 127 .BR strncpy ()
 128 produces an unterminated string in \fIdest\fP.
 129 You can force termination using something like the following:
 130 .in +4n
 131 .nf
 132
 133 strncpy(buf, str, n);
 134 if (n > 0)
 135     buf[n \- 1]= \(aq\\0\(aq;
 136 .fi
 137 .in
 138 .PP
 139 (Of course, the above technique ignores the fact that
 140 information contained in
 141 .I src
 142 is lost in the copying to
 143 .IR dest .)
 144
 145 Some systems (the BSDs, Solaris, and others) provide the following function:
 146
 147     size_t strlcpy(char *dest, const char *src, size_t size);
 148
 149 .\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
 150 .\"     "strlcpy and strlcat - consistent, safe, string copy and concatenation"
 151 .\"     1999 USENIX Annual Technical Conference
 152 This function is similar to
 153 .BR strncpy (),
 154 but it copies at most
 155 .I size\-1
 156 bytes to
 157 .IR dest ,
 158 always adds a terminating null byte,
 159 and does not pad the target with (further) null bytes.
 160 This function fixes some of the problems of
 161 .BR strcpy ()
 162 and
 163 .BR strncpy (),
 164 but the caller must still handle the possibility of data loss if
 165 .I size
 166 is too small.
 167 The return value of the function is the length of
 168 .IR src ,
 169 which allows truncation to be easily detected:
 170 if the return value is greater than or equal to
 171 .IR size ,
 172 truncation occurred.
 173 If loss of data matters, the caller
 174 .I must
 175 either check the arguments before the call,
 176 or test the function return value.
 177 .BR strlcpy ()
 178 is not present in glibc and is not standardized by POSIX,
 179 .\" https://lwn.net/Articles/506530/
 180 but is available on Linux via the
 181 .IR libbsd
 182 library.
 183 .SH BUGS
 184 If the destination string of a
 185 .BR strcpy ()
 186 is not large enough, then anything might happen.
 187 Overflowing fixed-length string buffers is a favorite cracker technique
 188 for taking complete control of the machine.
 189 Any time a program reads or copies data into a buffer,
 190 the program first needs to check that there's enough space.
 191 This may be unnecessary if you can show that overflow is impossible,
 192 but be careful: programs can get changed over time,
 193 in ways that may make the impossible possible.
 194 .SH SEE ALSO
 195 .BR bcopy (3),
 196 .BR memccpy (3),
 197 .BR memcpy (3),
 198 .BR memmove (3),
 199 .BR stpcpy (3),
 200 .BR stpncpy (3),
 201 .BR strdup (3),
 202 .BR string (3),
 203 .BR wcscpy (3),
 204 .BR wcsncpy (3)