man3/strcpy.3

   1 .\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
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  22 .\"
  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\" Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faith@cs.unc.edu)
  28 .\" Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aeb@cwi.nl)
  29 .\" Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aeb@cwi.nl)
  30 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
  31 .\"     Improve discussion of strncpy().
  32 .\"
  33 .TH STRCPY 3  2012-07-19 "GNU" "Linux Programmer's Manual"
  34 .SH NAME
  35 strcpy, strncpy \- copy a string
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <string.h>
  39 .sp
  40 .BI "char *strcpy(char *" dest ", const char *" src );
  41 .sp
  42 .BI "char *strncpy(char *" dest ", const char *" src ", size_t " n );
  43 .fi
  44 .SH DESCRIPTION
  45 The
  46 .BR strcpy ()
  47 function copies the string pointed to by \fIsrc\fP,
  48 including the terminating null byte (\(aq\\0\(aq),
  49 to the buffer pointed to by \fIdest\fP.
  50 The strings may not overlap, and the destination string
  51 \fIdest\fP must be large enough to receive the copy.
  52 .IR "Beware of buffer overruns!"
  53 (See BUGS.)
  54 .PP
  55 The
  56 .BR strncpy ()
  57 function is similar, except that at most
  58 \fIn\fP bytes of \fIsrc\fP are copied.
  59 .BR Warning :
  60 If there is no null byte
  61 among the first \fIn\fP bytes of \fIsrc\fP,
  62 the string placed in \fIdest\fP will not be null-terminated.
  63 .PP
  64 If the length of
  65 .I src
  66 is less than
  67 .IR n ,
  68 .BR strncpy ()
  69 writes additional null bytes to
  70 .I dest
  71 to ensure that a total of
  72 .I n
  73 bytes are written.
  74 .PP
  75 A simple implementation of
  76 .BR strncpy ()
  77 might be:
  78 .in +4n
  79 .nf
  80
  81 char *
  82 strncpy(char *dest, const char *src, size_t n)
  83 {
  84     size_t i;
  85
  86     for (i = 0; i < n && src[i] != \(aq\\0\(aq; i++)
  87         dest[i] = src[i];
  88     for ( ; i < n; i++)
  89         dest[i] = \(aq\\0\(aq;
  90
  91     return dest;
  92 }
  93 .fi
  94 .in
  95 .SH RETURN VALUE
  96 The
  97 .BR strcpy ()
  98 and
  99 .BR strncpy ()
 100 functions return a pointer to
 101 the destination string \fIdest\fP.
 102 .SH CONFORMING TO
 103 SVr4, 4.3BSD, C89, C99.
 104 .SH NOTES
 105 Some programmers consider
 106 .BR strncpy ()
 107 to be inefficient and error prone.
 108 If the programmer knows (i.e., includes code to test!)
 109 that the size of \fIdest\fP is greater than
 110 the length of \fIsrc\fP, then
 111 .BR strcpy ()
 112 can be used.
 113
 114 One valid (and intended) use of
 115 .BR strncpy ()
 116 is to copy a C string to a fixed-length buffer
 117 while ensuring both that the buffer is not overflowed
 118 and that unused bytes in the target buffer are zeroed out
 119 (perhaps to prevent information leaks if the buffer is to be
 120 written to media or transmitted to another process via an
 121 interprocess communication technique).
 122
 123 If there is no terminating null byte in the first \fIn\fP
 124 bytes of \fIsrc\fP,
 125 .BR strncpy ()
 126 produces an unterminated string in \fIdest\fP.
 127 You can force termination using something like the following:
 128 .in +4n
 129 .nf
 130
 131 strncpy(buf, str, n);
 132 if (n > 0)
 133     buf[n \- 1]= \(aq\\0\(aq;
 134 .fi
 135 .in
 136 .PP
 137 (Of course, the above technique ignores the fact that
 138 information contained in
 139 .I src
 140 is lost in the copying to
 141 .IR dest .)
 142
 143 Some systems (the BSDs, Solaris, and others) provide the following function:
 144
 145     size_t strlcpy(char *dest, const char *src, size_t size);
 146
 147 .\" http://static.usenix.org/event/usenix99/full_papers/millert/millert_html/index.html
 148 .\"     "strlcpy and strlcat - consistent, safe, string copy and concatenation"
 149 .\"     1999 USENIX Annual Technical Conference
 150 This function is similar to
 151 .BR strncpy (),
 152 but it copies at most
 153 .I size\-1
 154 bytes to
 155 .IR dest ,
 156 always adds a terminating null byte,
 157 and does not pad the target with (further) null bytes.
 158 This function fixes some of the problems of
 159 .BR strcpy ()
 160 and
 161 .BR strncpy (),
 162 but the caller must still handle the possibility of data loss if
 163 .I size
 164 is too small.
 165 The return value of the function is the length of
 166 .IR src ,
 167 which allows truncation to be easily detected:
 168 if the return value is greater than or equal to
 169 .IR size ,
 170 truncation occurred.
 171 If loss of data matters, the caller
 172 .I must
 173 either check the arguments before the call,
 174 or test the function return value.
 175 .BR strlcpy ()
 176 is not present in glibc and is not standardized by POSIX,
 177 .\" https://lwn.net/Articles/506530/
 178 but is available on Linux via the
 179 .IR libbsd
 180 library.
 181 .SH BUGS
 182 If the destination string of a
 183 .BR strcpy ()
 184 is not large enough, then anything might happen.
 185 Overflowing fixed-length string buffers is a favorite cracker technique
 186 for taking complete control of the machine.
 187 Any time a program reads or copies data into a buffer,
 188 the program first needs to check that there's enough space.
 189 This may be unnecessary if you can show that overflow is impossible,
 190 but be careful: programs can get changed over time,
 191 in ways that may make the impossible possible.
 192 .SH SEE ALSO
 193 .BR bcopy (3),
 194 .BR memccpy (3),
 195 .BR memcpy (3),
 196 .BR memmove (3),
 197 .BR stpcpy (3),
 198 .BR stpncpy (3),
 199 .BR strdup (3),
 200 .BR string (3),
 201 .BR wcscpy (3),
 202 .BR wcsncpy (3)