man3/strcat.3

   1 .\" Copyright 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 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
  28 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
  29 .\"     Improve discussion of strncat().
  30 .TH STRCAT 3  2012-07-19 "GNU" "Linux Programmer's Manual"
  31 .SH NAME
  32 strcat, strncat \- concatenate two strings
  33 .SH SYNOPSIS
  34 .nf
  35 .B #include <string.h>
  36 .sp
  37 .BI "char *strcat(char *" dest ", const char *" src );
  38 .sp
  39 .BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
  40 .fi
  41 .SH DESCRIPTION
  42 The
  43 .BR strcat ()
  44 function appends the \fIsrc\fP string to the
  45 \fIdest\fP string,
  46 overwriting the terminating null byte (\(aq\\0\(aq) at the end of
  47 \fIdest\fP, and then adds a terminating null byte.
  48 The strings may not overlap, and the \fIdest\fP string must have
  49 enough space for the result.
  50 If
  51 .I dest
  52 is not large enough, program behavior is unpredictable;
  53 .IR "buffer overruns are a favorite avenue for attacking secure programs" .
  54 .PP
  55 The
  56 .BR strncat ()
  57 function is similar, except that
  58 .IP * 3
  59 it will use at most \fIn\fP bytes from \fIsrc\fP; and
  60 .IP *
  61 \fIsrc\fP does not need to be null-terminated if it contains
  62 \fIn\fP or more bytes.
  63 .PP
  64 As with
  65 .BR strcat (),
  66 the resulting string in \fIdest\fP is always null-terminated.
  67 .PP
  68 If \fIsrc\fP contains \fIn\fP or more bytes,
  69 .BR strncat ()
  70 writes \fIn+1\fP bytes to \fIdest\fP (\fIn\fP
  71 from \fIsrc\fP plus the terminating null byte).
  72 Therefore, the size of \fIdest\fP must be at least
  73 \fIstrlen(dest)+n+1\fP.
  74
  75 A simple implementation of
  76 .BR strncat ()
  77 might be:
  78 .in +4n
  79 .nf
  80
  81 char*
  82 strncat(char *dest, const char *src, size_t n)
  83 {
  84     size_t dest_len = strlen(dest);
  85     size_t i;
  86
  87     for (i = 0 ; i < n && src[i] != \(aq\\0\(aq ; i++)
  88         dest[dest_len + i] = src[i];
  89     dest[dest_len + i] = \(aq\\0\(aq;
  90
  91     return dest;
  92 }
  93 .fi
  94 .in
  95 .SH RETURN VALUE
  96 The
  97 .BR strcat ()
  98 and
  99 .BR strncat ()
 100 functions return a pointer to the resulting string \fIdest\fP.
 101 .SH CONFORMING TO
 102 SVr4, 4.3BSD, C89, C99.
 103 .SH NOTES
 104 Some systems (the BSDs, Solaris, and others) provide the following function:
 105
 106     size_t strlcat(char *dest, const char *src, size_t size);
 107
 108 This function appends the null-terminated string
 109 .I src
 110 to the string
 111 .IR dest ,
 112 copying at most
 113 .IR "size\-strlen(dest)\-1"
 114 from
 115 .IR src ,
 116 and adds a null terminator to the result,
 117 .I unless
 118 .IR size
 119 is less than
 120 .IR strlen(dest) .
 121 This function fixes the buffer overrun problem of
 122 .BR strcat (),
 123 but the caller must still handle the possibility of data loss if
 124 .I size
 125 is too small.
 126 The function returns the length of the string
 127 .BR strlcat ()
 128 tried to create; if the return value is greater than or equal to
 129 .IR size ,
 130 data loss occurred.
 131 If data loss matters, the caller
 132 .I must
 133 either check the arguments before the call, or test the function return value.
 134 .BR strlcat ()
 135 is not present in glibc and is not standardized by POSIX,
 136 .\" https://lwn.net/Articles/506530/
 137 but is available on Linux via the
 138 .IR libbsd
 139 library.
 140 .SH SEE ALSO
 141 .BR bcopy (3),
 142 .BR memccpy (3),
 143 .BR memcpy (3),
 144 .BR strcpy (3),
 145 .BR string (3),
 146 .BR strncpy (3),
 147 .BR wcscat (3),
 148 .BR wcsncat (3)