man3/strcat.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  24 .\"
  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 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
  30 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
  31 .\"     Improve discussion of strncat().
  32 .TH STRCAT 3  2019-08-02 "GNU" "Linux Programmer's Manual"
  33 .SH NAME
  34 strcat, strncat \- concatenate two strings
  35 .SH SYNOPSIS
  36 .nf
  37 .B #include <string.h>
  38 .PP
  39 .BI "char *strcat(char *" dest ", const char *" src );
  40 .PP
  41 .BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
  42 .fi
  43 .SH DESCRIPTION
  44 The
  45 .BR strcat ()
  46 function appends the
  47 .I src
  48 string to the
  49 .I dest
  50 string,
  51 overwriting the terminating null byte (\(aq\e0\(aq) at the end of
  52 .IR dest ,
  53 and then adds a terminating null byte.
  54 The strings may not overlap, and the
  55 .I dest
  56 string must have
  57 enough space for the result.
  58 If
  59 .I dest
  60 is not large enough, program behavior is unpredictable;
  61 .IR "buffer overruns are a favorite avenue for attacking secure programs" .
  62 .PP
  63 The
  64 .BR strncat ()
  65 function is similar, except that
  66 .IP * 3
  67 it will use at most
  68 .I n
  69 bytes from
  70 .IR src ;
  71 and
  72 .IP *
  73 .I src
  74 does not need to be null-terminated if it contains
  75 .I n
  76 or more bytes.
  77 .PP
  78 As with
  79 .BR strcat (),
  80 the resulting string in
  81 .I dest
  82 is always null-terminated.
  83 .PP
  84 If
  85 .IR src
  86 contains
  87 .I n
  88 or more bytes,
  89 .BR strncat ()
  90 writes
  91 .I n+1
  92 bytes to
  93 .I dest
  94 .RI ( n
  95 from
  96 .I src
  97 plus the terminating null byte).
  98 Therefore, the size of
  99 .I dest
 100 must be at least
 101 .IR "strlen(dest)+n+1" .
 102 .PP
 103 A simple implementation of
 104 .BR strncat ()
 105 might be:
 106 .PP
 107 .in +4n
 108 .EX
 109 char *
 110 strncat(char *dest, const char *src, size_t n)
 111 {
 112     size_t dest_len = strlen(dest);
 113     size_t i;
 114
 115     for (i = 0 ; i < n && src[i] != \(aq\e0\(aq ; i++)
 116         dest[dest_len + i] = src[i];
 117     dest[dest_len + i] = \(aq\e0\(aq;
 118
 119     return dest;
 120 }
 121 .EE
 122 .in
 123 .SH RETURN VALUE
 124 The
 125 .BR strcat ()
 126 and
 127 .BR strncat ()
 128 functions return a pointer to the resulting string
 129 .IR dest .
 130 .SH ATTRIBUTES
 131 For an explanation of the terms used in this section, see
 132 .BR attributes (7).
 133 .TS
 134 allbox;
 135 lbw19 lb lb
 136 l l l.
 137 Interface       Attribute       Value
 138 T{
 139 .BR strcat (),
 140 .BR strncat ()
 141 T}      Thread safety   MT-Safe
 142 .TE
 143 .SH CONFORMING TO
 144 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
 145 .SH NOTES
 146 Some systems (the BSDs, Solaris, and others) provide the following function:
 147 .PP
 148     size_t strlcat(char *dest, const char *src, size_t size);
 149 .PP
 150 This function appends the null-terminated string
 151 .I src
 152 to the string
 153 .IR dest ,
 154 copying at most
 155 .IR "size\-strlen(dest)\-1"
 156 from
 157 .IR src ,
 158 and adds a terminating null byte to the result,
 159 .I unless
 160 .IR size
 161 is less than
 162 .IR strlen(dest) .
 163 This function fixes the buffer overrun problem of
 164 .BR strcat (),
 165 but the caller must still handle the possibility of data loss if
 166 .I size
 167 is too small.
 168 The function returns the length of the string
 169 .BR strlcat ()
 170 tried to create; if the return value is greater than or equal to
 171 .IR size ,
 172 data loss occurred.
 173 If data loss matters, the caller
 174 .I must
 175 either check the arguments before the call, or test the function return value.
 176 .BR strlcat ()
 177 is not present in glibc and is not standardized by POSIX,
 178 .\" https://lwn.net/Articles/506530/
 179 but is available on Linux via the
 180 .IR libbsd
 181 library.
 182 .\"
 183 .SH EXAMPLES
 184 Because
 185 .BR strcat ()
 186 and
 187 .BR strncat ()
 188 must find the null byte that terminates the string
 189 .I dest
 190 using a search that starts at the beginning of the string,
 191 the execution time of these functions
 192 scales according to the length of the string
 193 .IR dest .
 194 This can be demonstrated by running the program below.
 195 (If the goal is to concatenate many strings to one target,
 196 then manually copying the bytes from each source string
 197 while maintaining a pointer to the end of the target string
 198 will provide better performance.)
 199 .\"
 200 .SS Program source
 201 \&
 202 .EX
 203 #include <string.h>
 204 #include <time.h>
 205 #include <stdio.h>
 206
 207 int
 208 main(int argc, char *argv[])
 209 {
 210 #define LIM 4000000
 211     int j;
 212     char p[LIM + 1];    /* +1 for terminating null byte */
 213     time_t base;
 214
 215     base = time(NULL);
 216     p[0] = \(aq\e0\(aq;
 217
 218     for (j = 0; j < LIM; j++) {
 219         if ((j % 10000) == 0)
 220             printf("%d %ld\en", j, (long) (time(NULL) \- base));
 221         strcat(p, "a");
 222     }
 223 }
 224 .EE
 225 .\"
 226 .SH SEE ALSO
 227 .BR bcopy (3),
 228 .BR memccpy (3),
 229 .BR memcpy (3),
 230 .BR strcpy (3),
 231 .BR string (3),
 232 .BR strncpy (3),
 233 .BR wcscat (3),
 234 .BR wcsncat (3)