strncat(3)

NAME

   strcat, strncat - concatenate two strings

SYNOPSIS

   #include <string.h>

   char *strcat(char *dest, const char *src);

   char *strncat(char *dest, const char *src, size_t n);

DESCRIPTION

   The  strcat()  function  appends  the  src  string  to the dest string,
   overwriting the terminating null byte ('\0') at the end  of  dest,  and
   then  adds  a  terminating null byte.  The strings may not overlap, and
   the dest string must have enough space for the result.  If dest is  not
   large  enough, program behavior is unpredictable; buffer overruns are a
   favorite avenue for attacking secure programs.

   The strncat() function is similar, except that

   *  it will use at most n bytes from src; and

   *  src does not need to be null-terminated if it  contains  n  or  more
      bytes.

   As  with  strcat(),  the  resulting  string  in  dest  is  always null-
   terminated.

   If src contains n or more bytes, strncat() writes n+1 bytes to dest  (n
   from  src plus the terminating null byte).  Therefore, the size of dest
   must be at least strlen(dest)+n+1.

   A simple implementation of strncat() might be:

       char *
       strncat(char *dest, const char *src, size_t n)
       {
           size_t dest_len = strlen(dest);
           size_t i;

           for (i = 0 ; i < n && src[i] != '\0' ; i++)
               dest[dest_len + i] = src[i];
           dest[dest_len + i] = '\0';

           return dest;
       }

RETURN VALUE

   The strcat() and strncat() functions return a pointer to the  resulting
   string dest.

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   ┌────────────────────┬───────────────┬─────────┐
   │Interface           │ Attribute     │ Value   │
   ├────────────────────┼───────────────┼─────────┤
   │strcat(), strncat() │ Thread safety │ MT-Safe │
   └────────────────────┴───────────────┴─────────┘

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

NOTES

   Some systems (the BSDs, Solaris,  and  others)  provide  the  following
   function:

       size_t strlcat(char *dest, const char *src, size_t size);

   This  function  appends  the  null-terminated  string src to the string
   dest,  copying  at  most  size-strlen(dest)-1  from  src,  and  adds  a
   terminating  null  byte  to  the  result,  unless  size  is  less  than
   strlen(dest).  This  function  fixes  the  buffer  overrun  problem  of
   strcat(), but the caller must still handle the possibility of data loss
   if size is too small.  The function returns the length  of  the  string
   strlcat() tried to create; if the return value is greater than or equal
   to size, data loss occurred.  If data loss  matters,  the  caller  must
   either check the arguments before the call, or test the function return
   value.  strlcat() is not present in glibc and is  not  standardized  by
   POSIX, but is available on Linux via the libbsd library.

EXAMPLE

   Because  strcat() and strncat() must find the null byte that terminates
   the string dest using a search that starts  at  the  beginning  of  the
   string,  the  execution time of these functions scales according to the
   length of the string dest.  This can be  demonstrated  by  running  the
   program  below.   (If  the  goal  is to concatenate many strings to one
   target, then manually copying the bytes from each source  string  while
   maintaining  a  pointer  to  the  end of the target string will provide
   better performance.)

   Program source

   #include <string.h>
   #include <time.h>
   #include <stdio.h>

   int
   main(int argc, char *argv[])
   {
   #define LIM 4000000
       int j;
       char p[LIM];
       time_t base;

       base = time(NULL);
       p[0] = '\0';

       for (j = 0; j < LIM; j++) {
           if ((j % 10000) == 0)
               printf("%d %ld\n", j, (long) (time(NULL) - base));
           strcat(p, "a");
       }
   }

SEE ALSO

   bcopy(3),  memccpy(3),  memcpy(3),  strcpy(3),  string(3),  strncpy(3),
   wcscat(3), wcsncat(3)

COLOPHON

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   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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