strncat

STRCAT(3)                  Linux Programmer's Manual                 STRCAT(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, over-
       writing 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-termi-
       nated.

       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 at-
       tributes(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  termi-
       nating  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.  strl-
       cat() 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 tar-
       get, then manually copying the bytes  from  each  source  string  while
       maintaining a pointer to the end of the target string will provide bet-
       ter 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 + 1];    /* +1 for terminating null byte */
           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),  wc-
       scat(3), wcsncat(3)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       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/.

GNU                               2019-08-02                         STRCAT(3)