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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
     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 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];
         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");
         } }