STRCAT(3)                  Linux Programmer's Manual                 STRCAT(3)

       strcat, strncat - concatenate two strings

       #include <string.h>

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

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

       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

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

       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;

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

       For an explanation of the terms used in this section, see

       │Interface           Attribute     Value   │
       │strcat(), strncat() │ Thread safety │ MT-Safe │
       POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

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

           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.

       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>

       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");

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

       This page is part of release 5.06 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

GNU                               2019-08-02                         STRCAT(3)