strcat(3) — Linux manual page


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

NAME         top

       strcat, strncat - concatenate two strings

SYNOPSIS         top

       #include <string.h>

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

DESCRIPTION         top

       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-

       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         top

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

ATTRIBUTES         top

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

       │Interface                             Attribute     Value   │
       │strcat(), strncat()                   │ Thread safety │ MT-Safe │

CONFORMING TO         top

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

NOTES         top

       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.

EXAMPLES         top

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

       main(int argc, char *argv[])
       #define LIM 4000000
           char p[LIM + 1];    /* +1 for terminating null byte */
           time_t base;

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

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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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                            2021-03-22                      STRCAT(3)

Pages that refer to this page: pmstrncat(3)string(3)wcscat(3)wcsncat(3)feature_test_macros(7)signal-safety(7)