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 *dest, const char *src);

       char *strncat(char *dest, const char *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

       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

           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

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 <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");

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 5.08 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                              2020-06-09                        STRCAT(3)

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