NAME

strncat - append non-null bytes from a source array to a string, and null-terminate the result

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <string.h>

char *strncat(char *restrict dst, const char src[restrict .ssize],
              size_t ssize);

DESCRIPTION

This function appends at most ssize non-null bytes from the array pointed to by src, followed by a null character, to the end of the string pointed to by dst. dst must point to a string contained in a buffer that is large enough, that is, the buffer size must be at least strlen(dst) + strnlen(src, ssize) + 1.

An implementation of this function might be:

char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{

    #define strnul(s)  (s + strlen(s))

    stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");

    return dst;
}

RETURN VALUE

strncat() returns dst.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

STANDARDS

C11, POSIX.1-2008.

HISTORY

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

CAVEATS

The name of this function is confusing; it has no relation to strncpy(3).

If the destination buffer does not already contain a string, or is not large enough, the behavior is undefined. See _FORTIFY_SOURCE in feature_test_macros(7).

BUGS

This function can be very inefficient. Read about Shlemiel the painter.

EXAMPLES

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))
int
main(void)
{

    size_t  n;

    // Null-padded fixed-size character sequences

    char    pre[4] = "pre.";

    char    new_post[50] = ".foo.bar";

    // Strings

    char    post[] = ".post";

    char    src[] = "some_long_body.post";

    char    *dest;

    n = nitems(pre) + strlen(src) - strlen(post) + nitems(new_post) + 1;

    dest = malloc(sizeof(*dest) * n);

    if (dest == NULL)

        err(EXIT_FAILURE, "malloc()");

    dest[0] = '\0';  // There's no 'cpy' function to this 'cat'.

    strncat(dest, pre, nitems(pre));

    strncat(dest, src, strlen(src) - strlen(post));

    strncat(dest, new_post, nitems(new_post));

    puts(dest);  // "pre.some_long_body.foo.bar"

    free(dest);

    exit(EXIT_SUCCESS);
}

SEE ALSO

string(3), string_copying(7)