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)