strncat

Section: C Library Functions (3)
Updated: 2023-02-05
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NAME

strncat - concatenate a null-padded character sequence into a string

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <string.h>

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

DESCRIPTION

This function catenates the input character sequence contained in a null-padded fixed-width buffer, into a string at the buffer pointed to by dst. The programmer is responsible for allocating a destination buffer large enough, that is, strlen(dst) + strnlen(src, sz) + 1.

An implementation of this function might be:

char * strncat(char *restrict dst, const char *restrict src, size_t sz) {
int len;
char *p;

    len = strnlen(src, sz);
    p = dst + strlen(dst);
    p = mempcpy(p, src, len);
    *p = '\0';

return dst; }

RETURN VALUE

strncat() returns dst.

ATTRIBUTES

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

Interface	Attribute	Value
strncat()	Thread safety	MT-Safe

STANDARDS

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

CAVEATS

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

If the destination buffer 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 maxsize;

    // Null-padded fixed-width character sequences
    char    pre[4] = "pre.";
    char    new_post[50] = ".foo.bar";

    // Strings
    char    post[] = ".post";
    char    src[] = "some_long_body.post";
    char    *dest;

    maxsize = nitems(pre) + strlen(src) - strlen(post) +
              nitems(new_post) + 1;
    dest = malloc(sizeof(*dest) * maxsize);
    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); }

This document was created by man2html, using the manual pages.
Time: 21:42:03 GMT, May 08, 2024