Man page - strncat(3)

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Manual

strncat

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
RETURN VALUE
ATTRIBUTES
STANDARDS
HISTORY
CAVEATS
BUGS
EXAMPLES
SEE ALSO

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).

Image grohtml-454838-1.png

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);
}