strncat

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).

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);
}

SEE ALSO

string(3), string_copying(3)