strncat(3) Library Functions Manual strncat(3)

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

Standard C library (libc, -lc)

#include <string.h>
char *strncat(char *restrict dst, const char src[restrict .sz],
               size_t sz);

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; }

strncat() returns dst.

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

Interface Attribute Value
strncat () Thread safety MT-Safe

C11, POSIX.1-2008.

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

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).

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

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

size_t maxsize;
// Null-padded fixed-width character sequences
char pre[4] = "pre.";
char new_post[50] = "";
// Strings
char post[] = ".post";
char src[] = "";
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); // ""

string(3), string_copying(3)

2023-07-20 Linux man-pages 6.05.01