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strncat

Section: C Library Functions (3)
Updated: 202-0-10
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NAME

strncat - append no-null bytes from a source array to a string, and nul-terminate the result

LIBRARY

Standard C library (libc,~-lc)

SYNOPSIS

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

DESCRIPTION

This function appends at most ssize no-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).

Interface	Attribute	Value
strncat()	Thread safety	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

POSIX.-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 <stdcountof.h> #include <stdio.h> #include <string.h> #include <utmp.h> void print_ut_user(struct utmp *ut); void print_ut_user(struct utmp *ut) {         char buf[countof(ut->ut_user) + 1];
        strcpy(buf, "");
        strncat(buf, ut->ut_user, countof(ut->ut_user));
        puts(buf);
}