stpncpy, strncpy - fill a fixed-size buffer with non-null bytes from a string, padding with null bytes as needed

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE

These functions copy non-null bytes from the string pointed to by src into the array pointed to by dst. If the source has too few non-null bytes to fill the destination, the functions pad the destination with trailing null bytes. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting character sequence is truncated. For the difference between the two functions, see RETURN VALUE.

An implementation of these functions might be:

char *
strncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
    stpncpy(dst, src, dsize);
    return dst;
}
char *
stpncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
    size_t  dlen;
    dlen = strnlen(src, dsize);
    return memset(mempcpy(dst, src, dlen), 0, dsize - dlen);
}

strncpy()

returns dst.

stpncpy()

returns a pointer to one after the last character in the destination character sequence.

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
    char    *p;
    char    buf1[20];
    char    buf2[20];
    size_t  len;
    if (sizeof(buf2) < strlen("Hello world!"))
        errx("strncpy: truncating character sequence");
    strncpy(buf2, "Hello world!", sizeof(buf2));
    len = strnlen(buf2, sizeof(buf2));
    printf("[len = %zu]: ", len);
    fwrite(buf2, 1, len, stdout);
    putchar('\n');
    if (sizeof(buf1) < strlen("Hello world!"))
        errx("stpncpy: truncating character sequence");
    p = stpncpy(buf1, "Hello world!", sizeof(buf1));
    len = p - buf1;
    printf("[len = %zu]: ", len);
    fwrite(buf1, 1, len, stdout);
    putchar('\n');
    exit(EXIT_SUCCESS);
}

wcpncpy(3), string_copying(7)