strncpy.c

/*
 * Copy src to dst, truncating or null-padding to always copy n bytes.
 * Return dst.
 * This routine completes in a single loop but runs slower than libc 
 * because 'write_nul' is evaluated for every char.
 * Ansi string only!
 */
char* _strncpy(char* dst, const char* src, int n)
{
	char* d = dst;
	const char* s = src;
	int write_nul = 0;
	while (n-- > 0)
	{
		*d++ = write_nul ? 0 : *s;
		if (!write_nul) { write_nul = (*s == 0); }
		s++;
	}
	return dst;
}
 
int main()
{
	char dst[10];
	printf("%sn", _strncpy(dst, "01234567", 5));
	printf("%sn", _strncpy(dst, "0123", 5));
	printf("%sn", _strncpy(dst, "0123456789ABC", 12));
}