stpncpy.c

#include <sysclib.h>
#include <stdint.h>
#include <limits.h>
 
#define ALIGN (sizeof(size_t)-1)
#define ONES ((size_t)-1/UCHAR_MAX)
#define HIGHS (ONES * (UCHAR_MAX/2+1))
#define HASZERO(x) (((x)-ONES) & ~(x) & HIGHS)
 
char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
{
#ifdef __GNUC__
    typedef size_t __attribute__((__may_alias__)) word;
    word *wd;
    const word *ws;
    if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
        for (; ((uintptr_t)s & ALIGN) && n && (*d=*s); n--, s++, d++);
        if (!n || !*s) goto tail;
        wd=(void *)d; ws=(const void *)s;
        for (; n>=sizeof(size_t) && !HASZERO(*ws);
               n-=sizeof(size_t), ws++, wd++) *wd = *ws;
        d=(void *)wd; s=(const void *)ws;
    }
#endif
    for (; n && (*d=*s); n--, s++, d++);
    // cppcheck-suppress unusedLabelConfiguration
tail:
    memset(d, 0, n);
    return d;
}