C字符串处理函数的实现（Linux）

C字符串处理函数的实现（Linux）
#include <stddef.h>
char * ___strtok = NULL;

char * strcpy(char * dest,const char *src)
{
char *tmp = dest;

while ((*dest++ = *src++) != '\0')
/* nothing */;
return tmp;
}

char * strncpy(char * dest,const char *src,size_t count)
{
char *tmp = dest;

while (count-- && (*dest++ = *src++) != '\0')
/* nothing */;

return tmp;
}

char * strcat(char * dest, const char * src)
{
char *tmp = dest;

while (*dest)
dest++;
while ((*dest++ = *src++) != '\0')
;
return tmp;
}

char * strncat(char *dest, const char *src, size_t count)
{
char *tmp = dest;
if (count) {
while (*dest)
dest++;
while ((*dest++ = *src++)) {
if (--count == 0)
break;
}
}

return tmp;
}
int strcmp(const char * cs,const char * ct)
{
register signed char __res;

while (1) {
if ((__res = *cs - *ct++) != 0 || !*cs++) /*is !*cs++ necessary? incase more cmp*/
break;
}

return __res;
}

int strncmp(const char * cs,const char * ct,size_t count)
{
register signed char __res = 0;

while (count) {
if ((__res = *cs - *ct++) != 0 || !*cs++)
break;
count--;
}

return __res;
}

char * strchr(const char * s,char c)
{
for(; *s != c; ++s)
if (*s == '\0')
return NULL;
return (char *) s;
}

size_t strlen(const char * s)
{
const char *sc;

for (sc = s; *sc != '\0'; ++sc)
/* nothing */;
return sc - s;
}

size_t strnlen(const char * s, size_t count)
{
const char *sc;

for (sc = s; *sc != '\0' && count--;