<系统函数实现>memcmp

#include <stdio.h>
#include <string.h>
/*
*int memcmp (const void *s1,const void *s2,size_t n);
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*p is src, q is des, len is length of p
*error     =-1
*p>q     >0
*p<q     <0
*p==q    =0
*/
int _memcmp(const void *p,const void *q,int len)
{
if(!p || !q)
{
return -1;
}

// while((*(char *)p != '\0') && (*(char *)q != '\0') && (len > 0))
　　　　while(len > 0)//this is strcmp , memcmp should't test \0
{
//printf("p %p:%d q %p:%d len %d\n",p,*(char *)p,q,*(char *)q,len);
if((*(char *)p - *(char *)q) != 0)
{

return (*(char *)p-*(char *)q);
}
p++;
q++;
len--;
}

if((len > 0 ) && (*(char *)p != '\0'))
{
return 1;
}

if((len > 0 ) && (*(char *)q != '\0'))
{
return -1;
}

return 0;
}

int main(int argc, char **argv)
{
char *p = "abcfgf";

char *q = "abcfg";

int val;

val = memcmp(p,q,6);
printf("sys val is %d\n",val);

val = _memcmp(p,q,6);

printf("my val is %d  \n",val);

return 0;
}