百试不爽~~~~~~~~

#include <assert.h>
#include <stdio.h>

char * strcpy(char * str_to, const char * str_from);
void * memcpy(void * str_to, const void * str_from, size_t size);

int main(int argc, char* argv[])
{
 char from[] = "this is the string will be copied! ";
 enum {size = sizeof(from)};
 char to[size] = {0}; 
 
 assert(to != 0);
 strcpy(to, from);
 printf("use the strcpy : %s./n", to);

    memcpy(to, from, sizeof(from));
 printf("use the memcpy : %s./n", to);
 
 return 0;
}

char * strcpy(char * str_to, const char * str_from)
{
 assert(str_to != 0);
 assert(str_from != 0);
 
 char * p_to = str_to;
 const char * p_from = str_from;
 
 while( (*(p_to++) = *(p_from++)) != '/0' );
 
 return p_to;
}

void * memcpy(void * str_to, const void * str_from, size_t size)
{
 assert(str_to != 0);
 assert(str_from != 0);

 typedef unsigned char byte;
 byte * p_to = (byte*)str_to;
 const byte * p_from = (const byte *)str_from;

 while( size-- > 0)
  *(p_to++) = *(p_from++);
 
 return p_to;
}