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;

}