1 需求说明

题目大意如下：

请参考标准C库对memcpy的申明定义，使用C语言的语法写出其实现源码。

2 源码实现

2.1 函数申明

通过查看man帮助，我们可以知道memcpy函数的功能及其简要申明。

NAME
       memcpy - copy memory area

SYNOPSIS
       #include 

       void *memcpy(void *dest, const void *src, size_t n);

DESCRIPTION
       The memcpy() function copies n bytes from memory area src to memory area dest.  The memory areas must not overlap.  Use memmove(3) if the memory
       areas do overlap.

RETURN VALUE
       The memcpy() function returns a pointer to dest.

2.2 功能实现

以下是我的一个简单实现源码，仅供参考：

char *my_memcopy(char* dest, const char *src, size_t len)
{
    assert(dest && src && (len > 0));

if (dest == src) {
;
} else {
        char *p = dest;
size_t i;
        for (i = 0; i < len; i++) {
            *p++ = *src++;
}
    } 

    return dest;
}

3 源码测试

简单的测试代码如下：

#include 
#include 

int main(void)
{
    char buf[30] = "123456789abcdef";

printf("before-memcpy-buf: %s
", buf);
    my_memcopy(buf + 5, buf, 3);
printf("after-memcpy-buf: %s
", buf);

printf("before-memcpy-buf: %s
", buf);
    my_memcopy(buf + 5, buf, 9);
printf("after-memcpy-buf: %s
", buf);

return 0;
}

简单写了build.sh脚本做编译测试：

#! /bin/bash -e

CFLAGS="-Wall -Werror"
cmd="gcc *.c $CFLAGS -o test"

if [ "$1" = "clean" ]; then
    rm -rf test
    echo "Clean build done !"
    exit 0
fi

echo $cmd && $cmd

执行编译后，运行小程序的结果：

c_c++/memmove$ ./test 
before-memcpy-buf: 123451239abcdef
after-memcpy-buf: 123451239abcdef

before-memcpy-buf: 12345123451239f
after-memcpy-buf: 12345123451234f