C语言使用技巧（六）：memcpy函数为数组、函数变量和结构体内部的数组赋值

1、单一变量的赋值

源码：

#include <stdio.h>

#include <string.h>

struct s1
{
    char *name;
    int age;
}stu1 = {"kangchou", 25};

void main(void)
{
    struct s1 s2;
    memcpy(&s2, &stu1, sizeof(stu1));
    printf("s2.name = %s\ns2.age = %d\n", s2.name, s2.age);
    getch();
}

执行结果：

2、字符串数组的赋值的覆盖

将一个数组的前几个元素覆盖另一个数组的前几个元素。

下面解决了常见报错的问题;error: ‘for’ loop initial declarations are only allowed in C99 mode

源码：

#include <stdio.h>

#include<string.h>

int main()
{
    char a[4] = "mmmm";
    char b[7] = "123455";
    memcpy(b, a, 3);
    printf("%d\n\r", sizeof(b));
    printf("%s\n", b);

    int i;
    for(i = 0; i < sizeof(b); i++)
    {
        printf("b[%d]的字符串是%c\n\r", i, b[i]);
    }
    getch();
    return 0;
}

执行结果：

3、给整形数组赋值也可以全部覆盖：

#include <stdio.h>
#include <string.h>
#include <stdint.h>
int main()
{
    int i;
    int a[4] = {1, 2, 3, 100};
    int b[3] = {4, 5, 6};
    memcpy(b, a, 4);               // 通常int = 4 * char, 所以复制的结果为b = {1, 5, 6}, 只复制了前4个char地址的内容
    memcpy(b, a, sizeof(int) * 3); // 这里的sizeof(int) * 3 == 12， 所以复制结果为b = {1, 2, 3};
    for (i = 0; i < 3; i++)
    {
        printf("%d ", b[i]);
    }
    getch();
    return 0;
}

执行结果：1 2 3

4、memcpy()二维数组拷贝

源码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void printarr2d(int (*a)[3], int row, int col);

/************************************************/
void printarr2d(int (*a)[3], int row, int col)
{
    int i, j;
    for (i = 0; i < row; i++)
    {
        for (j = 0; j < col; j++)
        {
            printf("%d ", a[i][j]);
        }
        printf("\n");
    }
}

int main()
{
    int i, j;
    int a[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int b[4][3] = {{0, 0, 0}, {0, 0, 0}};
    memcpy(b[2], a, sizeof(int) * 2 * 3);
    printarr2d(b, 4, 3);
    getch();
    return 0;
}

执行结果：

5、给结构体中的数组赋值

源码：

#include <stdio.h>

#include <string.h>

struct s1
{
    char *name;
    int age[2]; //memcpy给该数组赋值
}stu1 = {"kangchou", {25,24}};//初始化结构体变量

void main(void)
{
    struct s1 s2;
    memcpy(&s2, &stu1, sizeof(stu1));
    printf("s2.name = %s\ns2.age = %d\n", s2.name, s2.age[1]);
    getch();
    return 0;
}

执行结果：

6、附录：

ubuntu ->man men 附录memcpy函数用法：

C语言复制函数分为3种，strcpy，strncpy，memcpy，适用场景如下：
strcpy：字符串复制
strncpy：相同结构的指针数组复制
memcpy：对象复制，指针结构可以不同，指向的数组结构必须相

MEMCPY(3)                                                              Linux Programmer's Manual                                                             MEMCPY(3)

NAME
       memcpy - copy memory area

SYNOPSIS
       #include <string.h>

       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 over‐
       lap.

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

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │memcpy()  │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

NOTES
       Failure to observe the requirement that the memory areas do not overlap has been the source of real bugs.  (POSIX and the C standards are explicit that employ‐
       ing  memcpy()  with  overlapping  areas  produces  undefined  behavior.)   Most notably, in glibc 2.13 a performance optimization of memcpy() on some platforms
       (including x86-64) included changing the order in which bytes were copied from src to dest.

       This change revealed breakages in a number of applications that performed copying with overlapping areas.  Under the  previous  implementation,  the  order  in
       which  the  bytes  were  copied had fortuitously hidden the bug, which was revealed when the copying order was reversed.  In glibc 2.14, a versioned symbol was
       added so that old binaries (i.e., those linked against glibc versions earlier than 2.14) employed a memcpy() implementation that safely handles the overlapping
       buffers case (by providing an "older" memcpy() implementation that was aliased to memmove(3)).

SEE ALSO
       bcopy(3), memccpy(3), memmove(3), mempcpy(3), strcpy(3), strncpy(3), wmemcpy(3)

COLOPHON
       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the latest version of
       this page, can be found at http://www.kernel.org/doc/man-pages/.