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这个基础函数，主要见过两种用法：

1.内存初始化

2.考试！！！(鉴于经常考试翻车，立个flag，逢考必对～)

memset声明：

void * memset ( void * ptr, int value, size_t num );

memset入参：

ptr:

　　Pointer to the block of memory to fill.

value:

　　Value to be set. The value is passed as an int, but the function fills the block of memory using the unsigned char conversion of this value.

num:

　　Number of bytes to be set to the value.  size_t is an unsigned integral type.

由于是按照字节赋值，来看一个经典考试题：

 1 #include "stdio.h"
 2 #include "string.h"
 3 
 4 int main()
 5 {
 6     int i = 0;
 7     memset(&i, 256, sizeof(i));
 8 
 9     unsigned char* m = (unsigned char*)(&i);
10     for (int j = 0; j < sizeof(i); j++)
11     {
12         printf("%d\n", m[j]);
13     }
14     return 0;
15 }

output:0000

memset(&i, 128, sizeof(i));
output:
128
128
128
128

256是0x100，取一个无符号字节是0x00

128是0x80(0b1000 0000 或者 128)，取一个无符号字节是0x80