#include<stdio.h>
int main(void){
    int a = -12;
    int mSize = sizeof(a);
    printf("a=%d, sizeof(a)=%d.\n", a, mSize);
    printf("size of int is %ld.\n", sizeof(int));
    unsigned char *p = (unsigned char*) &a; //一定要用unsigned char * !!!
    printf("a in memory: ");
    for(int i=0; i<mSize; i++){
        printf("%02x  ", *(p+i)); //按十六进制输出，占两个字符宽度，补0
    }
    printf("\n");
    return 0;
}

在我电脑上运行的结果：

a=-12, sizeof(a)=4.
size of int is 4.
a in memory: f4  ff  ff  ff

解释：int类型大小为4字节，系统采用小端法，故实际顺序为ff ff ff f4，取反后为00 00 00 0b，加1后为00 00 00 0c，即-12。