linux开发--1.C语言常见问题总结

1. error: variable-sized object may not be initialized

#include <stdio.h>
#include <stdlib.h>
int main ( int argc, char *argv[] )
{
    const int n = 5;
    int a[n]={1,2,3,4,5};
    return EXIT_SUCCESS;
}

c规定数组大小为常量,变量n被修饰为只读变量，可惜再怎么修饰也不是常量
详细见: 
2. x ? : y   是什么鬼

x ? : y   等同于    x ? x : y

3. 指针运算 

#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[])
{
    int* ip1, *ip2, iv;
    char* cp1, *cp2, cv;
    ip1 = (int*)0x518;
    ip2 = (int*)0x500;
    iv = ip1 - ip2;
    printf("iv=%d\n", iv);

    cp1 = (char*)0x518;
    cp2 = (char*)0x500;
    cv = cp1 - cp2;
    printf("cv=%d\n", cv);

    return 0;
}

这个例子出自《经典C面试真题精讲》.(李亚锋) P11 107指针运算
iv=24/4=6 cv=24/1=24
对于整数指针当它自加或自减时，实际上移动的是4个字节
4.结构体

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct __HEAD__
{
    char c;
    int num;
}__attribute__ ((__packed__)) HEAD;

typedef struct __HEAD2__
{
    char c;
    int num;
}HEAD2;

int main (int argc, char *argv[])
{
    printf("pack=%d,nopack=%d\n", sizeof(HEAD),sizeof(HEAD2));
    return 0;
}

pack=5,nopack=8