C的struct大小以及offsetof

    下面的程序主要是测试struct的大小，另外注意到在gcc4.3.3编译时，得到堆中的内存块，即firstNode.data和psbNode指向的内存，每个字节默认自动初始化为0值；而在VS2008编译下，每个字节被初始化为0xcd。
    在struct DemoStruct{char cArray[];}；DemoStruct的大小不包含从cArray的大小，这种结构中的不定长数组不被计入结构体的大小。这就是C语言的不定内存问题。
[希望了解的朋友去查看一下gcc文档，是不是自动初始化堆内存的每个字节为0，如果此处有错，希望给出正确的解答！]
// 测试两种struct结构在指定对齐和默认对齐下的内存占用大小
// 此源自blueshycool的blog： http://blog.chinaunix.net/u/18584/showart_296056.html
// Created by LHJ at 2009-11-21

#include
#include
#include

#define SIZE 10

#pragma pack(1)
typedef struct FirstBlock_t
{
    int size;
    char *data;
}FirstBlock;

typedef struct SecondBlock_t
{
    int size;
    char data[1];
}SecondBlock;

void StructTest(void)
{
    FirstBlock firstNode;
    firstNode.size = SIZE;
     firstNode.data = (char*)malloc(firstNode.size * sizeof(char));
    
    SecondBlock *psbNode = (SecondBlock*)malloc(sizeof(SecondBlock) + SIZE);
    psbNode->size = SIZE;
   
    printf("\n\n");
    printf("------------- firstNode Information ----------------------\n");
    printf("offset: \n");
    printf("\toffsetof(FirstBlock, size): %d\n", offsetof(FirstBlock, size));
    printf("\toffsetof(FirstBlock, data): %d\n", offsetof(FirstBlock, data));
    printf("\tsizeof(FirstBlock): %d\n", sizeof(FirstBlock));
    printf("content: \n");
    printf("\tfirstNode.data[%d] Ascii: [%d]\n", firstNode.size-1, firstNode.data[firstNode.size-1]);
    printf("address info: \n");
    printf("\tfirstNode.data: 0x%08x\n", (unsigned int)firstNode.data);
    printf("\t&firstNode.data[0]: 0x%08x\n", (unsigned int)&firstNode.data[0]);
    printf("\t&firstNode.data[%d]: 0x%08x\n", (unsigned int)firstNode.size-1,
                        (unsigned int)&firstNode.data[firstNode.size-1]);
   
    printf("\n-------------- *psbNode Information ----------------------\n");
    printf("offset: \n");
    printf("\toffsetof(*psbNode, size): %d\n", offsetof(SecondBlock, size));
    printf("\toffsetof(*psbNode, data): %d\n", offsetof(SecondBlock, data));
    printf("\tsizeof(SecondBlock): %d\n", sizeof(SecondBlock));
    printf("content: \n");
    printf("\tpsbNode->data[%d] Ascii: [%d]\n", psbNode->size-1, psbNode->data[psbNode->size-1]);
    printf("address info: \n");
    printf("\tpsbNode: 0x%08x\n", (unsigned int)psbNode);
    printf("\t&psbNode->data[0]: 0x%08x\n", (unsigned int)&psbNode->data[0]);
    printf("\tpsbNode->data[%d]: 0x%08x\n", (unsigned int)psbNode->size + 100000,
                        (unsigned int)psbNode->data[psbNode->size + 100000]);

    // free malloc's mem
    free(firstNode.data);
    free(psbNode);
    printf("\n-------------- After free ----------------------\n");
    printf("address info: \n");
    printf("\tfirstNode.data: 0x%08x\n", (unsigned int)firstNode.data);
    // invalid use wild pointers
    //printf("\t&firstNode.data[0]: 0x%08x\n", (unsigned int)&firstNode.data[0]);

    printf("\tpsbNode: 0x%08x\n", (unsigned int)psbNode);
    // invalid use wild pointers
    //printf("\t&psbNode->data[0]: 0x%08x\n", (unsigned int)&psbNode->data[0]);
   
    firstNode.data = NULL;
    psbNode = NULL;
    printf("after reset pointers: \n");
    printf("\tfirstNode.data: 0x%08x\n", (unsigned int)firstNode.data);
    printf("\tpsbNode: 0x%08x\n", (unsigned int)psbNode);
    printf("\n\n");
}

int main(int argc, char *argv[])
{
    StructTest();
}

#pragma pack()

此段程序在linux，gcc4.3.3环境中的结果：
------------- firstNode Information ----------------------
offset:
    offsetof(FirstBlock, size): 0
    offsetof(FirstBlock, data): 4
    sizeof(FirstBlock): 8
content:
    firstNode.data[9] Ascii: [0]
address info:
    firstNode.data: 0x095a0008
    &firstNode.data[0]: 0x095a0008
    &firstNode.data[9]: 0x095a0011

-------------- *psbNode Information ----------------------
offset:
    offsetof(*psbNode, size): 0
    offsetof(*psbNode, data): 4
    sizeof(SecondBlock): 5
content:
    psbNode->data[9] Ascii: [0]
address info:
    psbNode: 0x095a0018
    &psbNode->data[0]: 0x095a001c
    psbNode->data[100010]: 0x00000000

-------------- After free ----------------------
address info:
    firstNode.data: 0x095a0008
    psbNode: 0x095a0018
after reset pointers:
    firstNode.data: 0x00000000
    psbNode: 0x00000000