为何NULL检查在VS C11触发访问违例，Debian GCC C11却正常？

我写了一段用于设置Block相邻块指针的代码，在Debian环境用GCC C11编译运行完全正常，但放到Visual Studio C11环境下就触发了读取访问违例异常，代码如下：

#include <stdio.h>
#include <stdlib.h>
#define CHUNK_WIDTH 16
#define CHUNK_HEIGHT 64
#define CHUNK_DEPTH 16

struct Block
{
    // data
    struct
    {
        struct Block* above;
        struct Block* below;
        struct Block* left;
        struct Block* right;
        struct Block* front;
        struct Block* behind;
    } neighbors;
};

struct Chunk
{
    struct Block blocks[CHUNK_HEIGHT][CHUNK_DEPTH][CHUNK_WIDTH];
};

void SetNeighboringBlocks(struct Chunk* chunk, int blockIndexX, int blockIndexY, int blockIndexZ)
{
    struct Block* curBlock = &chunk->blocks[blockIndexY][blockIndexZ][blockIndexX];

    curBlock->neighbors.left = (blockIndexX != 0)? &chunk->blocks[blockIndexY][blockIndexZ][blockIndexX - 1] : NULL;
    curBlock->neighbors.right = (blockIndexX != CHUNK_WIDTH - 1)? &chunk->blocks[blockIndexY][blockIndexZ][blockIndexX + 1] : NULL;
    curBlock->neighbors.above = (blockIndexY != CHUNK_HEIGHT - 1)? &chunk->blocks[blockIndexY + 1][blockIndexZ][blockIndexX] : NULL;
    curBlock->neighbors.below = (blockIndexY != 0)? &chunk->blocks[blockIndexY - 1][blockIndexZ][blockIndexX] : NULL;
    curBlock->neighbors.front = (blockIndexZ != 0)? &chunk->blocks[blockIndexY][blockIndexZ - 1][blockIndexX] : NULL;
    curBlock->neighbors.behind = (blockIndexZ != CHUNK_DEPTH - 1)? &chunk->blocks[blockIndexY][blockIndexZ + 1][blockIndexX] : NULL;

}

struct Chunk* CreateChunk(int x, int z)
{
    struct Chunk* ret = (struct Chunk*)malloc(sizeof(struct Chunk));

    for (int z = 0; z < CHUNK_DEPTH; ++z)
        for (int y = 0; y < CHUNK_HEIGHT; ++y)
            for (int x = 0; x < CHUNK_WIDTH; ++x)
            {
                SetNeighboringBlocks(ret, x, y, z);
            }

    return ret;
}

int main(void)
{
    struct Chunk* chunk = CreateChunk(0, 0);
    for (int z = 0; z < CHUNK_DEPTH; ++z)
        for (int y = 0; y < CHUNK_HEIGHT; ++y)
            for (int x = 0; x < CHUNK_WIDTH; ++x)
            {
                // If it has neighbors in all direction, then it is inside
                struct Block* cur = &chunk->blocks[y][z][x];
                if (
                    cur->neighbors.above &&
                    cur->neighbors.below &&
                    cur->neighbors.left &&
                    cur->neighbors.right &&
                    cur->neighbors.front &&
                    cur->neighbors.behind
                    )
                    // Do something
            }

    return 0;
}

问题原因 #

核心问题是CreateChunk函数中的变量命名冲突：函数参数为int x, int z，但循环内部又定义了同名的局部变量int z和int x。

• GCC对这种嵌套同名变量的处理是内部变量覆盖外部参数，循环能正常执行，但这属于C标准中的未定义行为，只是GCC的实现恰好兼容这种写法。
• Visual Studio的C编译器对栈变量的布局更严格，变量覆盖后导致后续内存访问出现越界，最终触发读取访问违例。

修复方案 #

修改循环内的变量名，避免与函数参数冲突，比如将循环中的z改为dz，x改为dx：

struct Chunk* CreateChunk(int x, int z)
{
    struct Chunk* ret = (struct Chunk*)malloc(sizeof(struct Chunk));

    for (int dz = 0; dz < CHUNK_DEPTH; ++dz)
        for (int y = 0; y < CHUNK_HEIGHT; ++y)
            for (int dx = 0; dx < CHUNK_WIDTH; ++dx)
            {
                SetNeighboringBlocks(ret, dx, y, dz);
            }

    return ret;
}

修改后，代码在Visual Studio C11环境下可以正常运行，同时也符合C标准规范，消除了未定义行为的风险。

内容的提问来源于stack exchange，提问作者Hexa