UB24

author: Xiangzhi Liu.

Definition

Conversion between two pointer types produces a result that is incorrectly aligned (6.3.2.3).

Description

将一种指针类型（T）转换为另一种指针类型（P）时，该指针对象的值不满足 P 类型的对齐要求，那么结果是未定义的。

Code

UB24.c

#include <stdio.h>
int main() {
    char a[16] = {1,2,3,4,5,6,7,8};
    int* p = (int*)a;
    int* q = (int*)(a + 1); // Either this line or previous line produces a UB
    printf("0x%04X, 0x%04X\n", *p, *q);
    return 0;
}

Configurations

Configuration 1.Configuration 2.

gcc version 4.9.2

Target: x86_64-w64-mingw32

MSVC _MSC_VER = 1928

Target: x86_64

Behaviors

Configuration 1.Configuration 2.

通过编译无警报 输出：0x4030201, 0x5040302

通过编译无警报 输出：0x4030201, 0x5040302

Advice

保证转换前指针对象的值符合转换后类型的对齐要求。