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UB39⚓︎

Author: Kai Xu

Definition⚓︎

The operand of the unary * operator has an invalid value (6.5.3.2).

一元*运算符的操作数是无效值

Description⚓︎

The standard provides three examples:

  1. a null pointer
  2. an address inappropriately aligned for the type of object pointed to
  3. the address of an object after the end of its lifetime

标准给出了三个示例：

  1. 空指针
  2. 指针没有和指向对象类型对齐
  3. 指向对象结束生命周期

Code⚓︎

UB39.c
#include "stdio.h"
#include "stdlib.h"

void test_example_1() {
    int *p1;

    printf("p1 = %p\n", p1);
    printf("*p1 = %d\n", *p1);
}

void test_example_2() {
    int a;
    int *p2 = (int*)((char*)&a + 1);

    printf("&a = 0x%p\n", &a);
    printf("p2 = %p\n", p2);
    printf("*p2 = %d\n", *p2);
}

void test_example_3() {
    int *p3 = (int*) malloc(4);

    printf("p3 = %p\n", p3);

    free(p3);
    printf("*p3 = %d\n", *p3);
}

int main() {
    test_example_1();
//    test_example_2();
//    test_example_3();
}

Configurations⚓︎

OS: Microsoft Windows 10 22H2

gcc -v: gcc version 8.1.0 x86_64-w64-mingw32

compile and run commands: gcc -o UB39.exe UB39.c && UB39.exe

OS: arm64-apple-darwin20.6.0

clang -v: Apple clang version 13.0.0 (clang-1300.0.29.30)

compile and run commands: clang -o UB39.out UB39.c && ./UB39.out

Behaviors⚓︎

test_example_1: p1 = 0x0000000100000000 进程已结束,退出代码-1073741819 (0xC0000005)

test_example_2: &a = 0x00000035b21ff9f4 p2 = 0x00000035b21ff9f5 *p2 = -184549249

test_example_3: p3 = 0x0000020f1c5e5df0 *p3 = 475922768

test_example_1: p1 = 0x0 zsh: segmentation fault ./UB39.out

test_example_2: &a = 0x0x16dc3b40c p2 = 0x16dc3b40d *p2 = 536870912

test_example_3: p3 = 0x1346067f0 *p3 = 0

Advice⚓︎

UB39是绝对不应该出现在程序的