使用setjump loopjump在线程中跳转的问题

Question

使用setjump loopjump在线程中跳转的问题

使用setjump到另一个线程中跳转,跳转成功了,后面的代码不执行了

#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include<setjmp.h>
jmp_buf buf;


void *print_a(void *);

void *print_b(void *);

//线程A方法
void *print_a(void *a) {


    for (int i = 0; i < 5; ++i) {
        sleep(1);
        printf("aaaa \n");

        int jmpret = setjmp(buf);
        if (!jmpret) {
            printf("!jmpret  \n");
        } else {
            printf("继续执行线程1\n");
        }

    }
    return NULL;
}

//线程B方法
void *print_b(void *b) {

    for (int i = 0; i < 5; ++i) {
        sleep(1);
        printf("bbbb \n");

        if (i == 3) {
            printf("进入线程2 \n");
            longjmp(buf, 1);
        }
    }
    return NULL;
}

int main() {

    pthread_t t0;
    pthread_t t1;

    //创建线程A
    if (pthread_create(&t0, NULL, print_a, NULL) == -1) {
        printf("fail to create pthread t0 \n");
        exit(1);
    }

    if (pthread_create(&t1, NULL, print_b, NULL) == -1) {
        printf("fail to create pthread t1 \n");
        exit(1);
    }

    //等待线程结束
    void *result;
    if (pthread_join(t0, &result) == -1) {
        printf("fail to recollect t0 \n");
        exit(1);
    } else {
        printf("t0 success \n");
    }

    if (pthread_join(t1, &result) == -1) {
        printf("fail to recollect t1 \n");
        exit(1);
    } else {
        printf("t1 success \n");
    }

    getchar();
    return 0;
}

主线程的getchar也不执行了
这是为何?

Answer 1

讲真，我没想到还能这么用....

setjmp/longjmp is implemented by saving the register (including stack and code pointer etc) when first passed, and restoring them jumping.

当线程b通过 longjmp跳到print_a时，它的栈帧和线程a的栈帧所在的内存就是一个地方了 (原来栈的内存没人管)。当线程a退出时，自然要弹出所有栈帧，那么线程b之后对栈访问就是对非法内存的访问了。

就像你把双胞胎中的弟弟的头嫁接到哥哥身上，哥哥变成了连体婴儿，哥哥挂掉之后，弟弟也活不成了，反之也是

Answer 2

longjmp 不能跨线程。

C11 7.13.2.1 / 2:

The longjmp function restores the environment saved by the most recent invocation of
the setjmp macro in the same invocation of the program with the corresponding
jmp_buf argument. If there has been no such invocation, or if the invocation was from
another thread of execution, or if the function containing the invocation of the setjmp
macro has terminated execution in the interim, or if the invocation of the setjmp
macro was within the scope of an identifier with variably modified type and execution has
left that scope in the interim, the behavior is undefined