vfork 导致内存 "free()" 崩溃,但是 fork 却没有,这是怎么回事?

vfork leads to a memory "free()" crash, but fork doesn't, how it happened?

我试图查看 vfork 是否创建了一个与父进程共享内存的子进程,如下所示:

#include<stdio.h>
#include<unistd.h>
int main()
{
    int* pi = new int(5);
    int i = 5;
    pid_t id = vfork();
    if (id > 0) //father
    {
        *pi = 4;
        i = 4;
        printf("father set i=%d, *pi=%d\n", i, *pi);
        sleep(2);
        printf("father get i=%d, *pi=%d\n", i, *pi);
        delete pi;
    }
    else //child
    {
        sleep(1);
        printf("child get i=%d, *pi=%d\n", i, *pi);
        i = 3;
        *pi = 3;
        printf("child set i=%d, *pi=%d\n", i, *pi);
    }
    return 0;
}

我原以为 i 和 *pi 的值在父进程和 chid 进程之间共享,但是当我执行它时: $ g++ myvshare.cpp && ./a.out

child get i=5, *pi=5
child set i=3, *pi=3
father set i=4, *pi=4
father get i=4, *pi=4
*** Error in `./a.out': free(): invalid pointer: 0xb75f9000 ***
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(+0x67257)[0xb74ae257]
/lib/i386-linux-gnu/libc.so.6(+0x6d577)[0xb74b4577]
/lib/i386-linux-gnu/libc.so.6(+0x6dd31)[0xb74b4d31]
/usr/lib/i386-linux-gnu/libstdc++.so.6(_ZdlPv+0x18)[0xb766bd98]
./a.out[0x8048621]
/lib/i386-linux-gnu/libc.so.6(+0x15f2d4)[0xb75a62d4]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:01 1055312    /home/x/cpp/a.out
08049000-0804a000 r--p 00000000 08:01 1055312    /home/x/cpp/a.out
0804a000-0804b000 rw-p 00001000 08:01 1055312    /home/x/cpp/a.out
0859e000-085c3000 rw-p 00000000 00:00 0          [heap]
b7200000-b7221000 rw-p 00000000 00:00 0
b7221000-b7300000 ---p 00000000 00:00 0
b73d3000-b73d5000 rw-p 00000000 00:00 0
b73d5000-b73f1000 r-xp 00000000 08:01 1181015    /lib/i386-linux-gnu/libgcc_s.so.1
b73f1000-b73f2000 rw-p 0001b000 08:01 1181015    /lib/i386-linux-gnu/libgcc_s.so.1
b73f2000-b7445000 r-xp 00000000 08:01 1181047    /lib/i386-linux-gnu/libm-2.23.so
b7445000-b7446000 r--p 00052000 08:01 1181047    /lib/i386-linux-gnu/libm-2.23.so
b7446000-b7447000 rw-p 00053000 08:01 1181047    /lib/i386-linux-gnu/libm-2.23.so
b7447000-b75f6000 r-xp 00000000 08:01 1180977    /lib/i386-linux-gnu/libc-2.23.so
b75f6000-b75f7000 ---p 001af000 08:01 1180977    /lib/i386-linux-gnu/libc-2.23.so
b75f7000-b75f9000 r--p 001af000 08:01 1180977    /lib/i386-linux-gnu/libc-2.23.so
b75f9000-b75fa000 rw-p 001b1000 08:01 1180977    /lib/i386-linux-gnu/libc-2.23.so
b75fa000-b75fd000 rw-p 00000000 00:00 0
b75fd000-b776a000 r-xp 00000000 08:01 400094     /usr/lib/i386-linux-gnu/libstdc++.so.6.0.21
b776a000-b776b000 ---p 0016d000 08:01 400094     /usr/lib/i386-linux-gnu/libstdc++.so.6.0.21
b776b000-b7770000 r--p 0016d000 08:01 400094     /usr/lib/i386-linux-gnu/libstdc++.so.6.0.21
b7770000-b7771000 rw-p 00172000 08:01 400094     /usr/lib/i386-linux-gnu/libstdc++.so.6.0.21
b7771000-b7774000 rw-p 00000000 00:00 0
b7788000-b778b000 rw-p 00000000 00:00 0
b778b000-b778d000 r--p 00000000 00:00 0          [vvar]
b778d000-b778e000 r-xp 00000000 00:00 0          [vdso]
b778e000-b77b0000 r-xp 00000000 08:01 1180949    /lib/i386-linux-gnu/ld-2.23.so
b77b0000-b77b1000 rw-p 00000000 00:00 0
b77b1000-b77b2000 r--p 00022000 08:01 1180949    /lib/i386-linux-gnu/ld-2.23.so
b77b2000-b77b3000 rw-p 00023000 08:01 1180949    /lib/i386-linux-gnu/ld-2.23.so
bf9ca000-bf9eb000 rw-p 00000000 00:00 0          [stack]
Terminated (core dupm)

我困惑的是:

  1. I expect that vfork should differ from fork in that i and *pi are shared. But still vfork seem to have COW for i and *pi, when father sets the value, child still gets old value, and vice versa.

  2. Why there's a core dump saying "free()" failed? I tried to change vfork to fork, no such problem. How did this happen, inside libc, or glibc?

谢谢!

你错了,vfork(),child 不与 parent 共享内存。记忆只属于parent。 vfork()fork() 不同,因为它不复制内存,但行为未定义是您尝试使用 parent 的内存,但 pid_t [=46] =] 通过 vfork()。另外,child 不能调用 exit()exec() 家族的另一个函数。例如,在 child 中调用 printf() 是未定义的行为。 child需要exit()exec()之前完成。

最后,parent 线程被阻塞,直到 child 调用 exit()exec().

fork()一样,vfork()创建的进程继承了文件描述符、信号配置和当前工作目录。

所有信息都在 manual of vfork.

TLDR:这是未定义的行为。