vfork never return

Why this program will never return and continuing create child processes?

int main()
{
    pid_t pid;
    int foo1 = 1, foo2 = 2;
    printf("before fork()\n");

    if ((pid = vfork()) < 0 ) {
            printf("fork failed.\n");
    }else if (pid == 0) {
            foo1++;
            foo2++;
            printf("child process: pid is %d, my parent pid is  %d\n", getpid(), getppid());
    }else if (pid > 0){
            printf("parent process: pid is %d\n", getpid());
    }

    printf("%s: foo1 is %d, foo2 is %d\n",pid == 0 ? "child process" : "parent process", foo1, foo2);
    return 0;
}

the output is like:

before fork()
child process: pid is 17244, my parent pid is  15839
child process: foo1 is 2, foo2 is 3
parent process: pid is 15839
parent process: foo1 is -1079005816, foo2 is -1218256081
before fork()
child process: pid is 17245, my parent pid is  15839
child process: foo1 is 2, foo2 is 3
parent process: pid is 15839
parent process: foo1 is -1079005816, foo2 is -1218256081
before fork()
.....
.....

If a开发者_如何学Cdd an _exit in the second if block then it'ok. I know the vfork share the same address space with the parent process, but it is more reasonable if the progrem ends with a crash not the endless loop.

vfork is a very tricky system call and its only intended usage is to immediately have an execve in the child - for other kinds of uses it is dangerous and unpredictable.

Also note that unlike with fork, the parent is suspended until the child exits or calls execve.

from the manual:

the behaviour is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions.

Because parent and child share the address space, you must not return from the function that called vfork(); doing so can corrupt the parent's stack.

Inside a vfork child you can only call functions safe to call in signal handlers, so printf from inside vfork is a very bad idea.