why does vfork() giving segmentation fault

Question

When I run the following code

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
    pid_t pid;
    pid = vfork();
    printf("hello world\n");
}
Output:
hello world
hello world
hello world
Segmentation fault

I know that unless exec() or _exit() is called then vfork() can behave in strange manner if we try to modify any variable but can someone please explain what exactly is happening?? why hello world is getting printed 3 times? Is it because printf() is getting buffered? and finally why a seg fault is occuring just when parent is trying to return?

Answer 1

(From POSIX.1) The vfork() function has the same effect as fork(2), except that the behavior 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(2) or one of the exec(3) family of functions.

Seems like you violate all the conditions for using vfork. So then it doesn't work.