vfork() atexit assertion failed

Question

I am trying to understand the following piece of code

#include<stdio.h>
#include<unistd.h>
#include<sys/types.h>

int main()
{
pid_t pid ;
unsigned int i=0;
pid=vfork();
switch(pid)
{
    case -1: // some sort of error
        puts("fork error");
        break;
    case 0: // the child process 
        while(i<100)
        {
            printf("%d\n", i);
            i++;
        }
        break;
    default: //parent
        while(i<1000)
        {
            printf("%d\n", i);
            i++;
        }
        break;
}
//  _exit(0);
}

And please don't tell me that vfork() is bad and these kind of things .I know it is , but what is happening exactly in this code that is causing this kind of error . Thanks in advance

Answer 1

It's unclear what you are trying to do or understand, but here is a slightly edited quote from the manual:

The vfork() function has the same effect as fork(2), except that the behavior is undefined if the process created by vfork() either

1. modifies any data other than a variable of type pid_t used to store the return value from vfork()
2. returns from the function in which vfork() was called
3. calls any other function before successfully calling _exit(2) or one of the exec(3) family of functions

You are doing both 1: i++ and 3 printf("%d\n", i). Whatever you expect, it won't work.

As a side note, vfork isn't bad. Just tricky, dangerous, almost useless and removed from SUSv4.