Questions on fork() [duplicate]

Question

I am trying to understand fork(), and so I put together the following example:

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

void main()
{
   if(fork()==0)
   {
      printf("2");

      if(fork()==0)
      {
          printf("4");
      }
      else
      {
          printf("3");
      }
   } 
   else
   {
      printf("1");
   }
}

When I was tracing this on paper, I drew the following sketch:

So I believe the output should be 1234. However, when I run this code, the output is 12324. Why is that? Where is my mistake?

Update:

After reading the comments, it was suggested to do any of the following

1. Add \n to each printf statement
2. OR: Add fflush(stdout); after each printf statement
3. OR: Add setbuf(stdout, NULL); <---- this is what I ended-up doing :)

After updating my code, the output was indeed 1234.

Answer 1

printf() output is usually line-buffered. So when you fork(), the new processes gets the copy of the buffer as well. When the process exits, the whole buffer is flushed (or anytime it's flushed in the code or whenever the buffer becomes full). Hence, you see a copy of printf's output one more time.

1) You can flush it after each printf() call by using fflush(stdout);

2) or using \n for example:

  printf("2\n");

---

Another way is to disable the buffering with:

setbuf(stdout, NULL);