fork is confusing me

Question

Can anyone, please, explain how does this code work ?

int main()
{
  printf("Hello");
  fork();
  printf("World");
}

Prints:

HelloWorldHelloWorld

My exact question is, why hello is printed twice. Isn't hello printed first, then fork() is executed ?

Also, sometimes it prints:

 HelloWorld 
// then the reports....process exited with return value 0..etc etc.. then...//
HelloWorld

Why this output ?

score 18 · Accepted Answer · answered Aug 05 '14 at 15:04

18

The reason is: buffered output. "Hello" is in the buffer but not yet put out when you do the fork, so the forked process starts with the same buffer including the same word "Hello". Then, both the parent and the child output "World" so the total output is "HelloWorld" for both of them.

answered Aug 05 '14 at 15:04

Erich Kitzmueller

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… and the solution is to `fflush` before `fork`. – Potatoswatter Aug 05 '14 at 15:09
i'd say thank you :) but it is against the community guidelines i guess :) :D – guitar_geek Aug 05 '14 at 17:23
No, it's a valid question, but it has been asked before. Happens all the time. – Erich Kitzmueller Aug 06 '14 at 06:35

score 4 · Answer 2 · answered Aug 05 '14 at 15:09

4

Adding to @ammoQ answer:

int main()
{
    printf("Hello");
    fflush(stdout);
    fork();
    printf("World");
}

will get you to the expected result.

answered Aug 05 '14 at 15:09

Jeffery Thomas

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score 3 · Answer 3 · edited May 23 '17 at 12:11

3

Fork creates a copy of the process. And printf() may be buffered when the fork happens, this buffer would be copied.

Pretty solid explanation here: fork() branches more than expected?

edited May 23 '17 at 12:11

Community

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answered Aug 05 '14 at 15:05

davepmiller

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Hello laser_wizard. It seems like you spotted a duplicate. In this case you should flag the question as a duplicate instead of answering it. – Philipp Aug 05 '14 at 15:09

fork is confusing me

3 Answers3