Concepts on fork() system call

Question

If the call to fork() is executed successfully, Unix will make two identical copies of address spaces, one for the parent and the other for the child. Both processes will start their execution at the next statement following the fork() call.[Ref:http://www.csl.mtu.edu/cs4411.ck/www/NOTES/process/fork/create.html].

So,if I execute the following code:

#include <stdio.h> int main(void) { printf("Hello\n"); fork(); printf("World\n"); return 0; }

I think it will print

Hello World World

But when I run the program it prints

Hello World Hello World

Please explain where am I lacking in concept?

I think if you use printf() you can't have 'Hello World World' o/p.correct me if I am wrong. — Hailey
– Hailey, Commented Aug 9, 2016 at 6:19
You will have "Hello World World" as output,I simply copied and compiled the code,and got the same and this is what I expected. — Rusty
– Rusty, Commented Aug 9, 2016 at 6:23
check these too. stackoverflow.com/questions/34978568/… stackoverflow.com/questions/8255388/fork-behavior-in-linux — Rusty
– Rusty, Commented Aug 9, 2016 at 6:28
Also posted on CS.SE. Please do not post the same question on multiple sites. Each community should have an honest shot at answering without anybody's time being wasted. — D.W.
– D.W., Commented Aug 9, 2016 at 6:52

Gilles 'SO- stop being evil' · Accepted Answer · 2016-08-09 09:16:14Z

2

This is about buffering. When you print "hello" it doesn't go to output immediately. Instead, it goes to a buffer. It's still there during the fork, so, when each task terminates and flushes its buffer, there are two copies to send to output.

To counter this, you could specify unbuffered I/O or call fflush before the fork to flush the buffer.

edited Aug 9, 2016 at 9:16

Gilles 'SO- stop being evil'

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answered Aug 9, 2016 at 6:05

Andrew

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3 Comments

Andrew Over a year ago

@Gilles removed my mention of using terminal input to flush terminal output. I still think it's worth mentioning. fflush works as well, of course. It's just fancier.

Gilles 'SO- stop being evil' Over a year ago

I don't see what makes it “fancier”. fflush has the advantage of being simpler and of always working. Using terminal input only works if you're reading terminal input, which is often not the case.

Andrew Over a year ago

@Gilles, my idea is that OP is a beginner, so not acquainted with a wide spectrum of system calls. I want to reply in simple terms so that he gets the idea without having to delve into new documentation. Terminal I/O is where most people start out.

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