Understanding why fork gives different result in C

Question

Although there are some similar questions like this and this

I still cannot understand why fork gives different output with the following two codes

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

Gives output

Hello World

Where as this code

#include<stdio.h> 
void main()
{
    printf("Hello World");
    fork();
}

Gives output

Hello WorldHello World

The second case is clear to me from the other questions, that both processes get a copy of the same buffer. So, after the fork, both processes eventually flush the buffer and print the contents to screen separately.

But I am not clear why the first case is so.

Tip: What do you suspect would happen if it were flushed already before the call to fork? — AliciaBytes, May 10 '15 at 02:21
the difference is the `\n` -- http://stackoverflow.com/questions/3723795/is-stdout-line-buffered-unbuffered-or-indeterminate-by-default — Diego, May 10 '15 at 02:27

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

Let me explain it in simple words: Consider,these two statements:

printf("Hello World")

and

printf("Hello World\n")

The STDOUT is line buffered that is printf will be executed only when the buffer is full or when it is terminated by a new line character

printf("Hello World") will not be guaranteed to display the output unless the

Buffer is full or terminated by new line character..Thus when a fork() is called,

Fork will create a copy of a process,this means that after fork, there are two identical processes,

each having a copy of the FILE* STDOUT Therefore, each process will have the same buffer contents of STDOUT

Since in your second program there is no newline character,

each process has the string in its buffer.

Then each process prints another string, and all of the contents are printed since it is terminated by a new line character. This is why you see two identical outputs.

But

In your first program,there is a new line character and it displays output

immediately after the statement is executed and hence it flushes the contents of buffer..thus when a fork() is called

The process tries to access the buffer but since the buffer is empty,it does not print anything

Thus,the different response is due to Buffering behavior of stdout

Hope it helps!!

score 2 · Answer 2 · answered May 10 '15 at 02:27

It's due to the buffering behavior.

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

flushes the output before you call fork, so the printing is already finished when it was still only a single process.

As to why printf("Hello World\n"); gets flushed immediately and printf("Hello World"); doesn't isn't that easy. It depends on the situation. In your case you probably ran it in the commandline where line buffering is common, line buffering means that it will be flushed once you got a newline. If you write to files it might buffer more before you get any output and you might see 2 outputs in the first case too.

If you want consistent behavior with flushing you might want to do it yourself.

Understanding why fork gives different result in C

2 Answers2