I try to learn the function fork(). However, the results performed using Code Runner in VScode are different from the results performed in a shell. Just like blew pictures show, I used same commands but get different results. I know that the second output in shell is right and I would like to know why is the first output printed by using Code Runner? Is something wrong about the plugin?

The code is like this.

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

CodePudding user response：

It's a buffering issue.

When stdout is connected to an actual terminal it will be line buffered That means the output is actually written to the terminal when the buffer is full, is explicitly flushed, or when you print a newline ('\n').

This is what happens in the second case.

In the first case, VSCode will not run it directly in a terminal, instead stdout will be connected to a pipe which VSCode will then output in its "terminal". When connected to a pipe (or anything else that isn't a direct terminal) then stdout will be fully buffered, where the output is flushed only when the buffer is full or explicitly flushed.

In this first case the buffer for stdout will be inherited by the child processes, so each child-process you fork will have its own copy of the existing buffer. And when each of the child-processes exits, their buffer will be flushed and actually written. Since there are a total of eight processes running, you will get the output eight times.