Using fork() in simplest form need to hit enter to finish execution

Question

I want to run some simple example to find out how fork() system call work.

Its work but need to hit enter to exit the program after printing child process.

#include <stdio.h>
#include <unistd.h> 

int main()
{
    pid_t pid;
    pid = fork();

    if(pid == 0){
        printf("child process\n");
    }
    else{
        printf("parent process\n");
    }
    return 0;
}

compile, run & output

ss@ss:~$ gcc dd.c -o dd
ss@ss:~$ ./dd
parent process
ss@ss:~$ child process

after print child process on screen program need to hit enter key to return to the terminal.

Terminal wait for enter new command, if just hit enter return, write ss@ss: and OK.

Or we can enter any other terminal command and it executes.

I want to know why this behavior occur and if really program finished after printing child process why terminal not show ss@ss:~$

Here is my machine information

gcc --version

gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

uname -a

Linux ss 4.8.0-36-generic #36~16.04.1-Ubuntu SMP Sun Feb 5 09:39:41 UTC 2017 i686 i686 i686 GNU/Linux

EDIT

Besides Felix Guo answer by adding wait(NULL) after printf("Parent process"), forced parent to wait for child process.

I find another way for it from by redirecting output of program to an arbitrary file with > operator in terminal.

In execute command:

./dd > out.txt

it redirects outputs to out.txt file. if we cat this file:

$cat out.txt
parent process
child process

this is another way to see correct output of a program with fork().

[Edit 2]

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

int main()
{
    pid_t pid;
    pid = fork();

    if(pid == 0){
        printf("child process: \n\t");
        printf("PID: %d\n\tPPID: %d\n", getpid(), getppid());
    }
    else{
        printf("parent process: \n\t");
        printf("PID: %d\n\tPPID: %d\n", getpid(), getppid());
        wait(NULL);
    }

    return 0;
}

Output

parent process: 
    PID: 4344
    PPID: 4336    < --- OS
child process: 
    PID: 4345
    PPID: 4344    < --- parent process

https://stackoverflow.com/questions/54849218/output-from-fork-in-c?noredirect=1&lq=1 — EsmaeelE, Mar 09 '19 at 18:06
https://stackoverflow.com/questions/23714398/do-i-need-to-wait-in-the-parent-process-after-a-fork?rq=1 — EsmaeelE, Mar 09 '19 at 18:06

score 7 · Accepted Answer · answered Aug 18 '17 at 17:40

Your parent process exits before waiting for your child process to complete, so the parent process will complete, which is what the terminal is waiting for, so the terminal will return to it's command input state, thus outputting ss@ss.... Then your child processes prints and flushes stdout to the screen, which then outputs child process to the terminal, but since the terminal already outputted the prompt ss@ss, it ends up after the prompt.

The way to solve this is to wait for the child process to exit before return 0 in the parent process. You can use wait(NULL) after printf("Parent process") to wait for child process as described here: Make parent wait for all child processes

OK its true, thanks in advance to you and dbush. using `wait(NULL)` need ` #include `, but i am not accept it to see others opinion. — EsmaeelE, Aug 18 '17 at 18:08

score 2 · Answer 2 · answered Aug 18 '17 at 17:41

It may not look like it, but the terminal is ready to accept new commands right after the parent process finishes.

What is happening is that the shell is printing the prompt, then the child process (which is running concurrently with the shell) outputs a string. If you were to enter another command without pressing the enter key first to see another prompt, you will see that the command will run.

For example:

[dbush@dbush ~]$ ./x1
parent process
[dbush@dbush ~]$ child process
echo hello
hello
[dbush@dbush ~]$

Here, I type echo hello while the cursor is at the far left and the command is run.

Using fork() in simplest form need to hit enter to finish execution

compile, run & output

2 Answers2

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