Why does execl require me to hit "Enter" after running a process?

Question

In bash, when I type ls and hit enter, the binary ls will run and I will return to shell prompt again without doing anything from my side.

However this program, written in C will block:

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

int main(void)
{
    pid_t other = fork();
    // other will be 0 for the child process
    // other will be the childs process' value in the parent process.

    switch(other) {
        case 0:
            printf("%s %i\n", "I am the child process!", other);
            execl("/bin/ls","ls",NULL);         
            return 0;
        default:
            printf("%s %i\n", "I am the parent process!", other);
            return 1;
    }

}

Why?

The output is as follows:

Korays-MacBook-Pro:~ koraytugay$ ./a.out 
I am the parent process! 40309
I am the child process! 0
Korays-MacBook-Pro:~ koraytugay$ AndroidStudioProjects  Movies          happyko         koray.i
Applications        Music           hello.c         koray.o
ClionProjects       Pictures        hello.sh        koray.s
Code            Public          innbound        mssql
Desktop         TheElementsFiles    innbound-pf     nono.txt
Documents       VirtualBox VMs      innbound_usage.log  svn-key
Downloads       a.out           k.txt           tugay.c
IdeaProjects        asm.asm         klinnck         webtoolkit
Koray.class     asm.hack        klinnck-pf
Koray.java      cexamples       koray.a
Library         fifa.sql        koray.c

At this point I will need to hit Enter so that I return to bash prompt. Why?

Sourav Ghosh · Accepted Answer · 2015-06-02T08:21:28.000

At this point I will need to hit ENTER so that I return to bash prompt.

Actually, you're already back to the prompt, you just did not realize it.

To elaborate, the problem you facing here is, the parent does not wait for the child to exit and returns beforehand the child finishes execution. So, the shell prompt comes back, and then the output from the chlid process (output of ls) gets printed on the output.

If you notice properly, You've already got the prompt back, and your output appears later.

Korays-MacBook-Pro:~ koraytugay$ ./a.out 
I am the parent process! 40309
I am the child process! 0
****Korays-MacBook-Pro:~ koraytugay$***** AndroidStudioProjects  Movies          happyko         koray.i
Applications        Music           hello.c         koray.o
ClionProjects       Pictures        hello.sh        koray.s
Code            Public          innbound        mssql
Desktop         TheElementsFiles    innbound-p

Above, please note the **** marked line. There, you got your shell prompt back.

score 4 · Answer 2 · answered Jun 02 '15 at 08:06

4

At this point I will need to hit Enter so that I return to bash.

Except no, you're already in bash. But all that ls output after the prompt makes it seem that you are not. Go ahead, try another command.

answered Jun 02 '15 at 08:06

Ignacio Vazquez-Abrams

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But why do not I see Korays-MacBook-Pro:~ koraytugay$ ? – Koray Tugay Jun 02 '15 at 08:07
@KorayTugay But you do, right after the "I am the child process" output. – Some programmer dude Jun 02 '15 at 08:08
What are you talking about? It's up above, before the output of ls. – Ignacio Vazquez-Abrams Jun 02 '15 at 08:08

Why does execl require me to hit "Enter" after running a process?

2 Answers2