CODEWITHSUNDEEP
cshellexec
Jack
Jack

Reputation: 13

If execv launches a process using "/bin/sh -c", will there be a new process?

I want execute another program in C program. Since I want use redirection, I use execl function to execute /bin/sh and pass the new command to it. Code is like this:

execl("/bin/sh", "/bin/sh", "-c", "/myprogram > a.out", NULL);

I know C program process will turn to be the shell(/bin/sh) process in the first stage. What confuses me is that whether the new shell process forks a new process to execute "/myprogram > a.out" or the shell process itself parses and turns to be "/myprogram > a.out".

I did a small test on this. Result seems to support the latter hypothesis. Test is like this:

  1. I start a zsh on my computer, run the command echo $$, result is 56341. This shell's pid is 56341

    $ echo $$
56341

  2. Then run command /bin/sh -c "/Users/Jack/dead_loop" in this shell. It does not return because the program is a forever loop.

    $ /bin/sh -c "/Users/Jack/dead_loop"
█

    The dead_loop source code is:

    int main(){
    while(1){
        ;
    }
}

  3. I start a new terminal and run command ps -ef|grep dead_loop. Result shows that the dead_loop program's parent process is the first shell process.

    enter image description here

What I expect is that the dead_loop program's parent process is another new shell process. So this "the shell process itself parses and turns to be /myprogram > a.out" idea comes to my mind.

Upvotes: 0

Views: 542

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409176

Remember that the exec family of functions replaces the current process.

You must use fork to create a new process to run exec (if you want your program to continue).

And it's the same for the shell: It must also fork a new process to call exec for the programs it runs.

Upvotes: 1

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