Thread: Combine execvp result

Hi. I have problems with the execvp command. I would like to run multiple commands. For example:

>ls | wc

Right now it works fine if I write:

Code:

har *argv[2] = {"ls",NULL};
execvp("ls", argv);

But how do I add the "wc" command to this?
Have tried things like:

Code:

char *argv[3] = {"wc","ls",NULL};
execvp("ls", argv);

and

Code:

char *argv[3] = {"wc",[ls result],NULL};
execvp("wc", argv);

But cant get it to work.
Any ideas?

Hey,

once the exec*(2) call is successful, exec*(2) won't return to your "original" program.

To realize the command "ls | wc", you could fork(2) two processes and connect them by a pipe(2) and finally execute exec*(2) for each of the two new processes.

- Andi -

Originally Posted by ForzaItalia2006

Hey,

once the exec*(2) call is successful, exec*(2) won't return to your "original" program.

To realize the command "ls | wc", you could fork(2) two processes and connect them by a pipe(2) and finally execute exec*(2) for each of the two new processes.

- Andi -

Yes. That is what I have tried to do. Have a working fork together with pipe to send the result from execvp to the parent process. But when I have executed "ls" for the first process I need to use this somehow for the second process.
Tried to run:

Code:

execvp("wc", [result from child],NULL)

But this gives:

wc: : No such file or directory

Any tips what could be wrong?

Originally Posted by MK27

Actually, here's an idea:

Code:

#!/bin/bash

if [ $# != 2 ]; then
	exit;
fi

$1 | $2

Christen this "pipe" (since there is no such standard shell command) and you can use it:

Code:

char *args[] = {"pipe", "ls", "wc", NULL};
execvp("pipe", args);

You can replace ls and wc with whatever you want.

Seems nice. But I would prefer not to use scripting. Any hint on how it would look in C?

Originally Posted by Salem

You also need to look up the pipe(2) system call as well, along with close() and dup().
A pipe gives you two connected file descriptors, one for reading (will become a stdin), and one for writing (will become a stdout).


Essentially, you do this
- create a pipe
- call fork() to create a separate child process
--- in the child process (which will become say 'wc'
---- close stdin
---- dup() one end of the pipe to being the new stdin for the child process
---- close other fd's that are of no interest.
---- exec the 'wc' command
- call fork() again in the parent to create another child
--- in this child process, we will run 'ls'
---- close stdout
---- dup() the other end of the pipe to being stdout
---- close other fd's that are of no interest.
---- exec the 'ls' command
- wait() for both child processes to exit





Try using strace with a command line version so you can see most (if not all) of this happening.

I might be wrong but it looks to me that it would be executed as two separate commands by doing that way. Or?
"---- exec the 'wc' command"
will run and die before we go on to the
"---- exec the 'ls' command"
By typing "ls | wc" you should actually run wc on the result from ls. But it is maybe what happens with what you suggested?

Originally Posted by Hunter_wow

I might be wrong but it looks to me that it would be executed as two separate commands by doing that way. Or?
"---- exec the 'wc' command"
will run and die before we go on to the
"---- exec the 'ls' command"
By typing "ls | wc" you should actually run wc on the result from ls. But it is maybe what happens with what you suggested?

Yes, you execute two separate commands, just like a shell would do. But, you connect both programs by the pipe. Though, to clarify what salem already explained:

ls -- write --> | PIPE | -- read --> wc

Regarding ls, by using close(1)/dup(pipe[1]), stdout is redirected to the writing part of the pipe. In turn, regarding wc, by using close(0), dup(pipe[0]), stdin is redirected to the reading part of the pipe. So, that what ls prints to "stdout"/pipe is read by wc via stdin/pipe.

Hope that is clear,
- Andi -

Originally Posted by MK27

Here's a C version of that shell script. Just turn it into a function.

Code:

#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[]) {
	char byte;
	FILE *lsin = popen(argv[1],"r"),
		*wcout = popen(argv[2],"w");
	
	while (fread(&byte,1,1,lsin)) fwrite(&byte,1,1,wcout);

	pclose(lsin);
	pclose(wcout);

	return 0;
}

./a.out ls wc

Seems to work great. Thanks a lot mate.

Originally Posted by Salem

You also need to look up the pipe(2) system call as well, along with close() and dup().
A pipe gives you two connected file descriptors, one for reading (will become a stdin), and one for writing (will become a stdout).


Essentially, you do this
- create a pipe
- call fork() to create a separate child process
--- in the child process (which will become say 'wc'
---- close stdin
---- dup() one end of the pipe to being the new stdin for the child process
---- close other fd's that are of no interest.
---- exec the 'wc' command
- call fork() again in the parent to create another child
--- in this child process, we will run 'ls'
---- close stdout
---- dup() the other end of the pipe to being stdout
---- close other fd's that are of no interest.
---- exec the 'ls' command
- wait() for both child processes to exit





Try using strace with a command line version so you can see most (if not all) of this happening.

Hello again . I have restarted the problem but having a hard time getting it to work without the popen. It seems like wc gets the wrong result from ls,

wc: standard input: Bad file descriptor

Any ideas why?

Code:

//create a pipe
int pipep2[2];
pipe(pipep2);
int pread2 = pipep2[0];
int pwrite2 = pipep2[1];

// call fork() to create a separate child process
int PID2 = fork();
if(!PID2){//child
	close(0); //close stdin
	// dup() one end of the pipe to being the new stdin for the child process
	dup(pwrite2);
	// exec the 'wc' command
	char *cmd1[10] = {"wc",NULL};
	execvp("wc", cmd1);
}
else if(PID2)//parent
{
	int pipep[2];
	pipe(pipep);
	int pread = pipep[0];
	int pwrite = pipep[1];
	//call fork() again in the parent to create another child
	int PID = fork();
	if(!PID){//sub-child
		close(1); //close stdout
		//dup() the other end of the pipe to being stdout
		dup(pwrite);
		close(pread);
		// exec the 'ls' command
		char *cmd2[10] = {"ls",NULL};
		execvp("ls", cmd2);
	}
}

Thanks in advance!

Thanks Salem! Just one more thing. If I add a loop to do that twice it seems like the result are not printed out directly. Instead it puts it into the buffer until the loop/main are finished before it prints it out.
Any idea how I can fflush() or something similar after execvp to get the result directly?