C/C++ - popen

revof11 · 02-11-2008, 10:11 AM

All,

I can successfully launch popen and grab the stdout using the following code snippit:

Code:

string getStdoutFromCommand(string cmd)
{
  // setup
  string data;
  FILE *stream;
  char buffer[MAX_BUFFER];

  // do it
  stream = popen(cmd.c_str(), "r");
  while ( fgets(buffer, MAX_BUFFER, stream) != NULL )
    data.append(buffer);
  pclose(stream);

  // exit
  return trim(data);
}

What I want to do is launch cmd in a specific directory. Basically, I want to change to the directory and launch the command. The "raw shell" equivalent (*nix or Windows) being:

Code:

cd "SomeDir"
cmd

Is there any way to do this with the popen or exe/execl commands? I couldn't find anything in the man pages or online about it.

jim mcnamara · 02-11-2008, 10:57 AM

popen executes the shell in a subprocess -- concatenate two commands into one

Suppose your command looks like this now:

Code:

ls -l somefile

Make the command string look like this:

Code:

cd /path/to/somewhere && ls -l somefile

Matir · 02-11-2008, 11:17 AM

Or you could have the parent process cwd, then launch via popen/exec*.

revof11 · 02-11-2008, 01:04 PM

Quote:

Originally Posted by Matir

Or you could have the parent process cwd, then launch via popen/exec*.

I wasn't sure if changing directories on the parent process would do it or not. After a quick test, you are right... it does. I will use this solution for run.

Thanks for the quick replies!

bramac2 · 02-26-2009, 07:50 PM

Quote:

Originally Posted by jim mcnamara

popen executes the shell in a subprocess -- concatenate two commands into one

Suppose your command looks like this now:

Code:

ls -l somefile

Make the command string look like this:

Code:

cd /path/to/somewhere && ls -l somefile

hello..
i tried using && and it works ex:cd /tmp && ls -l.. thank you..
but i was trying this.. chroot /var/chroot && ls -l.. here the ls -l works only after i give an 'exit'
any suggestions, plz..
thanks..

ta0kira · 02-27-2009, 12:45 AM

Quote:

Originally Posted by bramac2

hello..
i tried using && and it works ex:cd /tmp && ls -l.. thank you..
but i was trying this.. chroot /var/chroot && ls -l.. here the ls -l works only after i give an 'exit'
any suggestions, plz..
thanks..

That's because chroot will give you a sub-shell by default when you don't specify a command to execute. In this case, it's like typing bash && ls.
Kevin Barry

bramac2 · 03-03-2009, 10:26 PM

thank you very much, kevin.

bramac2 · 03-11-2009, 11:30 PM

i have another question.. i used popen in my c file to chroot and do some actions..
the executable generated from my c file when run from terminal works fine..
but when i call the executable from a php file (which is a part of my Wordpress blog plugin run on Apache server) the chroot does not work and the popen returns 32512 which i think is a failure to fork a subshell for chroot..
any suggestions plz..
thanks, bala

ddreggors · 03-17-2009, 12:30 AM

revof11 I found your code quite handy...

Code:

string getStdoutFromCommand(string cmd) {
	string data;
	FILE *stream;
	int MAX_BUFFER = 256;
	char buffer[MAX_BUFFER];
	cmd.append(" 2>&1");
	stream = popen(cmd.c_str(), "r");
	if (!stream){
		exit(1);
	}
	while (!feof(stream)){
		if (fgets(buffer, MAX_BUFFER, stream) != NULL){
			data.append(buffer);
		}
	}
	pclose(stream);
	return data;
}

One question I have though is if a command is issued that does not return or takes too long is there a way to kill the command and break from the function.

What I am doing with the code above:

I have written a client server application that allows me to exec a command on many servers at once and get the output from that command at the client machine as follows:

Code:

[root@mobile-david ~]# for HOST in `echo "localhost thor"`
> do
> rcomm $HOST hostname -s
> done
localhost: mobile-david
thor: thor

which works amazingly well until I have a jr admin that does something like grepping our entire NFS tree for a string.

I need a way to kill that process from C++ code on say a timer event.

The timer portion is no problem, but killing that process has done little more than leaving it <defunct> until I restart the server, and if it gets called again and again... many <defunct> processes.

rriggs · 03-26-2009, 07:42 PM

Quote:

Originally Posted by ddreggors

I need a way to kill that process from C++ code on say a timer event.

The timer portion is no problem, but killing that process has done little more than leaving it <defunct> until I restart the server, and if it gets called again and again... many <defunct> processes.

You need to wait() for your defunct child processes. Call waitpid() with the PID of the process you just killed.

ddreggors · 03-27-2009, 02:57 PM

I'm sorry, I did not mention that I have been using waitpid(); The problem was resolved once I removed that popen() from a function outside main() and placed it directly in the main function.

Seems that for some reason if I have it external to main and call the function it is in from main I cannot escape from it properly. Is probably just my poor code though.

I'm not an advanced developer, merely a tinkering tester

rriggs · 03-29-2009, 05:11 PM

Oh -- with popen(), you don't need to kill or wait for the subprocess. pclose() does that for you. Just pclose() the stream and you should be OK. Closing the stream sends a SIGPIPE to the subprocess which should cause it to exit. The process will not get the SIGPIPE until it actually tries to write to the pipe which, for long running processes, may be a while. If you really do need to kill the subprocess, make sure you still call pclose().

You probably want to change the reader while() loop in your code to something like this:

Code:

    while (fgets(buffer, MAX_BUFFER, stream) != NULL) {
        data.append(buffer);
    }
    if (ferror(stream)) {
        // Handle error.
    }
    pclose(stream);

I'm not sure that feof() is true when fgets() returns due to an error, such as the subprocess being killed.

You should consider using exceptions in your code. Don't exit() from C++ applications. It will prevent some destructors from being called. Well-formed C++ applications always return from main().