Is there a way to not wait for a system() command to finish? (in c)

Question

Similar to:
program not executing anything after a call to system()

I am fairly new to using C but basically, I want to execute the following line:

int a = system("python -m plotter");

which will launch a python module I developed. However, I want the rest of my c program to keep running instead of waiting for the command to finish executing (the python app is on an infinite loop so it wont close automatically). Is there any way to do this using C/C++?

Update: the solution was:

int a = system("start python -m plotter &");

Solution 1

system() simply passes its argument to the shell (on Unix-like systems, usually /bin/sh).

Try this:

int a = system("python -m plotter &");

Of course the value returned by system() won't be the exit status of the python script, since it won't have finished yet.

This is likely to work only on Unix-like systems (probably including MacOS); in particular, it probably won't work on MS Windows, unless you're running under Cygwin.

On Windows, system() probably invokes cmd.exe, which doesn't accept commands with the same syntax used on Unix-like systems. But the Windows start command should do the job:

int a = system("start python -m plotter");

As long as you're writing Windows-specific code (start won't work on Unix unless you happen to have a start command in your $PATH), you might consider using some lower-level Windows feature, perhaps by calling StartProcess. That's more complicated, but it's likely to give you more control over how the process executes. On the other hand, if system() meets your requirements, you might as well use it.

Solution 2

I believe if you add a '&' to the end of the command it will work. '&' tells the command to run in the background

int a = system("python -m plotter &");

Solution 3

There's no way in the standard library, but you can fork out or create a separate thread that would run it on the background, if your OS supports it.

Solution 4

"System" command on Linux will let the rest of code execute only when it has done executing itself.

You should use fork() if you want simultaneous processing.

Solution 5

The word for this is an Asynchronous function/method call. C and C++ don't have such a feature so you have to either call the function in a separate thread or use a system call that will run it in a separate process. Both of these methods tend to be platform specific, although there are cross platform threading libraries.

I know that in unix to do this in a separate process you would use fork(2) to create a new process and exec(3) to execute a new program in that process. I do not know what system calls would be required in Windows, but should be something similar.

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Comments

[littleadv]

I ran a small test on my linux, and if the shell terminates, the background command will also terminate, so that doesn't necessarily solve the problem

[Keith Thompson]

What happens if you change it to int a = system("nohup python -m plotter &");?

[Simon]

Complement: man page for the exec family of functions

[gh4x]

I tried both, with simply the & as well as with both the nohup and the & but neither works. I am programming in Windows by the way.

[Keith Thompson]

Ok, that explains it. system() probably invokes cmd.exe, which doesn't accept the same syntax that Unix shells do. I should have thought of that earlier, but it would have been helpful if you'd told us in your question what platform you're using. A quick Google search found this, which might be useful; try start rather than nohup.

[gh4x]

start seems to have fixed it! I'm sorry I didn't realize this kind of thing was platform specific and not C specific.

[tripleee]

Any vaguely U*x-compatible shell.