How can a C/C++ program put itself into background?

Question

What\'s the best way for a running C or C++ program that\'s been launched from the command line to put itself into the background, equivalent to if the user had launched fro

Answer 1

A process cannot put itself into the background, because it isn't the one in charge of background vs. foreground. That would be the shell, which is waiting for process exit. If you launch a process with an ampersand "&" at the end, then the shell does not wait for process exit.

But the only way the process can escape the shell is to fork off another child and then let its original self exit back to the waiting shell.

From the shell, you can background a process with Control-Z, then type "bg".

Answer 2

The most common way of doing this under Linux is via forking. The same should work on Mac, as for Windows I'm not 100% sure but I believe they have something similar.

Basically what happens is the process splits itself into two processes, and then the original one exits (returning control to the shell or whatever), and the second process continues to run in the background.

Answer 3

Under Windows, the closing thing you're going to get to fork() is loading your program as a Windows service, I think.

Here is a link to an intro article on Windows services... CodeProject: Simple Windows Service Sample

Answer 4

On UNIX, you need to fork twice in a row and let the parent die.

Answer 5

Here is some pseudocode for Linux/UNIX:

initialization_code()
if(failure) exit(1)
if( fork() > 0 ) exit(0)
setsid()
setup_signal_handlers()
for(fd=0; fd<NOFILE; fd++) close(fd)
open("/dev/null", O_RDONLY)
open("/dev/null", O_WRONLY)
open("/dev/null", o_WRONLY)
chdir("/")

And congratulations, your program continues as an independent "daemonized" process without a controlling TTY and without any standard input or output.

Now, in Windows you simply build your program as a Win32 application with WinMain() instead of main(), and it runs without a console automatically. If you want to run as a service, you'll have to look that up because I've never written one and I don't really know how they work.

Answer 6

You edited your question, but you may still be missing the point that your question is a syntax error of sorts -- if the process wasn't put in the background to begin with and you want the PID to stay the same, you can't ignore the fact that the program which started the process is waiting on that PID and that is pretty much the definition of being in the foreground.

I think you need to think about why you want to both put something in the background and keep the PID the same. I suggest you probably don't need both of those constraints.