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

As others mentioned, fork() is how to do it on *nix. You can get fork() on Windows by using MingW or Cygwin libraries. But those will require you to switch to using GCC as your compiler.

In pure Windows world, you'd use CreateProcess (or one of its derivatives CreateProcessAsUser, CreateProcessWithLogonW).

Answer 2

In Unix, I have learned to do that using fork(). If you want to put a running process into the background, fork it twice.

Answer 3

My advice: don't do this, at least not under Linux/UNIX.

GUI programs under Linux/UNIX traditionally do not auto-background themselves. While this may occasionally be annoying to newbies, it has a number of advantages:

• Makes it easy to capture standard error in case of core dumps / other problems that need debugging.

• Makes it easy for a shell script to run the program and wait until it's completed.

• Makes it easy for a shell script to run the program in the background and get its process id:

  gui-program &
  pid=$!
  # do something with $pid later, such as check if the program is still running
  

  If your program forks itself, this behavior will break.

"Scriptability" is useful in so many unexpected circumstances, even with GUI programs, that I would hesitate to explicitly break these behaviors.

Windows is another story. AFAIK, Windows programs automatically run in the background--even when invoked from a command shell--unless they explicitly request access to the command window.

Answer 4

To followup on your edited question:

I was wondering if there was a way to put the EXISTING process into the background.

In a Unix-like OS, there really is not a way to do this that I know of. The shell is blocked because it is executing one of the variants of a wait() call, waiting for the child process to exit. There is not a way for the child process to remain running but somehow cause the shell's wait() to return with a "please stop watching me" status. The reason you have the child fork and exit the original is so the shell will return from wait().

Answer 5

If you need a script to have the PID of the program, you can still get it after a fork.

When you fork, save the PID of the child in the parent process. When you exit the parent process, either output the PID to STD{OUT,ERR} or simply have a return pid; statement at the end of main(). A calling script can then get the pid of the program, although it requires a certain knowledge of how the program works.

Answer 6

On Linux, daemon() is what you're looking for, if I understand you correctly.