Programmatically check if a process is being run in the background

Question

2 questions:

1) Is there any Linux/Posix API to know if a process has been invoked as a background process?

linux> myprogram &

Can the code for myprogram detect that it has been invoked to run in the background (via &) ?

2) Is there any Linux/Posix API to make a process run in the background even if it has been started as a foreground process? I.E. somehow 'detach' from the shell at runtime.. (either detach itself from the shell completely, or run as a background process of the shell).

linux> myprogram
**** starting myprogram as a background job ****
linux>

The shell prompt should come right back to me since myprogram has detached from the shell and is running in the background

Answer 1

1) there are two ways to know whether a process in background

1. have a signal handler for SIGTTIN /SIGTTOUT and do a non-blocking read/write depending on which signal handler(stdin/stdout).

2. check the process-group and match it with the terminals' getpgrp() == tcgetpgrp(STDOUT_FILENO)

you will need to repeat the check, as the process can be foregrounded or backgrounded anytime.

2) There is a daemon function to put the process in background. its advisable to redirect the application prints to syslog or some other file while daemonizing.

if (daemonize) {
//redirect all prints to syslog or some other logfile
    daemon(0, 0);
}

where daemonize can be an arguement to the application whether to go into background or not.

Answer 2

To answer your second part, that's usually called a daemon and they're built something like this.

main()
    pid = fork()
    if pid is child
          run program
    else we are the parent process
       exit to command prompt