Question

In my C program I want to know if my executable is run in foreground like this

$./a.out

or like this

$./a.out &

Answer 1

If you are the foreground job,

getpgrp() == tcgetpgrp(STDOUT_FILENO)

or STDIN_FILENO or STDERR_FILENO or whichever file descriptor you’re attached to your controlling terminal by. (If you’re not sure, open("/dev/tty") will always get you a file descriptor to your controlling terminal, if one exists.)

This is what openssh does, and is a bit easier than handling SIGTTIN/SIGTTOU if you just want a quick check.

On the other hand, you may have been backgrounded

$ ./a.out
^Z
[1]+  Stopped                 ./a.out
$ bg
[1]+ ./a.out &

or foregrounded

$ fg
./a.out

at any point in time. You cannot expect that you can check this once and it will still be true (or false) later.