get foreground process group
function returns the value of the process group ID of the foreground process
group associated with the terminal device. If there is no foreground process
tcgetpgrp() returns an invalid process
If an error occurs,
tcgetpgrp() returns -1
and the global variable errno is set to indicate the
error, as follows:
- The fd argument is not a valid file descriptor.
- The calling process does not have a controlling terminal or the underlying terminal device represented by fd is not the controlling terminal.
setpgid(2), setsid(2), tcsetpgrp(3)
tcgetpgrp() function is expected to be
compliant with the IEEE Std 1003.1-1988