get foreground process group ID
() 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 group,
() returns an invalid process ID.
If an error occurs,
() returns -1
and the global variable errno
is set to
indicate the error, as follows:
- The fd argument is not a valid file
- The calling process does not have a controlling terminal or the underlying
terminal device represented by fd is not
the controlling terminal.
() function is expected to be
compliant with the IEEE Std 1003.1-1988