get foreground process group ID
tcgetpgrp() 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,
tcgetpgrp() returns an invalid process ID.
If an error occurs,
tcgetpgrp() returns -1 and the
global variable errno is set to indicate the error, as
- 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
tcgetpgrp() function is expected to be compliant
with the IEEE Std 1003.1-1988