pid_t tcgetpgrp(int fildes);
IEEE Std 1003.1-2017
tcgetpgrp() - get the foreground process group ID
tcgetpgrp() function shall return the value of the process group ID of the foreground process group associated with the terminal.
If there is no foreground process group,
tcgetpgrp() shall return a value greater than
1 that does not match the process group ID of any existing process group.
tcgetpgrp() function is allowed from a process that is a member of a background process group; however, the information may be subsequently changed by a process that is a member of a foreground process group.
The functions can never return. Upon successful completion,
tcgetpgrp() shall return the value of the process group ID of the foreground process associated with the terminal. Otherwise,
-1 shall be returned and
errno set to indicate the error.
tcgetpgrp() function shall fail if:
EBADF- The fildes argument is not a valid file descriptor.
ENOTTY- The calling process does not have a controlling terminal, or the file is not the controlling terminal.