The tcgetsid() function obtains the process group ID of the session for which the terminal specified by fildes is the controlling terminal.
RETURN VALUES
Upon successful completion, tcgetsid() returns the process group ID associated with the terminal. Otherwise, a value of (pid_t)-1 is returned and errno is set to indicate the error.
ERRORS
The tcgetsid() function will fail if:
EACCES
The fildes argument is not associated with a controlling terminal.
EBADF
The fildes argument is not a valid file descriptor.
ENOTTY
The file associated with fildes is not a terminal.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes: