You can use the ctermid function to get a file name that you can
use to open the controlling terminal. In the GNU library, it returns
the same string all the time: "/dev/tty". That is a special
“magic” file name that refers to the controlling terminal of the
current process (if it has one). To find the name of the specific
terminal device, use ttyname; see Is It a Terminal.
The function ctermid is declared in the header file
— Function: char * ctermid (char *string)
The ctermid function returns a string containing the file name of
the controlling terminal for the current process. If string is
not a null pointer, it should be an array that can hold at least
L_ctermid characters; the string is returned in this array.
Otherwise, a pointer to a string in a static area is returned, which
might get overwritten on subsequent calls to this function.
An empty string is returned if the file name cannot be determined for
any reason. Even if a file name is returned, access to the file it
represents is not guaranteed.
— Macro: int L_ctermid
The value of this macro is an integer constant expression that
represents the size of a string large enough to hold the file name
returned by ctermid.