ctermid(3) -- Linux man page
NAMEctermid - get controlling terminal name
#include <stdio.h> char *ctermid(char *s);
DESCRIPTIONctermid() returns a string which is the pathname for the current controlling terminal for this process. If s is NULL, a static buffer is used, otherwise s points to a buffer used to hold the terminal pathname. The symbolic constant L_ctermid is the maximum number of characters in the returned pathname.
RETURN VALUEThe pointer to the pathname.
CONFORMING TOSVID, Issue 1; POSIX.1
BUGSThe path returned may not uniquely identify the controlling terminal; it may, for example, be /dev/tty.