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(libc.info)Identifying the Terminal

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Identifying the Controlling Terminal
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   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'; Note: Is It a Terminal.

   The function `ctermid' is declared in the header file `stdio.h'.

 - 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'.

   See also the `isatty' and `ttyname' functions, in Note: Is It a
Terminal.