This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The ctermid() function shall generate a string that, when used as a
pathname, refers to the current controlling terminal for the current
process. If ctermid() returns a pathname, access to the file is not
The ctermid() function need not be thread-safe if called with a NULL
If s is a null pointer, the string shall be generated in an area that
may be static, the address of which shall be returned. The
application shall not modify the string returned. The returned
pointer might be invalidated or the string content might be
overwritten by a subsequent call to ctermid(). If s is not a null
pointer, s is assumed to point to a character array of at least
L_ctermid bytes; the string is placed in this array and the value of
s shall be returned. The symbolic constant L_ctermid is defined in
<stdio.h>, and shall have a value greater than 0.
The ctermid() function shall return an empty string if the pathname
that would refer to the controlling terminal cannot be determined, or
if the function is unsuccessful.
Determining the Controlling Terminal for the Current Process
The following example returns a pointer to a string that identifies
the controlling terminal for the current process. The pathname for
the terminal is stored in the array pointed to by the ptr argument,
which has a size of L_ctermid bytes, as indicated by the term
#include <stdio.h>...char term[L_ctermid];char *ptr;
ptr = ctermid(term);
The difference between ctermid() and ttyname() is that ttyname() must
be handed a file descriptor and return a path of the terminal
associated with that file descriptor, while ctermid() returns a
string (such as "/dev/tty") that refers to the current controlling
terminal if used as a pathname.
L_ctermid must be defined appropriately for a given implementation
and must be greater than zero so that array declarations using it are
accepted by the compiler. The value includes the terminating null
Conforming applications that use multiple threads cannot call
ctermid() with NULL as the parameter. If s is not NULL, the ctermid()
function generates a string that, when used as a pathname, refers to
the current controlling terminal for the current process. If s is
NULL, the return value of ctermid() is undefined.
There is no additional burden on the programmer—changing to use a
hypothetical thread-safe version of ctermid() along with allocating a
buffer is more of a burden than merely allocating a buffer.
Application code should not assume that the returned string is short,
as some implementations have more than two pathname components before
reaching a logical device name.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 CTERMID(3P)