CTERMID(3P) POSIX Programmer's Manual CTERMID(3P)PROLOG
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.
ctermid — generate a pathname for the controlling terminal
char *ctermid(char *s);
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 con‐
stant L_ctermid is defined in <stdio.h>, and shall have a value greater
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.
No errors are defined.
The following sections are informative.
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 ter‐
minal is stored in the array pointed to by the ptr argument, which has
a size of L_ctermid bytes, as indicated by the term argument.
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 associ‐
ated 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 byte.
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 hypo‐
thetical thread-safe version of ctermid() along with allocating a buf‐
fer 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.
The Base Definitions volume of POSIX.1‐2008, <stdio.h>
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 Electri‐
cal 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 https://www.ker‐
IEEE/The Open Group 2013 CTERMID(3P)