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 guaranteed.
If the application uses any of the _POSIX_THREAD_SAFE_FUNCTIONS or
_POSIX_THREADS functions, it shall ensure that the ctermid() function
is called with a non-NULL parameter.
If s is a null pointer, the string shall be generated in an area that may
be static (and therefore may be overwritten by each call), the address of
which shall be returned. Otherwise, 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
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.
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 argument.
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 byte.
Conforming applications that use threads cannot call ctermid() with NULL
as the parameter if either _POSIX_THREAD_SAFE_FUNCTIONS or _POSIX_THREADS is
defined. 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, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html