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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
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NAME
ctermid — generate a pathname for the controlling terminal
SYNOPSIS
#include <stdio.h> char *ctermid(char *s);
DESCRIPTION
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.
The ctermid() function need not be thread-safe if called with a NULL parameter.
RETURN VALUE
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(). The returned
pointer might also be invalidated if the calling thread is terminated. 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.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
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 argument.
#include <stdio.h>
...
char term[L_ctermid];
char *ptr;
ptr = ctermid(term);
APPLICATION USAGE
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.
RATIONALE
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 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.
FUTURE DIRECTIONS
None.
SEE ALSO
ttyname()
The Base Definitions volume of POSIX.1‐2017, <stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .