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       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.

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 .