The tcgetattr() function shall get the parameters associated with the
terminal referred to by fildes and store them in the termios
structure referenced by termios_p. The fildes argument is an
open file descriptor associated with a terminal.
The termios_p argument is a pointer to a termios structure.
The tcgetattr() operation is allowed from any process.
If the terminal device supports different input and output baud rates, the baud
rates stored in the termios structure returned by tcgetattr()
shall reflect the actual baud rates, even if they are equal. If differing baud
rates are not supported, the rate returned as the output baud rate shall be
the actual baud rate. If the terminal device does not support split baud
rates, the input baud rate stored in the termios structure shall be the
output rate (as one of the symbolic values).
Care must be taken when changing the terminal attributes. Applications should
always do a tcgetattr(), save the termios structure values
returned, and then do a tcsetattr(), changing only the necessary
fields. The application should use the values saved from the
tcgetattr() to reset the terminal state whenever it is done with the
terminal. This is necessary because terminal attributes apply to the
underlying port and not to each individual open instance; that is, all
processes that have used the terminal see the latest attribute changes.
A program that uses these functions should be written to catch all signals and
take other appropriate actions to ensure that when the program terminates,
whether planned or not, the terminal device's state is restored to its
Existing practice dealing with error returns when only part of a request can be
honored is based on calls to the ioctl() function. In historical BSD
and System V implementations, the corresponding ioctl() returns zero if
the requested actions were semantically correct, even if some of the requested
changes could not be made. Many existing applications assume this behavior and
would no longer work correctly if the return value were changed from zero to
-1 in this case.
Note that either specification has a problem. When zero is returned, it implies
everything succeeded even if some of the changes were not made. When -1 is
returned, it implies everything failed even though some of the changes were
Applications that need all of the requested changes made to work properly should
follow tcsetattr() with a call to tcgetattr() and compare the
appropriate field values.
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