The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
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tcgetattr - get the parameters associated with the terminal


#include <termios.h>

int tcgetattr(int
fildes, struct termios *termios_p);


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).


Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.


The tcgetattr() function shall fail if:

The fildes argument is not a valid file descriptor.
The file associated with fildes is not a terminal.

The following sections are informative.






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 original state.

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 made.

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.




tcsetattr(), the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 11, General Terminal Interface, <termios.h>


First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.

Issue 6

In the DESCRIPTION, the rate returned as the input baud rate shall be the output rate. Previously, the number zero was also allowed but was obsolescent.

End of informative text.

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