The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


tcsetattr - set the parameters associated with the terminal


#include <termios.h>

int tcsetattr(int fildes, int optional_actions,
    const struct termios *termios_p);


The tcsetattr() function sets the parameters associated with the terminal referred to by the open file descriptor fildes (an open file descriptor associated with a terminal) from the termios structure referenced by termios_p as follows:

If the output baud rate stored in the termios structure pointed to by termios_p is the zero baud rate, B0, the modem control lines will no longer be asserted. Normally, this will disconnect the line.

If the input baud rate stored in the termios structure pointed to by termios_p is 0, the input baud rate given to the hardware will be the same as the output baud rate stored in the termios structure.

The tcsetattr() function will return successfully if it was able to perform any of the requested actions, even if some of the requested actions could not be performed. It will set all the attributes that implementation supports as requested and leave all the attributes not supported by the implementation unchanged. If no part of the request can be honoured, it will return -1 and set errno to [EINVAL]. If the input and output baud rates differ and are a combination that is not supported, neither baud rate is changed. A subsequent call to tcgetattr() will return the actual state of the terminal device (reflecting both the changes made and not made in the previous tcsetattr() call). The tcsetattr() function will not change the values in the termios structure whether or not it actually accepts them.

The effect of tcsetattr() is undefined if the value of the termios structure pointed to by termios_p was not derived from the result of a call to tcgetattr() on fildes; an application should modify only fields and flags defined by this specification between the call to tcgetattr() and tcsetattr(), leaving all other fields and flags unmodified.

No actions defined by this specification, other than a call to tcsetattr() or a close of the last file descriptor in the system associated with this terminal device, will cause any of the terminal attributes defined by this specification to change.

Attempts to use tcsetattr() from a process which is a member of a background process group on a fildes associated with its controlling terminal, will cause the process group to be sent a SIGTTOU signal. If the calling process is blocking or ignoring SIGTTOU signals, the process is allowed to perform the operation, and no signal is sent.


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


The tcsetattr() function will fail if:
The fildes argument is not a valid file descriptor.
A signal interrupted tcsetattr().
The optional_actions argument is not a supported value, or an attempt was made to change an attribute represented in the termios structure to an unsupported value.
The file associated with fildes is not a terminal.

The tcsetattr() function may fail if:

The process group of the writing process is orphaned, and the writing process is not ignoring or blocking SIGTTOU.




If trying to change baud rates, applications should call tcsetattr() then call tcgetattr() in order to determine what baud rates were actually selected.


Using an input baud rate of 0 to set the input rate equal to the output rate will not necessarily be supported in future issues of this document.

In the ISO POSIX-1 standard, the possibility of an [EIO] error occurring is described in XBD specification, Terminal Access Control , but it is not mentioned in the tcsetattr() interface definition. It has become clear that this omission was unintended, so it is likely that the [EIO] error will be reclassified as a "will fail" in a future issue of the POSIX standard.


cfgetispeed(), tcgetattr(), <termios.h>, <unistd.h>, the XBD specification, General Terminal Interface .


Derived from the POSIX.1-1988 standard.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
[ Main Index | XSH | XCU | XBD | XCURSES | XNS ]