Section: POSIX Programmer's Manual (3P)
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.
--- set the parameters associated with the terminal
int tcsetattr(int fildes, int optional_actions,
const struct termios *termios_p);
function shall set the parameters associated with the terminal referred
to by the open file descriptor
(an open file descriptor associated with a terminal) from the
structure referenced by
is TCSANOW, the change shall occur immediately.
is TCSADRAIN, the change shall occur after all output written to
is transmitted. This function should be used when changing parameters
that affect output.
is TCSAFLUSH, the change shall occur after all output written to
is transmitted, and all input so far received but not read shall be
discarded before the change is made.
If the output baud rate stored in the
structure pointed to by
is the zero baud rate, B0, the modem control lines shall no longer
be asserted. Normally, this shall disconnect the line.
If the input baud rate stored in the
structure pointed to by
is 0, the input baud rate given to the hardware is the same as the
output baud rate stored in the
function shall 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 shall set all the attributes that the implementation
supports as requested and leave all the attributes not supported by
the implementation unchanged. If no part of the request can be honored,
it shall return -1 and set
If the input and output baud rates differ and are a combination that is
not supported, neither baud rate shall be changed. A subsequent call to
shall return the actual state of the terminal device (reflecting both
the changes made and not made in the previous
function shall not change the values found in the
structure under any circumstances.
The effect of
is undefined if the value of the
structure pointed to by
was not derived from the result of a call to
an application should modify only fields and flags defined by this volume of POSIX.1-2008
between the call to
leaving all other fields and flags unmodified.
No actions defined by this volume of POSIX.1-2008, other than a call to
a close of the last file descriptor in the system associated with this
terminal device, or an open of the first file descriptor in the system
associated with this terminal device (using the O_TTY_INIT flag if it
is non-zero and the device is not a pseudo-terminal), shall cause any
of the terminal attributes defined by this volume of POSIX.1-2008 to change.
is called from a process which is a member of a background process
group on a
associated with its controlling terminal:
If the calling thread is blocking SIGTTOU signals or the process is
ignoring SIGTTOU signals, the operation completes normally and no signal
Otherwise, a SIGTTOU signal shall be sent to the process group.
Upon successful completion, 0 shall be returned. Otherwise,
-1 shall be returned and
set to indicate the error.
function shall fail if:
argument is not a valid file descriptor.
A signal interrupted
argument is not a supported value, or an attempt was made to change an
attribute represented in the
structure to an unsupported value.
The process group of the writing process is orphaned, the calling thread
is not blocking SIGTTOU, and the process is not ignoring SIGTTOU.
The file associated with
is not a terminal.
The following sections are informative.
If trying to change baud rates, applications should call
in order to determine what baud rates were actually selected.
In general, there are two reasons for an application to change the
parameters associated with a terminal device:
The device already has working parameter settings but the application
needs a different behavior, such as non-canonical mode instead of
canonical mode. The application sets (or clears) only a few flags or
values. Typically, the terminal device in this case is either the
controlling terminal for the process or a pseudo-terminal.
The device is a modem or similar piece of equipment connected by a serial
line, and it was not open before the application opened it. In this case,
the application needs to initialize all of the parameter settings ``from
scratch''. However, since the
structure may include both standard and non-standard parameters, the
application cannot just initialize the whole structure in an arbitrary
way (e.g., using
as this may cause some of the non-standard parameters to be set
incorrectly, resulting in non-conforming behavior of the terminal
device. Conversely, the application cannot just set the standard
parameters, assuming that the non-standard parameters will already have
suitable values, as the device might previously have been used with
non-conforming parameter settings (and some implementations retain the
settings from one use to the next). The solution is to open the terminal
device using the O_TTY_INIT flag to initialize the terminal device to
have conforming parameter settings, obtain those settings using
and then set all of the standard parameters to the desired settings.
function can be interrupted in the following situations:
It is interrupted while waiting for output to drain.
It is called from a process in a background process group and SIGTTOU
See also the RATIONALE section in
Using an input baud rate of 0 to set the input rate equal to the output
rate may not necessarily be supported in a future version of this volume of POSIX.1-2008.
The Base Definitions volume of POSIX.1-2008,
Chapter 11, General Terminal Interface,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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
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