TCSETATTR(3P) POSIX Programmer's Manual TCSETATTR(3P)PROLOG
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.
NAMEtcsetattr — set the parameters associated with the terminal
SYNOPSIS
#include <termios.h>
int tcsetattr(int fildes, int optional_actions,
const struct termios *termios_p);
DESCRIPTION
The tcsetattr() function shall set 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 ref‐
erenced by termios_p as follows:
* If optional_actions is TCSANOW, the change shall occur immediately.
* If optional_actions is TCSADRAIN, the change shall occur after all
output written to fildes is transmitted. This function should be
used when changing parameters that affect output.
* If optional_actions is TCSAFLUSH, the change shall occur after all
output written to fildes 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 termios structure pointed to by
termios_p 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 termios structure pointed to by
termios_p is 0, the input baud rate given to the hardware is the same
as the output baud rate stored in the termios structure.
The tcsetattr() 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 errno to [EINVAL].
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
tcgetattr() shall return the actual state of the terminal device
(reflecting both the changes made and not made in the previous tcse‐
tattr() call). The tcsetattr() function shall not change the values
found in the termios structure under any circumstances.
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 volume of POSIX.1‐2008 between the call to
tcgetattr() and tcsetattr(), leaving all other fields and flags unmodi‐
fied.
No actions defined by this volume of POSIX.1‐2008, other than a call to
tcsetattr(), a close of the last file descriptor in the system associ‐
ated 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-termi‐
nal), shall cause any of the terminal attributes defined by this volume
of POSIX.1‐2008 to change.
If tcsetattr() is called from a process which is a member of a back‐
ground process group on a fildes associated with its controlling termi‐
nal:
* If the calling thread is blocking SIGTTOU signals or the process is
ignoring SIGTTOU signals, the operation completes normally and no
signal is sent.
* Otherwise, a SIGTTOU signal shall be sent to the process group.
RETURN VALUE
Upon successful completion, 0 shall be returned. Otherwise, −1 shall be
returned and errno set to indicate the error.
ERRORS
The tcsetattr() function shall fail if:
EBADF The fildes argument is not a valid file descriptor.
EINTR A signal interrupted tcsetattr().
EINVAL 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.
EIO The process group of the writing process is orphaned, the call‐
ing thread is not blocking SIGTTOU, and the process is not
ignoring SIGTTOU.
ENOTTY The file associated with fildes is not a terminal.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
If trying to change baud rates, applications should call tcsetattr()
then call tcgetattr() in order to determine what baud rates were actu‐
ally selected.
In general, there are two reasons for an application to change the
parameters associated with a terminal device:
1. The device already has working parameter settings but the applica‐
tion needs a different behavior, such as non-canonical mode instead
of canonical mode. The application sets (or clears) only a few
flags or c_cc[] values. Typically, the terminal device in this case
is either the controlling terminal for the process or a pseudo-ter‐
minal.
2. 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 param‐
eter settings ``from scratch''. However, since the termios struc‐
ture may include both standard and non-standard parameters, the
application cannot just initialize the whole structure in an arbi‐
trary way (e.g., using memset()) as this may cause some of the non-
standard parameters to be set incorrectly, resulting in non-con‐
forming behavior of the terminal device. Conversely, the applica‐
tion 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 parame‐
ter 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 con‐
forming parameter settings, obtain those settings using tcge‐
tattr(), and then set all of the standard parameters to the desired
settings.
RATIONALE
The tcsetattr() function can be interrupted in the following situa‐
tions:
* It is interrupted while waiting for output to drain.
* It is called from a process in a background process group and SIGT‐
TOU is caught.
See also the RATIONALE section in tcgetattr().
FUTURE DIRECTIONS
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 vol‐
ume of POSIX.1‐2008.
SEE ALSOcfgetispeed(), tcgetattr()
The Base Definitions volume of POSIX.1‐2008, Chapter 11, General Termi‐
nal Interface, <termios.h>
COPYRIGHT
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 Electri‐
cal 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 http://www.unix.org/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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 TCSETATTR(3P)
