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.
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
referenced by termios_p as follows:
* If optional_actions is TCSANOW, the change shall occur
* 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
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
tcsetattr() 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
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
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.
If tcsetattr() is called from a process which is a member of a
background process group on a fildes associated with its controlling
* 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.
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
calling thread is not blocking SIGTTOU, and the process is not
ENOTTY The file associated with fildes is not a terminal.
The following sections are informative.
If trying to change baud rates, applications should call tcsetattr()
then call tcgetattr() in order to determine what baud rates were
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
application 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-terminal.
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
parameter settings ``from scratch''. However, since the termios
structure may include both standard and non-standard parameters,
the application cannot just initialize the whole structure in an
arbitrary way (e.g., using memset()) 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 tcgetattr(), and then set all of the
standard parameters to the desired settings.
The tcsetattr() function can be interrupted in the following
* It is interrupted while waiting for output to drain.
* It is called from a process in a background process group and
SIGTTOU is caught.
See also the RATIONALE section in tcgetattr(3p).
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 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
IEEE/The Open Group 2013 TCSETATTR(3P)