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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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TCSETATTR(3P) POSIX Programmer's Manual TCSETATTR(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.
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 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
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 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.1‐2017
between the call to tcgetattr() and tcsetattr(), leaving all other
fields and flags unmodified.
No actions defined by this volume of POSIX.1‐2017, 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.1‐2017 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 terminal:
* 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.
Upon successful completion, 0 shall be returned. Otherwise, -1
shall be returned and errno set to indicate the error.
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 ignoring SIGTTOU.
ENOTTY The file associated with fildes is not a terminal.
The following sections are informative.
None.
If trying to change baud rates, applications should call
tcsetattr() then call tcgetattr() 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:
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
situations:
* 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).
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.1‐2017.
cfgetispeed(3p), tcgetattr(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General
Terminal Interface, termios.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 TCSETATTR(3P)
Pages that refer to this page: termios.h(0p), cfsetispeed(3p), cfsetospeed(3p), tcgetattr(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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