TCSETATTR(3)               POSIX Programmer's Manual              TCSETATTR(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       tcsetattr -- 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
       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.

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
              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.

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
       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.

RATIONALE
       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().

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
       volume of POSIX.1-2017.

SEE ALSO
       cfgetispeed(), tcgetattr()

       The Base Definitions volume of POSIX.1-2017, Chapter 11, General
       Terminal Interface, <termios.h>

COPYRIGHT
       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(3)
