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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.

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.1‐2008
       between  the  call  to  tcgetattr()  and  tcsetattr(),  leaving all other fields and flags
       unmodified.

       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 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.1‐2008.

SEE ALSO

       cfgetispeed(), tcgetattr()

       The Base Definitions volume of  POSIX.1‐2008,  Chapter  11,  General  Terminal  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 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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .