CFGETISPEED(3)             POSIX Programmer's Manual            CFGETISPEED(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
       cfgetispeed -- get input baud rate

SYNOPSIS
       #include <termios.h>

       speed_t cfgetispeed(const struct termios *termios___p);

DESCRIPTION
       The cfgetispeed() function shall extract the input baud rate from the
       termios structure to which the termios___p argument points.

       This function shall return exactly the value in the termios data
       structure, without interpretation.

RETURN VALUE
       Upon successful completion, cfgetispeed() shall return a value of type
       speed_t representing the input baud rate.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       The term ``baud'' is used historically here, but is not technically
       correct. This is properly ``bits per second'', which may not be the
       same as baud. However, the term is used because of the historical usage
       and understanding.

       The cfgetospeed(), cfgetispeed(), cfsetospeed(), and cfsetispeed()
       functions do not take arguments as numbers, but rather as symbolic
       names. There are two reasons for this:

        1. Historically, numbers were not used because of the way the rate was
           stored in the data structure. This is retained even though a
           function is now used.

        2. More importantly, only a limited set of possible rates is at all
           portable, and this constrains the application to that set.

       There is nothing to prevent an implementation accepting as an extension
       a number (such as 126), and since the encoding of the Bxxx symbols is
       not specified, this can be done to avoid introducing ambiguity.

       Setting the input baud rate to zero was a mechanism to allow for split
       baud rates. Clarifications in this volume of POSIX.1-2017 have made it
       possible to determine whether split rates are supported and to support
       them without having to treat zero as a special case. Since this
       functionality is also confusing, it has been declared obsolescent.  The
       0 argument referred to is the literal constant 0, not the symbolic
       constant B0. This volume of POSIX.1-2017 does not preclude B0 from
       being defined as the value 0; in fact, implementations would likely
       benefit from the two being equivalent. This volume of POSIX.1-2017 does
       not fully specify whether the previous cfsetispeed() value is retained
       after a tcgetattr() as the actual value or as zero. Therefore,
       conforming applications should always set both the input speed and
       output speed when setting either.

       In historical implementations, the baud rate information is
       traditionally kept in c_cflag.  Applications should be written to
       presume that this might be the case (and thus not blindly copy
       c_cflag), but not to rely on it in case it is in some other field of
       the structure. Setting the c_cflag field absolutely after setting a
       baud rate is a non-portable action because of this. In general, the
       unused parts of the flag fields might be used by the implementation and
       should not be blindly copied from the descriptions of one terminal
       device to another.

FUTURE DIRECTIONS
       None.

SEE ALSO
       cfgetospeed(), cfsetispeed(), cfsetospeed(), 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                       CFGETISPEED(3)
