bionic (3) cfgetispeed.3posix.gz

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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
       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.1‐2008 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.1‐2008 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.1‐2008 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.1‐2008, Chapter 11, General Terminal Interface, <termios.h>


       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 .

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       https://www.kernel.org/doc/man-pages/reporting_bugs.html .