CFGETISPEED(3P)                                     POSIX Programmer's Manual                                    CFGETISPEED(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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
       IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001  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 IEEE Std 1003.1-2001 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  IEEE Std 1003.1-2001,  Chapter
       11, General Terminal Interface, <termios.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                               CFGETISPEED(3P)