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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 (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 struc-
       ture, 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 func-
           tion 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  func-
       tionality  is  also confusing, it has been declared obsolescent.  The 0
       argument referred to is the literal constant 0, not the  symbolic  con-
       stant  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, conform-
       ing applications should always set both  the  input  speed  and  output
       speed when setting either.
       In  historical implementations, the baud rate information is tradition-
       ally 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-porta-
       ble 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 Termi-
       nal 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 Electri-
       cal  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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                      CFGETISPEED(3P)