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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 dif-
       fer (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 applica-
           tion 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 vol-
       ume 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. There-
       fore, 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                      CFGETISPEED(3P)