cfgetispeed(3p) — Linux manual page


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

PROLOG         top

       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         top

       cfgetispeed — get input baud rate

SYNOPSIS         top

       #include <termios.h>

       speed_t cfgetispeed(const struct termios *termios_p);

DESCRIPTION         top

       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         top

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

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top




RATIONALE         top

       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

       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.



SEE ALSO         top

       cfgetospeed(3p), cfsetispeed(3p), cfsetospeed(3p), tcgetattr(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 11, General
       Terminal Interface, termios.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                     CFGETISPEED(3P)

Pages that refer to this page: termios.h(0p)cfgetospeed(3p)cfsetispeed(3p)cfsetospeed(3p)tcsetattr(3p)