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

       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.1‐2017
       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.1‐2017 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.1‐2017 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.1‐2017, Chapter 11, General
       Terminal Interface, termios.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       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               2017                   CFGETISPEED(3P)

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