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.
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.
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.
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 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
IEEE/The Open Group 2013 CFGETISPEED(3P)