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.
NAMEcfgetispeed — get input baud rate
speed_t cfgetispeed(const struct termios *termios_p);
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.
Upon successful completion, cfgetispeed() shall return a value of type
speed_t representing the input baud rate.
No errors are defined.
The following sections are informative.
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
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.
SEE ALSOcfgetospeed(), cfsetispeed(), cfsetospeed(), tcgetattr()
The Base Definitions volume of POSIX.1‐2008, Chapter 11, General Termi‐
nal Interface, <termios.h>
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
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IEEE/The Open Group 2013 CFGETISPEED(3P)