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CFGETISPEED(P) CFGETISPEED(P)
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 rep-
resenting 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 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 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. 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.
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, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications 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 Standard, 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 .
POSIX 2003 CFGETISPEED(P)
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