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.TH CFGETISPEED "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH 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.

.SH NAME

cfgetispeed

\(em get input baud rate

.SH SYNOPSIS

.LP

.nf

#include <termios.h>

.P

speed_t cfgetispeed(const struct termios *\fItermios_p\fP);

.fi

.SH DESCRIPTION

The

\fIcfgetispeed\fR()

function shall extract the input baud rate from the

.BR termios

structure to which the

.IR termios_p

argument points.

.P

This function shall return exactly the value in the

.BR termios

data structure, without interpretation.

.SH "RETURN VALUE"

Upon successful completion,

\fIcfgetispeed\fR()

shall return a value of type

.BR speed_t

representing the input baud rate.

.SH ERRORS

No errors are defined.

.LP

.IR "The following sections are informative."

.SH EXAMPLES

None.

.SH "APPLICATION USAGE"

None.

.SH 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.

.P

The

\fIcfgetospeed\fR(),

\fIcfgetispeed\fR(),

\fIcfsetospeed\fR(),

and

\fIcfsetispeed\fR()

functions do not take arguments as numbers, but rather as symbolic

names. There are two reasons for this:

.IP " 1." 4

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.

.IP " 2." 4

More importantly, only a limited set of possible rates is at all

portable, and this constrains the application to that set.

.P

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.

.P

Setting the input baud rate to zero was a mechanism to allow for split

baud rates. Clarifications in this volume of POSIX.1\(hy2008 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\(hy2008 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\(hy2008 does not fully specify whether the previous

\fIcfsetispeed\fR()

value is retained after a

\fItcgetattr\fR()

as the actual value or as zero. Therefore, conforming applications should

always set both the input speed and output speed when setting either.

.P

In historical implementations, the baud rate information is

traditionally kept in

.BR c_cflag .

Applications should be written to presume that this might be the case

(and thus not blindly copy

.BR c_cflag ),

but not to rely on it in case it is in some other field of the

structure. Setting the

.BR 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.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "\fIcfgetospeed\fR\^(\|)",

.IR "\fIcfsetispeed\fR\^(\|)",

.IR "\fIcfsetospeed\fR\^(\|)",

.IR "\fItcgetattr\fR\^(\|)"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "Chapter 11" ", " "General Terminal Interface",

.IR "\fB<termios.h>\fP"

.SH COPYRIGHT

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

https://www.kernel.org/doc/man-pages/reporting_bugs.html .