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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CFGETISPEED(3P) POSIX Programmer's Manual CFGETISPEED(3P)
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.
cfgetispeed — get input baud rate
#include <termios.h>
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
structure, 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.
None.
None.
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 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.
None.
cfgetospeed(3p), cfsetispeed(3p), cfsetospeed(3p), tcgetattr(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General
Terminal Interface, termios.h(0p)
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
http://www.opengroup.org/unix/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 .
IEEE/The Open Group 2017 CFGETISPEED(3P)
Pages that refer to this page: termios.h(0p), cfgetospeed(3p), cfsetispeed(3p), cfsetospeed(3p), tcsetattr(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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