получить входную скорость передачи данных (get input baud rate)
Пролог (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.
Имя (Name)
cfgetispeed — get input baud rate
Синопсис (Synopsis)
#include <termios.h>
speed_t cfgetispeed(const struct termios *termios_p);
Описание (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.
Возвращаемое значение (Return value)
Upon successful completion, cfgetispeed() shall return a value of
type speed_t representing the input baud rate.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
None.
Обоснование (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 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.
Будущие направления (Future directions)
None.
Смотри также (See also)
cfgetospeed(3p), cfsetispeed(3p), cfsetospeed(3p), tcgetattr(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 11, General
Terminal Interface, termios.h(0p)
