1CFGETISPEED(3P)            POSIX Programmer's Manual           CFGETISPEED(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       cfgetispeed - get input baud rate
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SYNOPSIS

15       #include <termios.h>
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17       speed_t cfgetispeed(const struct termios *termios_p);
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DESCRIPTION

21       The cfgetispeed() function shall extract the input baud rate  from  the
22       termios structure to which the termios_p argument points.
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24       This function shall return exactly the value in the termios data struc‐
25       ture, without interpretation.
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RETURN VALUE

28       Upon successful completion, cfgetispeed() shall return a value of  type
29       speed_t representing the input baud rate.
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ERRORS

32       No errors are defined.
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34       The following sections are informative.
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EXAMPLES

37       None.
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APPLICATION USAGE

40       None.
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RATIONALE

43       The  term "baud" is used historically here, but is not technically cor‐
44       rect. This is properly "bits per second", which may not be the same  as
45       baud.  However,  the  term  is used because of the historical usage and
46       understanding.
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48       The  cfgetospeed(),  cfgetispeed(),  cfsetospeed(),  and  cfsetispeed()
49       functions  do  not  take  arguments  as numbers, but rather as symbolic
50       names. There are two reasons for this:
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52        1. Historically, numbers were not used because of the way the rate was
53           stored  in the data structure. This is retained even though a func‐
54           tion is now used.
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56        2. More importantly, only a limited set of possible rates  is  at  all
57           portable, and this constrains the application to that set.
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59       There is nothing to prevent an implementation accepting as an extension
60       a number (such as 126), and since the encoding of the Bxxx  symbols  is
61       not specified, this can be done to avoid introducing ambiguity.
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63       Setting  the input baud rate to zero was a mechanism to allow for split
64       baud rates. Clarifications in this volume of IEEE Std 1003.1-2001  have
65       made  it possible to determine whether split rates are supported and to
66       support them without having to treat zero as a special case. Since this
67       functionality  is also confusing, it has been declared obsolescent. The
68       0 argument referred to is the literal constant 0, not the symbolic con‐
69       stant B0. This volume of IEEE Std 1003.1-2001 does not preclude B0 from
70       being defined as the value 0; in  fact,  implementations  would  likely
71       benefit    from   the   two   being   equivalent.    This   volume   of
72       IEEE Std 1003.1-2001  does  not  fully  specify  whether  the  previous
73       cfsetispeed() value is retained after a tcgetattr() as the actual value
74       or as zero. Therefore, conforming applications should always  set  both
75       the input speed and output speed when setting either.
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77       In  historical implementations, the baud rate information is tradition‐
78       ally kept in c_cflag. Applications should be written  to  presume  that
79       this  might be the case (and thus not blindly copy c_cflag), but not to
80       rely on it in case it is in some other field of the structure.  Setting
81       the  c_cflag field absolutely after setting a baud rate is a non-porta‐
82       ble action because of this. In general, the unused parts  of  the  flag
83       fields  might  be  used by the implementation and should not be blindly
84       copied from the descriptions of one terminal device to another.
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FUTURE DIRECTIONS

87       None.
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SEE ALSO

90       cfgetospeed(), cfsetispeed(), cfsetospeed(), tcgetattr(), the Base Def‐
91       initions  volume  of IEEE Std 1003.1-2001, Chapter 11, General Terminal
92       Interface, <termios.h>
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95       Portions of this text are reprinted and reproduced in  electronic  form
96       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
97       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
98       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
99       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
100       event of any discrepancy between this version and the original IEEE and
101       The Open Group Standard, the original IEEE and The Open Group  Standard
102       is  the  referee document. The original Standard can be obtained online
103       at http://www.opengroup.org/unix/online.html .
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107IEEE/The Open Group                  2003                      CFGETISPEED(3P)
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