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

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

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

31       No errors are defined.
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33       The following sections are informative.
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EXAMPLES

36       None.
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APPLICATION USAGE

39       None.
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RATIONALE

42       The  term  ``baud''  is  used historically here, but is not technically
43       correct. This is properly ``bits per second'', which  may  not  be  the
44       same as baud. However, the term is used because of the historical usage
45       and understanding.
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47       The  cfgetospeed(),  cfgetispeed(),  cfsetospeed(),  and  cfsetispeed()
48       functions  do  not  take  arguments  as numbers, but rather as symbolic
49       names. There are two reasons for this:
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51        1. Historically, numbers were not used because of the way the rate was
52           stored  in the data structure. This is retained even though a func‐
53           tion is now used.
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55        2. More importantly, only a limited set of possible rates  is  at  all
56           portable, and this constrains the application to that set.
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58       There is nothing to prevent an implementation accepting as an extension
59       a number (such as 126), and since the encoding of the Bxxx  symbols  is
60       not specified, this can be done to avoid introducing ambiguity.
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62       Setting  the input baud rate to zero was a mechanism to allow for split
63       baud rates. Clarifications in this volume of POSIX.1‐2017 have made  it
64       possible  to determine whether split rates are supported and to support
65       them without having to treat zero as a special case. Since  this  func‐
66       tionality  is  also confusing, it has been declared obsolescent.  The 0
67       argument referred to is the literal constant 0, not the  symbolic  con‐
68       stant  B0.  This volume of POSIX.1‐2017 does not preclude B0 from being
69       defined as the value 0; in fact, implementations would  likely  benefit
70       from  the  two  being  equivalent. This volume of POSIX.1‐2017 does not
71       fully specify whether the  previous  cfsetispeed()  value  is  retained
72       after a tcgetattr() as the actual value or as zero. Therefore, conform‐
73       ing applications should always set both  the  input  speed  and  output
74       speed when setting either.
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76       In  historical implementations, the baud rate information is tradition‐
77       ally kept in c_cflag.  Applications should be written to  presume  that
78       this  might be the case (and thus not blindly copy c_cflag), but not to
79       rely on it in case it is in some other field of the structure.  Setting
80       the  c_cflag field absolutely after setting a baud rate is a non-porta‐
81       ble action because of this. In general, the unused parts  of  the  flag
82       fields  might  be  used by the implementation and should not be blindly
83       copied from the descriptions of one terminal device to another.
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FUTURE DIRECTIONS

86       None.
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SEE ALSO

89       cfgetospeed(), cfsetispeed(), cfsetospeed(), tcgetattr()
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91       The Base Definitions volume of POSIX.1‐2017, Chapter 11, General Termi‐
92       nal Interface, <termios.h>
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95       Portions  of  this text are reprinted and reproduced in electronic form
96       from IEEE Std 1003.1-2017, Standard for Information Technology --  Por‐
97       table  Operating System Interface (POSIX), The Open Group Base Specifi‐
98       cations Issue 7, 2018 Edition, Copyright (C) 2018 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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105       Any  typographical  or  formatting  errors that appear in this page are
106       most likely to have been introduced during the conversion of the source
107       files  to  man page format. To report such errors, see https://www.ker
108       nel.org/doc/man-pages/reporting_bugs.html .
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112IEEE/The Open Group                  2017                      CFGETISPEED(3P)
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