1CFGETISPEED(3P) POSIX Programmer's Manual CFGETISPEED(3P)
2
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.
10
13 cfgetispeed — get input baud rate
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16 #include <termios.h>
17 speed_t cfgetispeed(const struct termios *termios_p);
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21 The cfgetispeed() function shall extract the input baud rate from the
22 termios structure to which the termios_p argument points.
23 This function shall return exactly the value in the termios data struc‐
25 ture, without interpretation.
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28 Upon successful completion, cfgetispeed() shall return a value of type
29 speed_t representing the input baud rate.
30
32 No errors are defined.
33 The following sections are informative.
35
37 None.
38
40 None.
41
43 The term ``baud'' is used historically here, but is not technically
44 correct. This is properly ``bits per second'', which may not be the
45 same as baud. However, the term is used because of the historical usage
46 and understanding.
47 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:
51 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.
55 2. More importantly, only a limited set of possible rates is at all
57 portable, and this constrains the application to that set.
58 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.
62 Setting the input baud rate to zero was a mechanism to allow for split
64 baud rates. Clarifications in this volume of POSIX.1‐2008 have made it
65 possible to determine whether split rates are supported and to support
66 them without having to treat zero as a special case. Since this func‐
67 tionality is also confusing, it has been declared obsolescent. The 0
68 argument referred to is the literal constant 0, not the symbolic con‐
69 stant B0. This volume of POSIX.1‐2008 does not preclude B0 from being
70 defined as the value 0; in fact, implementations would likely benefit
71 from the two being equivalent. This volume of POSIX.1‐2008 does not
72 fully specify whether the previous cfsetispeed() value is retained
73 after a tcgetattr() as the actual value or as zero. Therefore, conform‐
74 ing applications should always set both the input speed and output
75 speed when setting either.
76 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.
85
87 None.
88
90 cfgetospeed(), cfsetispeed(), cfsetospeed(), tcgetattr()
91 The Base Definitions volume of POSIX.1‐2008, Chapter 11, General Termi‐
93 nal Interface, <termios.h>
94
96 Portions of this text are reprinted and reproduced in electronic form
97 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
98 -- Portable Operating System Interface (POSIX), The Open Group Base
99 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
100 cal and Electronics Engineers, Inc and The Open Group. (This is
101 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
102 event of any discrepancy between this version and the original IEEE and
103 The Open Group Standard, the original IEEE and The Open Group Standard
104 is the referee document. The original Standard can be obtained online
105 at http://www.unix.org/online.html .
106 Any typographical or formatting errors that appear in this page are
108 most likely to have been introduced during the conversion of the source
109 files to man page format. To report such errors, see https://www.ker‐
110 nel.org/doc/man-pages/reporting_bugs.html .
111IEEE/The Open Group 2013 CFGETISPEED(3P)