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
12 cfgetispeed — get input baud rate
13
15 #include <termios.h>
16 speed_t cfgetispeed(const struct termios *termios_p);
18
20 The cfgetispeed() function shall extract the input baud rate from the
21 termios structure to which the termios_p argument points.
22 This function shall return exactly the value in the termios data struc‐
24 ture, without interpretation.
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27 Upon successful completion, cfgetispeed() shall return a value of type
28 speed_t representing the input baud rate.
29
31 No errors are defined.
32 The following sections are informative.
34
36 None.
37
39 None.
40
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.
46 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:
50 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.
54 2. More importantly, only a limited set of possible rates is at all
56 portable, and this constrains the application to that set.
57 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.
61 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.
75 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.
84
86 None.
87
89 cfgetospeed(), cfsetispeed(), cfsetospeed(), tcgetattr()
90 The Base Definitions volume of POSIX.1‐2017, Chapter 11, General Termi‐
92 nal Interface, <termios.h>
93
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 .
104 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 .
109IEEE/The Open Group 2017 CFGETISPEED(3P)