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107 lines
4.1 KiB
Plaintext
107 lines
4.1 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "CFGETISPEED" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" cfgetispeed
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.SH NAME
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cfgetispeed \- get input baud rate
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.SH SYNOPSIS
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.LP
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\fB#include <termios.h>
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.br
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.sp
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speed_t cfgetispeed(const struct termios *\fP\fItermios_p\fP\fB);
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIcfgetispeed\fP() function shall extract the input baud rate
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from the \fBtermios\fP structure to which the
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\fItermios_p\fP argument points.
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.LP
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This function shall return exactly the value in the \fBtermios\fP
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data structure, without interpretation.
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.SH RETURN VALUE
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.LP
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Upon successful completion, \fIcfgetispeed\fP() shall return a value
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of type \fBspeed_t\fP representing the input baud
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rate.
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.SH ERRORS
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.LP
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No errors are defined.
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.LP
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None.
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.SH APPLICATION USAGE
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.LP
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None.
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.SH RATIONALE
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.LP
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The term "baud" is used historically here, but is not technically
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correct. This is properly "bits per second", which may not
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be the same as baud. However, the term is used because of the historical
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usage and understanding.
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.LP
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The \fIcfgetospeed\fP(), \fIcfgetispeed\fP(), \fIcfsetospeed\fP(),
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and \fIcfsetispeed\fP()
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functions do not take arguments as numbers, but rather as symbolic
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names. There are two reasons for this:
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.IP " 1." 4
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Historically, numbers were not used because of the way the rate was
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stored in the data structure. This is retained even though a
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function is now used.
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.LP
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.IP " 2." 4
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More importantly, only a limited set of possible rates is at all portable,
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and this constrains the application to that set.
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.LP
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.LP
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There is nothing to prevent an implementation accepting as an extension
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a number (such as 126), and since the encoding of the
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Bxxx symbols is not specified, this can be done to avoid introducing
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ambiguity.
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.LP
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Setting the input baud rate to zero was a mechanism to allow for split
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baud rates. Clarifications in this volume of
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IEEE\ Std\ 1003.1-2001 have made it possible to determine whether
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split rates are supported and to support them without
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having to treat zero as a special case. Since this functionality is
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also confusing, it has been declared obsolescent. The 0
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argument referred to is the literal constant 0, not the symbolic constant
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B0. This volume of IEEE\ Std\ 1003.1-2001 does
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not preclude B0 from being defined as the value 0; in fact, implementations
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would likely benefit from the two being equivalent.
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This volume of IEEE\ Std\ 1003.1-2001 does not fully specify whether
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the previous \fIcfsetispeed\fP() value is retained after a \fItcgetattr\fP()
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as the actual value or as zero. Therefore, conforming applications
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should
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always set both the input speed and output speed when setting either.
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.LP
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In historical implementations, the baud rate information is traditionally
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kept in \fBc_cflag\fP. Applications should be written
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to presume that this might be the case (and thus not blindly copy
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\fBc_cflag\fP), but not to rely on it in case it is in some
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other field of the structure. Setting the \fBc_cflag\fP field absolutely
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after setting a baud rate is a non-portable action
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because of this. In general, the unused parts of the flag fields might
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be used by the implementation and should not be blindly
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copied from the descriptions of one terminal device to another.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIcfgetospeed\fP() , \fIcfsetispeed\fP() , \fIcfsetospeed\fP() ,
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\fItcgetattr\fP() , the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Chapter 11, General Terminal Interface, \fI<termios.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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