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termios.3: Add missing details on behaviour of PARMRK 

For a serial terminal, with a specific configuration, input bytes
with value 0377 are passed to the program as two bytes, 0377 0377.

This (correct) behaviour is described in the documentation of the
GNU C Library
(https://www.gnu.org/software/libc/manual/html_node/Input-Modes.html#Input-Modes)
but not in the termios.3 man page.

Problematic configuration: INPCK set, IGNPAR not set, PARMRK set,
ISTRIP not set.

This man page problem affects several users. Examples:
* http://sourceforge.net/p/ftdi-usb-sio/mailman/message/4079724/
* http://mailman.uclinux.org/pipermail/uclinux-dev/2006-November/040984.html
* ...

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Olivier TARTROU 2015-11-05 22:38:24 +01:00 committed by Michael Kerrisk
parent cc3407d12d
commit 2d7c8f1f78
1 changed files with 13 additions and 2 deletions
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15
man3/termios.3
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@ -36,6 +36,8 @@
.\" Added a section on canonical and noncanonical mode.
.\" Enhanced the discussion of "raw" mode for cfmakeraw().
.\" Document CMSPAR.
.\" 2015-11-04, Olivier TARTROU <olivier.tartrou@gmail.com>:
.\" Reworked description of PARMRK from https://www.gnu.org/software/libc/manual/html_node/Input-Modes.html#Input-Modes
.\"
.TH TERMIOS 3 2015-03-02 "Linux" "Linux Programmer's Manual"
.SH NAME
@ -133,8 +135,17 @@ in which case it reads as the sequence \\377 \\0 \\0.
Ignore framing errors and parity errors.
.TP
.B PARMRK
If \fBIGNPAR\fP is not set, prefix a character with a parity error or
framing error with \\377 \\0.
If this bit is set, input bytes with parity or framing errors are
marked when passed to the program. This bit is meaningful only when
\fBINPCK\fP is set and \fBIGNPAR\fP is not set.
The way erroneous bytes are marked is with two preceding bytes,
\\377 and \\0. Thus, the program actually reads three bytes for one
erroneous byte received from the terminal.
If a valid byte has the value \\377, and \fBISTRIP\fP (see below) is
not set, the program might confuse it with the prefix that marks a
parity error. So a valid byte \\377 is passed to the program as two
bytes, \\377 \\377, in this case.
If neither \fBIGNPAR\fP nor \fBPARMRK\fP
is set, read a character with a parity error or framing error
as \\0.