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>

diff --git a/man3/termios.3 b/man3/termios.3
index 1ea52369ca8732d89e681c372af1f15898a96d9c..3f38c90b128feded7e71cc4e00e11f4d4bc578b8 100644 (file)
--- a/man3/termios.3
+++ b/man3/termios.3
@@ -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.