On 18/09/2025 00:07, John Scott wrote:
Hello,
When using and debugging GNSS receivers, it's typical to get data over a serial
communications channel. The de facto standard format, known as NMEA-0183, sends
records as lines of US-ASCII text. Thus it is convenient for me to have this
terminal in canonical mode for the sake of reading lines but to otherwise not
have the data transformed so as to depart from the protocol: don't mangle
newline characters or do anything special with any control characters that may
be perceived, for example.
The raw combination mode to stty (but not cfmakeraw()) is in the Single UNIX
Specification under the XSI option. At
https://pubs.opengroup.org/onlinepubs/9799919799/utilities/stty.html#tag_20_116_05_06
one finds
[XSI]
raw (-raw or cooked)
Enable (disable) raw input and output. Raw mode shall be equivalent to setting:
stty cs8 erase ^- kill ^- intr ^- \
quit ^- eof ^- eol ^- -post⃰ -inpck
[/XSI]
[＊I do believe this is a mistake in the standard: "-post" should read "-opost".
I'll file an issue on The Austin Group's bug tracker soon for that.]
The GNU version of stty does more; at stty.c:1600 one finds the following:
{
/* Raw mode. */
mode->c_iflag = 0;
mode->c_oflag &= ~OPOST;
mode->c_lflag &= ~(ISIG | ICANON
#ifdef XCASE
| XCASE
#endif
);
mode->c_cc[VMIN] = 1;
mode->c_cc[VTIME] = 0;
}
The GNU behavior is non-conforming, but it's also inconvenient for me in
practice. BusyBox's stty, for comparison, doesn't have this quirk. The
command-line help on my system (maybe older than what's in Git) says
raw same as -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr
-icrnl -ixon -ixoff -icanon -opost -isig -iuclc -ixany -imaxbel -xcase min 1
time 0
and that's quite the mouthful; I'm not sure this is actually true.
In conclusion, I'd like the GNU stty raw mode to leave icanon alone, but if
this is seriously objectionable then guarding that behavior behind
POSIXLY_CORRECT would be an appreciated compromise.
Also related to this is cfmakeraw() of which there is a good summary at:
https://github.com/mitogen-hq/mitogen/wiki/cfmakeraw-notes
Note that no platforms leave ICANON set with cfmakeraw which is interesting.
It seems like ICANON is a higher level way to achieve
the same as the POSIX specified disabling of Ctrl char handling, i.e.:
erase ^- kill ^- intr ^- quit ^- eof ^- eol ^- ?
Why exactly do you need icanon enabled?
It seems like you have non standard requirements
which may be best served with specific stty settings,
which you could save with `stty -g` and set as required?
thanks,
Padraig
