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Suppose an application tries to print a double-width character
starting in the rightmost column of the screen, so that we apply our
emergency fix of wrapping to the next line immediately and printing
the character in the first two columns. Suppose they then backspace
twice, taking the cursor to the RHS and then the LHS of that
character. What should happen if they backspace a third time?
Our previous behaviour was to completely ignore the unusual situation,
and do the same thing we'd do in any other backspace from column 0:
anti-wrap the cursor to the last column of the previous line, leaving
it in the empty character cell that was skipped when the DW char
couldn't be printed in it.
But I think this isn't the best response, because it breaks the
invariant that printing N columns' worth of graphic characters and
then backspacing N times should leave the cursor on the first of those
characters. If I print "a가" (for example) and then backspace three
times, I want the cursor on the a, _even_ if weird line wrapping
behaviour happened somewhere in that sequence.
(Rationale: this helps naïve terminal applications which don't even
know what the terminal width is, and aren't tracking their absolute x
position. In particular, the simplistic line-based input systems that
appear in OS kernels and our own lineedit.c will want to emit a fixed
number of backspace-space-backspace sequences to delete characters
previously entered on to the line by the user. They still need to
check the wcwidth of the characters they're emitting, so that they can
BSB twice for a DW character or 0 times for a combining one, but it
would be *hugely* more awkward for them to ask the terminal where the
cursor is so that they can take account of difficult line wraps!)
We already have the ability to _recognise_ this situation: on a line
that was wrapped in this unusual way, we set the LATTR_WRAPPED2 line
attribute flag, to prevent the empty rightmost column from injecting
an unwanted space into copy-pastes from the terminal. Now we also use
the same flag to cause the backspace control character to do something
interesting.
This was the fix that inspired me to start writing test_terminal,
because I knew it was touching a delicate area. However, in the course
of writing this fix and its tests, I encountered two (!) further bugs,
which I'll fix in followup commits!
(cherry picked from commit
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charset | ||
cmake | ||
contrib | ||
crypto | ||
doc | ||
icons | ||
keygen | ||
otherbackends | ||
proxy | ||
ssh | ||
stubs | ||
terminal | ||
test | ||
unix | ||
utils | ||
windows | ||
.gitignore | ||
aqsync.c | ||
be_list.c | ||
Buildscr | ||
Buildscr.cv | ||
callback.c | ||
cgtest.c | ||
CHECKLST.txt | ||
clicons.c | ||
CMakeLists.txt | ||
cmdgen.c | ||
cmdline.c | ||
config.c | ||
console.c | ||
console.h | ||
defs.h | ||
dialog.c | ||
dialog.h | ||
errsock.c | ||
import.c | ||
LATEST.VER | ||
ldisc.c | ||
LICENCE | ||
licence.pl | ||
logging.c | ||
marshal.h | ||
misc.h | ||
mksrcarc.sh | ||
mkunxarc.sh | ||
mpint.h | ||
network.h | ||
pageant.c | ||
pageant.h | ||
pinger.c | ||
pscp.c | ||
psftp.c | ||
psftp.h | ||
psftpcommon.c | ||
psocks.c | ||
psocks.h | ||
putty.h | ||
puttymem.h | ||
README | ||
release.pl | ||
settings.c | ||
sign.sh | ||
ssh.h | ||
sshcr.h | ||
sshkeygen.h | ||
sshpubk.c | ||
sshrand.c | ||
storage.h | ||
timing.c | ||
tree234.h | ||
version.h | ||
x11disp.c |
This is the README for PuTTY, a free Windows and Unix Telnet and SSH client. PuTTY is built using CMake <https://cmake.org/>. To compile in the simplest way (on any of Linux, Windows or Mac), run these commands in the source directory: cmake . cmake --build . Then, to install in the simplest way on Linux or Mac: cmake --build . --target install On Unix, pterm would like to be setuid or setgid, as appropriate, to permit it to write records of user logins to /var/run/utmp and /var/log/wtmp. (Of course it will not use this privilege for anything else, and in particular it will drop all privileges before starting up complex subsystems like GTK.) The cmake install step doesn't attempt to add these privileges, so if you want user login recording to work, you should manually ch{own,grp} and chmod the pterm binary yourself after installation. If you don't do this, pterm will still work, but not update the user login databases. Documentation (in various formats including Windows Help and Unix `man' pages) is built from the Halibut (`.but') files in the `doc' subdirectory. If you aren't using one of our source snapshots, you'll need to do this yourself. Halibut can be found at <https://www.chiark.greenend.org.uk/~sgtatham/halibut/>. The PuTTY home web site is https://www.chiark.greenend.org.uk/~sgtatham/putty/ If you want to send bug reports or feature requests, please read the Feedback section of the web site before doing so. Sending one-line reports saying `it doesn't work' will waste your time as much as ours. See the file LICENCE for the licence conditions.