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Known Problems with GNU Emacs

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Copyright (C) 1987-1989, 1993-1999, 2001-2018 Free Software Foundation,
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Inc.
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See the end of the file for license conditions.


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This file describes various problems that have been encountered
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in compiling, installing and running GNU Emacs.  Try doing C-c C-t
and browsing through the outline headers.  (See C-h m for help on
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Outline mode.)  Information about systems that are no longer supported,
and old Emacs releases, has been removed.  Consult older versions of
this file if you are interested in that information.
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* Mule-UCS doesn't work in Emacs 23 onwards
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It's completely redundant now, as far as we know.

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* Emacs startup failures
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** Emacs fails to start, complaining about missing fonts.
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A typical error message might be something like
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  No fonts match ‘-*-fixed-medium-r-*--6-*-*-*-*-*-iso8859-1’
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This happens because some X resource specifies a bad font family for
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Emacs to use.  The possible places where this specification might be are:
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  - in the X server resources database, often initialized from
    ~/.Xresources (use $ xrdb -query to find out the current state)

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  - in your ~/.Xdefaults file
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  - client-side X resource file, such as  ~/Emacs or
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    /usr/share/X11/app-defaults/Emacs
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One of these files might have bad or malformed specification of a
fontset that Emacs should use.  To fix the problem, you need to find
the problematic line(s) and correct them.
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After correcting ~/.Xresources, the new data has to be merged into the
X server resources database.  Depending on the circumstances, the
following command may do the trick.  See xrdb(1) for more information.

  $ xrdb -merge ~/.Xresources

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** Emacs aborts while starting up, only when run without X.
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This problem often results from compiling Emacs with GCC when GCC was
installed incorrectly.  The usual error in installing GCC is to
specify --includedir=/usr/include.  Installation of GCC makes
corrected copies of the system header files.  GCC is supposed to use
the corrected copies in preference to the original system headers.
Specifying --includedir=/usr/include causes the original system header
files to be used.  On some systems, the definition of ioctl in the
original system header files is invalid for ANSI C and causes Emacs
not to work.
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The fix is to reinstall GCC, and this time do not specify --includedir
when you configure it.  Then recompile Emacs.  Specifying --includedir
is appropriate only in very special cases and it should *never* be the
same directory where system header files are kept.
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** Emacs does not start, complaining that it cannot open termcap database file.
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If your system uses Terminfo rather than termcap (most modern
systems do), this could happen if the proper version of
ncurses is not visible to the Emacs configure script (i.e. it
cannot be found along the usual path the linker looks for
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libraries).  It can happen because your version of ncurses is
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obsolete, or is available only in form of binaries.
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The solution is to install an up-to-date version of ncurses in
the developer's form (header files, static libraries and
symbolic links); in some GNU/Linux distributions (e.g. Debian)
it constitutes a separate package.
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** Emacs 20 and later fails to load Lisp files at startup.
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The typical error message might be like this:
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  "Cannot open load file: fontset"
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This could happen if you compress the file lisp/subdirs.el.  That file
tells Emacs what are the directories where it should look for Lisp
files.  Emacs cannot work with subdirs.el compressed, since the
Auto-compress mode it needs for this will not be loaded until later,
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when your .emacs file is processed.  (The package 'fontset.el' is
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required to set up fonts used to display text on window systems, and
it's loaded very early in the startup procedure.)
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Similarly, any other .el file for which there's no corresponding .elc
file could fail to load if it is compressed.
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The solution is to uncompress all .el files that don't have a .elc file.
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Another possible reason for such failures is stale *.elc files
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lurking somewhere on your load-path -- see the next section.
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** Emacs prints an error at startup after upgrading from an earlier version.
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An example of such an error is:
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  x-complement-fontset-spec: "Wrong type argument: stringp, nil"
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This can be another symptom of stale *.elc files in your load-path.
The following command will print any duplicate Lisp files that are
present in load-path:
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    emacs -batch -f list-load-path-shadows
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If this command prints any file names, some of these files are stale,
and should be deleted or their directories removed from your
load-path.
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* Crash bugs
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** Emacs crashes when running in a terminal, if compiled with GCC 4.5.0
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This version of GCC is buggy: see

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  https://debbugs.gnu.org/6031
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43904
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You can work around this error in gcc-4.5 by omitting sibling call
optimization.  To do this, configure Emacs with

 CFLAGS="-g -O2 -fno-optimize-sibling-calls" ./configure

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** Emacs compiled with GCC 4.6.1 crashes on MS-Windows when C-g is pressed

This is known to happen when Emacs is compiled with MinGW GCC 4.6.1
with the -O2 option (which is the default in the Windows build).  The
reason is a bug in MinGW GCC 4.6.1; to work around, either add the
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'-fno-omit-frame-pointer' switch to GCC or compile without
optimizations ('--no-opt' switch to the configure.bat script).
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** Emacs crashes in x-popup-dialog.
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This can happen if the dialog widget cannot find the font it wants to
use.  You can work around the problem by specifying another font with
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an X resource--for example, 'Emacs.dialog*.font: 9x15' (or any font that
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happens to exist on your X server).
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** Emacs crashes when you use Bibtex mode.
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This happens if your system puts a small limit on stack size.  You can
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prevent the problem by using a suitable shell command (often 'ulimit')
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to raise the stack size limit before you run Emacs.
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Patches to raise the stack size limit automatically in 'main'
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(src/emacs.c) on various systems would be greatly appreciated.
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** Error message 'Symbol’s value as variable is void: x', followed by
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a segmentation fault and core dump.
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This has been tracked to a bug in tar!  People report that tar erroneously
added a line like this at the beginning of files of Lisp code:
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   x FILENAME, N bytes, B tape blocks
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If your tar has this problem, install GNU tar--if you can manage to
untar it :-).
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** Emacs can crash when displaying PNG images with transparency.
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This is due to a bug introduced in ImageMagick 6.8.2-3.  The bug should
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be fixed in ImageMagick 6.8.3-10.  See <URL:https://debbugs.gnu.org/13867>.
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** Crashes when displaying GIF images in Emacs built with version
libungif-4.1.0 are resolved by using version libungif-4.1.0b1.
Configure checks for the correct version, but this problem could occur
if a binary built against a shared libungif is run on a system with an
older version.
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** Emacs aborts inside the function 'tparam1'.
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This can happen if Emacs was built without terminfo support, but the
terminal's capabilities use format that is only supported by terminfo.
If your system has ncurses installed, this might happen if your
version of ncurses is broken; upgrading to a newer version of ncurses
and reconfiguring and rebuilding Emacs should solve this.
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All modern systems support terminfo, so even if ncurses is not the
problem, you should look for a way to configure Emacs so that it uses
terminfo when built.
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** Emacs crashes when using some version of the Exceed X server.
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Upgrading to a newer version of Exceed has been reported to prevent
these crashes.  You should consider switching to a free X server, such
as Xming or Cygwin/X.
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** Emacs crashes with SIGSEGV in XtInitializeWidgetClass.
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It crashes on X, but runs fine when called with option "-nw".
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This has been observed when Emacs is linked with GNU ld but without passing
the -z nocombreloc flag.  Emacs normally knows to pass the -z nocombreloc
flag when needed, so if you come across a situation where the flag is
necessary but missing, please report it via M-x report-emacs-bug.
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On platforms such as Solaris, you can also work around this problem by
configuring your compiler to use the native linker instead of GNU ld.
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** When Emacs is compiled with Gtk+, closing a display kills Emacs.
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There is a long-standing bug in GTK that prevents it from recovering
from disconnects: http://bugzilla.gnome.org/show_bug.cgi?id=85715.
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Thus, for instance, when Emacs is run as a server on a text terminal,
and an X frame is created, and the X server for that frame crashes or
exits unexpectedly, Emacs must exit to prevent a GTK error that would
result in an endless loop.
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If you need Emacs to be able to recover from closing displays, compile
it with the Lucid toolkit instead of GTK.
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** Emacs compiled with GTK crashes at startup due to X protocol error.

This is known to happen on elementary OS GNU/Linux systems.

The error message is:

  X protocol error: BadMatch (invalid parameter attributes) on protocol request 140
  When compiled with GTK, Emacs cannot recover from X disconnects.
  This is a GTK bug: https://bugzilla.gnome.org/show_bug.cgi?id=85715
  For details, see etc/PROBLEMS.
  Fatal error 6: Aborted

followed by a C backtrace.  (Sometimes the offending protocol request
number is 139.)

The relevant bug report is here:

  https://bugs.launchpad.net/elementaryos/+bug/1355274

A workaround is to set XLIB_SKIP_ARGB_VISUALS=1 in the environment
before starting Emacs, or run Emacs as root.

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** Emacs crashes when you try to view a file with complex characters.
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For example, the etc/HELLO file (as shown by C-h h).
The message "symbol lookup error: /usr/bin/emacs: undefined symbol: OTF_open"
is shown in the terminal from which you launched Emacs.
This problem only happens when you use a graphical display (ie not
with -nw) and compiled Emacs with the "libotf" library for complex
text handling.

This problem occurs because unfortunately there are two libraries
called "libotf".  One is the library for handling OpenType fonts,
http://www.m17n.org/libotf/, which is the one that Emacs expects.
The other is a library for Open Trace Format, and is used by some
versions of the MPI message passing interface for parallel
programming.

For example, on RHEL6 GNU/Linux, the OpenMPI rpm provides a version
of "libotf.so" in /usr/lib/openmpi/lib.  This directory is not
normally in the ld search path, but if you want to use OpenMPI,
you must issue the command "module load openmpi".  This adds
/usr/lib/openmpi/lib to LD_LIBRARY_PATH.  If you then start Emacs from
the same shell, you will encounter this crash.
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Ref: <URL:https://bugzilla.redhat.com/show_bug.cgi?id=844776>
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There is no good solution to this problem if you need to use both
OpenMPI and Emacs with libotf support.  The best you can do is use a
wrapper shell script (or function) "emacs" that removes the offending
element from LD_LIBRARY_PATH before starting emacs proper.
Or you could recompile Emacs with an -Wl,-rpath option that
gives the location of the correct libotf.

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* General runtime problems
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** Lisp problems
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*** Changes made to .el files do not take effect.
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You may have forgotten to recompile them into .elc files.
Then the old .elc files will be loaded, and your changes
will not be seen.  To fix this, do M-x byte-recompile-directory
and specify the directory that contains the Lisp files.
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Emacs prints a warning when loading a .elc file which is older
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than the corresponding .el file.
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Alternatively, if you set the option 'load-prefer-newer' non-nil,
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Emacs will load whichever version of a file is the newest.

*** Watch out for the EMACSLOADPATH environment variable
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EMACSLOADPATH overrides which directories the function "load" will search.
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If you observe strange problems, check for this variable in your
environment.
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*** Using epop3.el package causes Emacs to signal an error.
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The error message might be something like this:
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  "Lisp nesting exceeds max-lisp-eval-depth"
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This happens because epop3 redefines the function gethash, which is a
built-in primitive beginning with Emacs 21.1.  We don't have a patch
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for epop3 to fix it, but perhaps a newer version of epop3 corrects that.
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*** Buffers from 'with-output-to-temp-buffer' get set up in Help mode.
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Changes in Emacs 20.4 to the hooks used by that function cause
problems for some packages, specifically BBDB.  See the function's
documentation for the hooks involved.  BBDB 2.00.06 fixes the problem.
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*** The Hyperbole package causes *Help* buffers not to be displayed in
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Help mode due to setting 'temp-buffer-show-hook' rather than using
'add-hook'.  Using '(add-hook 'temp-buffer-show-hook 'help-mode-finish)'
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after loading Hyperbole should fix this.
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** Keyboard problems
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*** Unable to enter the M-| key on some German keyboards.
Some users have reported that M-| suffers from "keyboard ghosting".
This can't be fixed by Emacs, as the keypress never gets passed to it
at all (as can be verified using "xev").  You can work around this by
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typing 'ESC |' instead.
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*** "Compose Character" key does strange things when used as a Meta key.
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If you define one key to serve as both Meta and Compose Character, you
will get strange results.  In previous Emacs versions, this "worked"
in that the key acted as Meta--that's because the older Emacs versions
did not try to support Compose Character.  Now Emacs tries to do
character composition in the standard X way.  This means that you
must pick one meaning or the other for any given key.
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You can use both functions (Meta, and Compose Character) if you assign
them to two different keys.
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*** C-z just refreshes the screen instead of suspending Emacs.
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You are probably using a shell that doesn't support job control, even
though the system itself is capable of it.  Either use a different shell,
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or set the variable 'cannot-suspend' to a non-nil value.
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** Mailers and other helper programs
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*** movemail compiled with POP support can't connect to the POP server.
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This problem can occur if you do not configure --with-mailutils,
and don't have GNU Mailutils installed.  Then Emacs uses its own
version of movemail, which doesn't support secure POP connections.
To solve this, install GNU Mailutils.

Also, make sure that the 'pop' entry in /etc/services, or in the
services NIS map if your machine uses NIS, has the same port number as
the entry on the POP server.  A common error is for the POP server to
be listening on port 110, the assigned port for the POP3 protocol,
while the client is trying to connect on port 109, the assigned port
for the old POP protocol.
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*** RMAIL gets error getting new mail.
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RMAIL gets new mail from /usr/spool/mail/$USER using a program
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called 'movemail'.  This program interlocks with /bin/mail using
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the protocol defined by /bin/mail.
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There are two different protocols in general use.  One of them uses
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the 'flock' system call.  The other involves creating a lock file;
'movemail' must be able to write in /usr/spool/mail in order to do
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this.  You control which one is used by defining, or not defining,
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the macro MAIL_USE_FLOCK in config.h.
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IF YOU DON'T USE THE FORM OF INTERLOCKING THAT IS NORMAL ON YOUR
SYSTEM, YOU CAN LOSE MAIL!
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If your system uses the lock file protocol, and fascist restrictions
prevent ordinary users from writing the lock files in /usr/spool/mail,
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you may need to make 'movemail' setgid to a suitable group such as
'mail'.  To do this,  use the following commands (as root) after doing the
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make install.
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        chgrp mail movemail
        chmod 2755 movemail
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Installation normally copies movemail from the build directory to an
installation directory which is usually under /usr/local/lib.  The
installed copy of movemail is usually in the directory
/usr/local/lib/emacs/VERSION/TARGET.  You must change the group and
mode of the installed copy; changing the group and mode of the build
directory copy is ineffective.
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*** rcs2log gives you the awk error message "too many fields".
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This is due to an arbitrary limit in certain versions of awk.
The solution is to use gawk (GNU awk).
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** Problems with hostname resolution
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*** Emacs does not know your host's fully-qualified domain name.
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For example, (system-name) returns some variation on
"localhost.localdomain", rather the name you were expecting.

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You need to configure your machine with a fully qualified domain name,
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(i.e., a name with at least one "."), either in /etc/hostname
or wherever your system calls for specifying this.
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If you cannot fix the configuration, you can set the Lisp variable
mail-host-address to the value you want.
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** NFS
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*** Emacs says it has saved a file, but the file does not actually
appear on disk.
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This can happen on certain systems when you are using NFS, if the
remote disk is full.  It is due to a bug in NFS (or certain NFS
implementations), and there is apparently nothing Emacs can do to
detect the problem.  Emacs checks the failure codes of all the system
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calls involved in writing a file, including 'close'; but in the case
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where the problem occurs, none of those system calls fails.
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** PSGML conflicts with sgml-mode.
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PSGML package uses the same names of some variables (like keymap)
as built-in sgml-mode.el because it was created as a replacement
of that package.  The conflict will be shown if you load
sgml-mode.el before psgml.el.  E.g. this could happen if you edit
HTML page and then start to work with SGML or XML file.  html-mode
(from sgml-mode.el) is used for HTML file and loading of psgml.el
(for sgml-mode or xml-mode) will cause an error.
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** PCL-CVS

*** Lines are not updated or new lines are added in the buffer upon commit.

When committing files located higher in the hierarchy than the examined
directory, some versions of the CVS program return an ambiguous message
from which PCL-CVS cannot extract the full location of the committed
files.  As a result, the corresponding lines in the PCL-CVS buffer are
not updated with the new revision of these files, and new lines are
added to the top-level directory.

This can happen with CVS versions 1.12.8 and 1.12.9.  Upgrade to CVS
1.12.10 or newer to fix this problem.

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** Miscellaneous problems
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*** Editing files with very long lines is slow.

For example, simply moving through a file that contains hundreds of
thousands of characters per line is slow, and consumes a lot of CPU.
This is a known limitation of Emacs with no solution at this time.

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*** Emacs uses 100% of CPU time

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This was a known problem with some old versions of the Semantic package.
The solution was to upgrade Semantic to version 2.0pre4 (distributed
with CEDET 1.0pre4) or later.  Note that Emacs includes Semantic since
23.2, and this issue does not apply to the included version.
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*** Self-documentation messages are garbled.
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This means that the file 'etc/DOC' doesn't properly correspond
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with the Emacs executable.  Redumping Emacs and then installing the
corresponding pair of files should fix the problem.
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*** Programs running under terminal emulator do not recognize 'emacs'
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terminal type.
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The cause of this is a shell startup file that sets the TERMCAP
environment variable.  The terminal emulator uses that variable to
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provide the information on the special terminal type that Emacs emulates.
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Rewrite your shell startup file so that it does not change TERMCAP
in such a case.  You could use the following conditional which sets
it only if it is undefined.
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    if ( ! ${?TERMCAP} ) setenv TERMCAP ~/my-termcap-file
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Or you could set TERMCAP only when you set TERM--which should not
happen in a non-login shell.
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*** In Shell mode, you get a ^M at the end of every line.
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This happens to people who use tcsh, because it is trying to be too
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smart.  It sees that the Shell uses terminal type 'unknown' and turns
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on the flag to output ^M at the end of each line.  You can fix the
problem by adding this to your .cshrc file:
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    if ($?INSIDE_EMACS && $?tcsh)
        unset edit
        stty -icrnl -onlcr -echo susp ^Z
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    endif
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*** In Shell buffers using ksh, resizing a window inserts random characters.

The characters come from the PS2 prompt, but they are not followed by
a newline, which messes up the next command you type.  This strange
effect is caused by Emacs 25 and later telling the shell that its
screen size changed.

To work around the problem, customize the option
'window-adjust-process-window-size-function' to "Do not adjust process
window sizes" (Lisp value 'ignore').

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*** In Inferior Python mode, input is echoed and native completion doesn't work.
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25753>

This happens when python uses a libedit based readline module, which
is the default on macOS.  This can be worked around by installing a
GNU readline based module instead, for example, using setuptools

    sudo easy_install gnureadline

And then rename the system's readline so that it won't be loaded:

    cd /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload
    mv readline.so readline.so.bak

See <https://pypi.python.org/pypi/gnureadline> for more details on
installation.

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*** Emacs startup on GNU/Linux systems (and possibly other systems) is slow.
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This can happen if the system is misconfigured and Emacs can't get the
full qualified domain name, FQDN.  You should have your FQDN in the
/etc/hosts file, something like this:
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127.0.0.1	localhost
129.187.137.82	nuc04.t30.physik.tu-muenchen.de	nuc04
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The way to set this up may vary on non-GNU systems.
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*** Visiting files in some auto-mounted directories causes Emacs to print
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'Error reading dir-locals: (file-error "Read error" "is a directory" ...'
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This can happen if the auto-mounter mistakenly reports that
.dir-locals.el exists and is a directory.  There is nothing Emacs can
do about this, but you can avoid the issue by adding a suitable entry
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to the variable 'locate-dominating-stop-dir-regexp'.  For example, if
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the problem relates to "/smb/.dir-locals.el", set that variable
to a new value where you replace "net\\|afs" with "net\\|afs\\|smb".
(The default value already matches common auto-mount prefixes.)
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See https://lists.gnu.org/r/help-gnu-emacs/2015-02/msg00461.html .
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*** Attempting to visit remote files via ange-ftp fails.
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If the error message is "ange-ftp-file-modtime: Specified time is not
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representable", then this could happen when 'lukemftp' is used as the
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ftp client.  This was reported to happen on Debian GNU/Linux, kernel
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version 2.4.3, with 'lukemftp' 1.5-5, but might happen on other
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systems as well.  To avoid this problem, switch to using the standard
ftp client.  On a Debian system, type
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  update-alternatives --config ftp
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and then choose /usr/bin/netkit-ftp.
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*** Dired is very slow.
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This could happen if invocation of the 'df' program takes a long
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time.  Possible reasons for this include:

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  - ClearCase mounted filesystems (VOBs) that sometimes make 'df'
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    response time extremely slow (dozens of seconds);

  - slow automounters on some old versions of Unix;

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  - slow operation of some versions of 'df'.
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To work around the problem, you could either (a) set the variable
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'directory-free-space-program' to nil, and thus prevent Emacs from
invoking 'df'; (b) use 'df' from the GNU Coreutils package; or
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(c) use CVS, which is Free Software, instead of ClearCase.

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*** ps-print commands fail to find prologue files ps-prin*.ps.
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This can happen if you use an old version of X-Symbol package: it
defines compatibility functions which trick ps-print into thinking it
runs in XEmacs, and look for the prologue files in a wrong directory.

The solution is to upgrade X-Symbol to a later version.

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*** On systems with shared libraries you might encounter run-time errors
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from the dynamic linker telling you that it is unable to find some
shared libraries, for instance those for Xaw3d or image support.
These errors mean Emacs has been linked with a library whose shared
library is not in the default search path of the dynamic linker.

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Similar problems could prevent Emacs from building, since the build
process invokes Emacs several times.

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On many systems, it is possible to set LD_LIBRARY_PATH in your
environment to specify additional directories where shared libraries
can be found.

Other systems allow to set LD_RUN_PATH in a similar way, but before
Emacs is linked.  With LD_RUN_PATH set, the linker will include a
specified run-time search path in the executable.

Please refer to the documentation of your dynamic linker for details.

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*** When you run Ispell from Emacs, it reports a "misalignment" error.
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This can happen if you compiled the Ispell program to use ASCII
characters only and then try to use it from Emacs with non-ASCII
characters, like Latin-1.  The solution is to recompile Ispell with
support for 8-bit characters.
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To see whether your Ispell program supports 8-bit characters, type
this at your shell's prompt:
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     ispell -vv
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and look in the output for the string "NO8BIT".  If Ispell says
"!NO8BIT (8BIT)", your speller supports 8-bit characters; otherwise it
does not.
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To rebuild Ispell with 8-bit character support, edit the local.h file
in the Ispell distribution and make sure it does _not_ define NO8BIT.
Then rebuild the speller.
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Another possible cause for "misalignment" error messages is that the
version of Ispell installed on your machine is old.  Upgrade.
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Yet another possibility is that you are trying to spell-check a word
in a language that doesn't fit the dictionary you choose for use by
Ispell.  (Ispell can only spell-check one language at a time, because
it uses a single dictionary.)  Make sure that the text you are
spelling and the dictionary used by Ispell conform to each other.
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If your spell-checking program is Aspell, it has been reported that if
you have a personal configuration file (normally ~/.aspell.conf), it
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can cause this error.  Remove that file, execute 'ispell-kill-ispell'
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in Emacs, and then try spell-checking again.
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*** TLS problems, e.g., Gnus hangs when fetching via imaps
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https://debbugs.gnu.org/24247
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gnutls-cli 3.5.3 (2016-08-09) does not generate a "- Handshake was
completed" message that tls.el relies upon, causing affected Emacs
functions to hang.  To work around the problem, use older or newer
versions of gnutls-cli, or use Emacs's built-in gnutls support.

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* Runtime problems related to font handling
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** Characters are displayed as empty boxes or with wrong font under X.

*** This can occur when two different versions of FontConfig are used.
For example, XFree86 4.3.0 has one version and Gnome usually comes
with a newer version.  Emacs compiled with Gtk+ will then use the
newer version.  In most cases the problem can be temporarily fixed by
stopping the application that has the error (it can be Emacs or any
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other application), removing ~/.fonts.cache-1, and then starting the
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application again.  If removing ~/.fonts.cache-1 and restarting
doesn't help, the application with problem must be recompiled with the
same version of FontConfig as the rest of the system uses.  For KDE,
it is sufficient to recompile Qt.

*** Some fonts have a missing glyph and no default character.  This is
known to occur for character number 160 (no-break space) in some
fonts, such as Lucida but Emacs sets the display table for the unibyte
and Latin-1 version of this character to display a space.

*** Some of the fonts called for in your fontset may not exist on your
X server.
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Each X font covers just a fraction of the characters that Emacs
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supports.  To display the whole range of Emacs characters requires
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many different fonts, collected into a fontset.  You can remedy the
problem by installing additional fonts.
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The intlfonts distribution includes a full spectrum of fonts that can
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display all the characters Emacs supports.  The etl-unicode collection
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of fonts (available from
<https://ftp.nluug.nl/windowing/X/contrib/fonts/>) includes fonts that
can display many Unicode characters; they can also be used by ps-print
and ps-mule to print Unicode characters.
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** Under X, some characters appear improperly aligned in their lines.
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You may have bad fonts.
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** Under X, an unexpected monospace font is used as the default font.

When compiled with XFT, Emacs tries to use a default font named
"monospace".  This is a "virtual font", which the operating system
(Fontconfig) redirects to a suitable font such as DejaVu Sans Mono.
On some systems, there exists a font that is actually named Monospace,
which takes over the virtual font.  This is considered an operating
system bug; see
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https://lists.gnu.org/r/emacs-devel/2008-10/msg00696.html
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If you encounter this problem, set the default font to a specific font
in your .Xresources or initialization file.  For instance, you can put
the following in your .Xresources:

Emacs.font: DejaVu Sans Mono 12

** Certain fonts make each line take one pixel more than it should.

This is because these fonts contain characters a little taller than
the font's nominal height.  Emacs needs to make sure that lines do not
overlap.
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** Font Lock displays portions of the buffer in incorrect faces.
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By far the most frequent cause of this is a parenthesis '(' or a brace
'{' in column zero.  Font Lock assumes that such a paren is outside of
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any comment or string.  This is of course not true in general, but the
vast majority of well-formatted program source files don't have such
parens, and therefore this assumption is used to allow optimizations
in Font Lock's syntactical analysis.  These optimizations avoid some
pathological cases where jit-lock, the Just-in-Time fontification
introduced with Emacs 21.1, could significantly slow down scrolling
through the buffer, especially scrolling backwards, and also jumping
to the end of a very large buffer.
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Beginning with version 22.1, a parenthesis or a brace in column zero
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is highlighted in bold-red face if it is inside a string or a comment,
to indicate that it could interfere with Font Lock (and also with
indentation) and should be moved or escaped with a backslash.
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If you don't use large buffers, or have a very fast machine which
makes the delays insignificant, you can avoid the incorrect
fontification by setting the variable
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'font-lock-beginning-of-syntax-function' to a nil value.  (This must
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be done _after_ turning on Font Lock.)
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Another alternative is to avoid a paren in column zero.  For example,
in a Lisp string you could precede the paren with a backslash.
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** Emacs pauses for several seconds when changing the default font.
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This has been reported for fvwm 2.2.5 and the window manager of KDE
2.1.  The reason for the pause is Xt waiting for a ConfigureNotify
event from the window manager, which the window manager doesn't send.
Xt stops waiting after a default timeout of usually 5 seconds.
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A workaround for this is to add something like
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emacs.waitForWM: false
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to your X resources.  Alternatively, add '(wait-for-wm . nil)' to a
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frame's parameter list, like this:
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   (modify-frame-parameters nil '((wait-for-wm . nil)))
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(this should go into your '.emacs' file).
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** Underlines appear at the wrong position.
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This is caused by fonts having a wrong UNDERLINE_POSITION property.
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Examples are the 7x13 font on XFree86 prior to version 4.1, or the jmk
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neep font from the Debian xfonts-jmk package prior to version 3.0.17.
To circumvent this problem, set x-use-underline-position-properties
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to nil in your '.emacs'.
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To see what is the value of UNDERLINE_POSITION defined by the font,
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type 'xlsfonts -lll FONT' and look at the font's UNDERLINE_POSITION property.
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** When using Exceed, fonts sometimes appear too tall.
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When the display is set to an Exceed X-server and fonts are specified
(either explicitly with the -fn option or implicitly with X resources)
then the fonts may appear "too tall".  The actual character sizes are
correct but there is too much vertical spacing between rows,  which
gives the appearance of "double spacing".
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To prevent this, turn off the Exceed's "automatic font substitution"
feature (in the font part of the configuration window).
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** Subscript/superscript text in TeX is hard to read.

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If 'tex-fontify-script' is non-nil, tex-mode displays
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subscript/superscript text in the faces subscript/superscript, which
are smaller than the normal font and lowered/raised.  With some fonts,
nested superscripts (say) can be hard to read.  Switching to a
different font, or changing your antialiasing setting (on an LCD
screen), can both make the problem disappear.  Alternatively, customize
the following variables: tex-font-script-display (how much to
lower/raise); tex-suscript-height-ratio (how much smaller than
normal); tex-suscript-height-minimum (minimum height).

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** Screen refresh is slow when there are special characters for which no suitable font is available

If the display is too slow in refreshing when you scroll to a new
region, or when you edit the buffer, it might be due to the fact that
some characters cannot be displayed in the default font, and Emacs is
spending too much time in looking for a suitable font to display them.

You can suspect this if you have several characters that are displayed
as small rectangles containing a hexadecimal code inside.

The solution is to install the appropriate fonts on your machine. For
instance if you are editing a text with a lot of math symbols, then
installing a font like 'Symbola' should solve this problem.

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Another reason for slow display is reportedly the nerd-fonts
installation, even when Symbola is installed as well.  Uninstalling
nerd-fonts was reported to solve the problem in that case.

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** Emacs running on GNU/Linux system with the m17n library Ver.1.7.1 or the
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earlier version has a problem with rendering Bengali script.
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The problem can be fixed by installing the newer version of the m17n
library (if any), or by following this procedure:

1. Locate the file BENG-OTF.flt installed on your system as part of the
m17n library.  Usually it is under the directory /usr/share/m17n.

2. Apply the following patch to BENG-OTF.flt

------------------------------------------------------------
diff --git a/FLT/BENG-OTF.flt b/FLT/BENG-OTF.flt
index 45cc554..0cc5e76 100644
--- a/FLT/BENG-OTF.flt
+++ b/FLT/BENG-OTF.flt
@@ -232,7 +232,7 @@
  (lang-forms
   (cond
    ("(.H)J" (1 :otf=beng=half+))
-   (".H" :otf=beng=blwf,half,vatu+)
+   (".+H" :otf=beng=blwf,half,vatu+)
    ("." =)))

  (post
------------------------------------------------------------

If you can't modify that file directly, copy it to the directory
~/.m17n.d/ (create it if it doesn't exist), and apply the patch.

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** Emacs running on GNU/Linux system with the m17n library Ver.1.7.1 or the
earlier version has a problem with rendering Lao script with OpenType font.

The problem can be fixed by installing the newer version of the m17n
library (if any), or by following this procedure:

1. Locate the file LAOO-OTF.flt installed on your system as part of the
m17n library.  Usually it is under the directory /usr/share/m17n.

2. Apply the following patch to LAOO-OTF.flt

------------------------------------------------------------
diff --git a/FLT/LAOO-OTF.flt b/FLT/LAOO-OTF.flt
index 5504171..431adf8 100644
--- a/FLT/LAOO-OTF.flt
+++ b/FLT/LAOO-OTF.flt
@@ -3,7 +3,7 @@
 ;; See the end for copying conditions.

 (font layouter laoo-otf nil
-      (font (nil phetsarath\ ot unicode-bmp)))
+      (font (nil nil unicode-bmp :otf=lao\ )))

 ;;; <li> LAOO-OTF.flt

------------------------------------------------------------

If you can't modify that file directly, copy it to the directory
~/.m17n.d/ (create it if it doesn't exist), and apply the patch.

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* Internationalization problems
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** M-{ does not work on a Spanish PC keyboard.

Many Spanish keyboards seem to ignore that combination.  Emacs can't
do anything about it.

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** International characters aren't displayed under X.

*** Missing X fonts
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XFree86 4 contains many fonts in iso10646-1 encoding which have
minimal character repertoires (whereas the encoding part of the font
name is meant to be a reasonable indication of the repertoire
according to the XLFD spec).  Emacs may choose one of these to display
characters from the mule-unicode charsets and then typically won't be
able to find the glyphs to display many characters.  (Check with C-u
C-x = .)  To avoid this, you may need to use a fontset which sets the
font for the mule-unicode sets explicitly.  E.g. to use GNU unifont,
include in the fontset spec:
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mule-unicode-2500-33ff:-gnu-unifont-*-iso10646-1,\
mule-unicode-e000-ffff:-gnu-unifont-*-iso10646-1,\
mule-unicode-0100-24ff:-gnu-unifont-*-i