[bpt/emacs.git] / man / trouble.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@iftex
@chapter Dealing with Common Problems

  If you type an Emacs command you did not intend, the results are often
mysterious.  This chapter tells what you can do to cancel your mistake or
recover from a mysterious situation.  Emacs bugs and system crashes are
also considered.
@end iftex

@node Quitting, Lossage, Customization, Top
@section Quitting and Aborting
@cindex quitting

@table @kbd
@item C-g
@itemx C-@key{BREAK}@r{ (MS-DOS only)}
Quit (@code{keyboard-quit}): cancel running or partially typed command.
@item C-]
Abort innermost recursive editing level and cancel the command which
invoked it (@code{abort-recursive-edit}).
@item @key{ESC} @key{ESC} @key{ESC}
Either quit or abort, whichever makes sense (@code{keyboard-escape-quit}).
@item M-x top-level
Abort all recursive editing levels that are currently executing.
@item C-x u
Cancel a previously made change in the buffer contents (@code{undo}).
@end table

  There are two ways of canceling commands which are not finished
executing: @dfn{quitting} with @kbd{C-g}, and @dfn{aborting} with
@kbd{C-]} or @kbd{M-x top-level}.  Quitting cancels a partially typed
command or one which is already running.  Aborting exits a recursive
editing level and cancels the command that invoked the recursive edit.
(@xref{Recursive Edit}.)

@cindex quitting
@findex keyboard-quit
@kindex C-g
  Quitting with @kbd{C-g} is used for getting rid of a partially typed
command, or a numeric argument that you don't want.  It also stops a
running command in the middle in a relatively safe way, so you can use
it if you accidentally give a command which takes a long time.  In
particular, it is safe to quit out of killing; either your text will
@emph{all} still be in the buffer, or it will @emph{all} be in the kill
ring (or maybe both).  Quitting an incremental search does special
things documented under searching; in general, it may take two
successive @kbd{C-g} characters to get out of a search
(@pxref{Incremental Search}).

  On MS-DOS, the character @kbd{C-@key{BREAK}} serves as a quit character
like @kbd{C-g}.  The reason is that it is not feasible, on MS-DOS, to
recognize @kbd{C-g} while a command is running, between interactions
with the user.  By contrast, it @emph{is} feasible to recognize
@kbd{C-@key{BREAK}} at all times.  @xref{MS-DOS Input}.

  @kbd{C-g} works by setting the variable @code{quit-flag} to @code{t}
the instant @kbd{C-g} is typed; Emacs Lisp checks this variable
frequently and quits if it is non-@code{nil}.  @kbd{C-g} is only
actually executed as a command if you type it while Emacs is waiting for
input.

  If you quit with @kbd{C-g} a second time before the first @kbd{C-g} is
recognized, you activate the ``emergency escape'' feature and return to
the shell.  @xref{Emergency Escape}.

@cindex NFS and quitting
  There may be times when you cannot quit.  When Emacs is waiting for
the operating system to do something, quitting is impossible unless
special pains are taken for the particular system call within Emacs
where the waiting occurs.  We have done this for the system calls that
users are likely to want to quit from, but it's possible you will find
another.  In one very common case---waiting for file input or output
using NFS---Emacs itself knows how to quit, but most NFS implementations
simply do not allow user programs to stop waiting for NFS when the NFS
server is hung.

@cindex aborting recursive edit
@findex abort-recursive-edit
@kindex C-]
  Aborting with @kbd{C-]} (@code{abort-recursive-edit}) is used to get
out of a recursive editing level and cancel the command which invoked
it.  Quitting with @kbd{C-g} does not do this, and could not do this,
because it is used to cancel a partially typed command @emph{within} the
recursive editing level.  Both operations are useful.  For example, if
you are in a recursive edit and type @kbd{C-u 8} to enter a numeric
argument, you can cancel that argument with @kbd{C-g} and remain in the
recursive edit.

@findex keyboard-escape-quit
@kindex ESC ESC ESC
  The command @kbd{@key{ESC} @key{ESC} @key{ESC}}
(@code{keyboard-escape-quit}) can either quit or abort.  This key was
defined because @key{ESC} is used to ``get out'' in many PC programs.
It can cancel a prefix argument, clear a selected region, or get out of
a Query Replace, like @kbd{C-g}.  It can get out of the minibuffer or a
recursive edit, like @kbd{C-]}.  It can also get out of splitting the
frame into multiple windows, like @kbd{C-x 1}.  One thing it cannot do,
however, is stop a command that is running.  That's because it executes
as an ordinary command, and Emacs doesn't notice it until it is ready
for a command.

@findex top-level
  The command @kbd{M-x top-level} is equivalent to ``enough'' @kbd{C-]}
commands to get you out of all the levels of recursive edits that you
are in.  @kbd{C-]} gets you out one level at a time, but @kbd{M-x
top-level} goes out all levels at once.  Both @kbd{C-]} and @kbd{M-x
top-level} are like all other commands, and unlike @kbd{C-g}, in that
they take effect only when Emacs is ready for a command.  @kbd{C-]} is
an ordinary key and has its meaning only because of its binding in the
keymap.  @xref{Recursive Edit}.

  @kbd{C-x u} (@code{undo}) is not strictly speaking a way of canceling
a command, but you can think of it as canceling a command that already
finished executing.  @xref{Undo}.

@node Lossage, Bugs, Quitting, Top
@section Dealing with Emacs Trouble

  This section describes various conditions in which Emacs fails to work
normally, and how to recognize them and correct them.

@menu
* DEL Gets Help::       What to do if @key{DEL} doesn't delete.
* Stuck Recursive::     `[...]' in mode line around the parentheses.
* Screen Garbled::      Garbage on the screen.
* Text Garbled::        Garbage in the text.
* Unasked-for Search::  Spontaneous entry to incremental search.
* Memory Full::         How to cope when you run out of memory.
* After a Crash::       Recovering editing in an Emacs session that crashed.
* Emergency Escape::    Emergency escape---
                          What to do if Emacs stops responding.
* Total Frustration::   When you are at your wits' end.
@end menu

@node DEL Gets Help
@subsection If @key{DEL} Fails to Delete

  If you find that @key{DEL} enters Help like @kbd{Control-h} instead of
deleting a character, your terminal is sending the wrong code for
@key{DEL}.  You can work around this problem by changing the keyboard
translation table (@pxref{Keyboard Translations}).

@node Stuck Recursive
@subsection Recursive Editing Levels

  Recursive editing levels are important and useful features of Emacs, but
they can seem like malfunctions to the user who does not understand them.

  If the mode line has square brackets @samp{[@dots{}]} around the parentheses
that contain the names of the major and minor modes, you have entered a
recursive editing level.  If you did not do this on purpose, or if you
don't understand what that means, you should just get out of the recursive
editing level.  To do so, type @kbd{M-x top-level}.  This is called getting
back to top level.  @xref{Recursive Edit}.

@node Screen Garbled
@subsection Garbage on the Screen

  If the data on the screen looks wrong, the first thing to do is see
whether the text is really wrong.  Type @kbd{C-l}, to redisplay the
entire screen.  If the screen appears correct after this, the problem
was entirely in the previous screen update.  (Otherwise, see @ref{Text
Garbled}.)

  Display updating problems often result from an incorrect termcap entry
for the terminal you are using.  The file @file{etc/TERMS} in the Emacs
distribution gives the fixes for known problems of this sort.
@file{INSTALL} contains general advice for these problems in one of its
sections.  Very likely there is simply insufficient padding for certain
display operations.  To investigate the possibility that you have this sort
of problem, try Emacs on another terminal made by a different manufacturer.
If problems happen frequently on one kind of terminal but not another kind,
it is likely to be a bad termcap entry, though it could also be due to a
bug in Emacs that appears for terminals that have or that lack specific
features.

@node Text Garbled
@subsection Garbage in the Text

  If @kbd{C-l} shows that the text is wrong, try undoing the changes to it
using @kbd{C-x u} until it gets back to a state you consider correct.  Also
try @kbd{C-h l} to find out what command you typed to produce the observed
results.

  If a large portion of text appears to be missing at the beginning or
end of the buffer, check for the word @samp{Narrow} in the mode line.
If it appears, the text you don't see is probably still present, but
temporarily off-limits.  To make it accessible again, type @kbd{C-x n
w}.  @xref{Narrowing}.

@node Unasked-for Search
@subsection Spontaneous Entry to Incremental Search

  If Emacs spontaneously displays @samp{I-search:} at the bottom of the
screen, it means that the terminal is sending @kbd{C-s} and @kbd{C-q}
according to the poorly designed xon/xoff ``flow control'' protocol.

  If this happens to you, your best recourse is to put the terminal in a
mode where it will not use flow control, or give it so much padding that
it will never send a @kbd{C-s}.  (One way to increase the amount of
padding is to set the variable @code{baud-rate} to a larger value.  Its
value is the terminal output speed, measured in the conventional units
of baud.)

@cindex flow control
@cindex xon-xoff
@findex enable-flow-control
  If you don't succeed in turning off flow control, the next best thing
is to tell Emacs to cope with it.  To do this, call the function
@code{enable-flow-control}.

@findex enable-flow-control-on
  Typically there are particular terminal types with which you must use
flow control.  You can conveniently ask for flow control on those
terminal types only, using @code{enable-flow-control-on}.  For example,
if you find you must use flow control on VT-100 and H19 terminals, put
the following in your @file{.emacs} file:

@example
(enable-flow-control-on "vt100" "h19")
@end example

  When flow control is enabled, you must type @kbd{C-\} to get the
effect of a @kbd{C-s}, and type @kbd{C-^} to get the effect of a
@kbd{C-q}.  (These aliases work by means of keyboard translations; see
@ref{Keyboard Translations}.)

@node Memory Full
@subsection Running out of Memory
@cindex memory full
@cindex out of memory

  If you get the error message @samp{Virtual memory exceeded}, save your
modified buffers with @kbd{C-x s}.  This method of saving them has the
smallest need for additional memory.  Emacs keeps a reserve of memory
which it makes available when this error happens; that should be enough
to enable @kbd{C-x s} to complete its work.

  Once you have saved your modified buffers, you can exit this Emacs job
and start another, or you can use @kbd{M-x kill-some-buffers} to free
space in the current Emacs job.  If you kill buffers containing a
substantial amount of text, you can safely go on editing.  Emacs refills
its memory reserve automatically when it sees sufficient free space
available, in case you run out of memory another time.

  Do not use @kbd{M-x buffer-menu} to save or kill buffers when you run
out of memory, because the buffer menu needs a fair amount memory
itself, and the reserve supply may not be enough.

@node After a Crash
@subsection Recovery After a Crash

  If Emacs or the computer crashes, you can recover the files you were
editing at the time of the crash from their auto-save files.  To do
this, start Emacs again and type the command @kbd{M-x recover-session}.

  This command initially displays a buffer which lists interrupted
session files, each with its date.  You must choose which session to
recover from.  Typically the one you want is the most recent one.  Move
point to the one you choose, and type @kbd{C-c C-c}.

  Then @code{recover-session} asks about each of the files that you were
editing during that session; it asks whether to recover that file.  If
you answer @kbd{y} for a file, it shows the dates of that file and its
auto-save file, then asks once again whether to recover that file.  For
the second question, you must confirm with @kbd{yes}.  If you do, Emacs
visits the file but gets the text from the auto-save file.

  When @code{recover-session} is done, the files you've chosen to
recover are present in Emacs buffers.  You should then save them.  Only
this---saving them---updates the files themselves.

@node Emergency Escape
@subsection Emergency Escape

  Because at times there have been bugs causing Emacs to loop without
checking @code{quit-flag}, a special feature causes Emacs to be suspended
immediately if you type a second @kbd{C-g} while the flag is already set,
so you can always get out of GNU Emacs.  Normally Emacs recognizes and
clears @code{quit-flag} (and quits!) quickly enough to prevent this from
happening.  (On MS-DOS and compatible systems, type @kbd{C-@key{BREAK}}
twice.)

  When you resume Emacs after a suspension caused by multiple @kbd{C-g}, it
asks two questions before going back to what it had been doing:

@example
Auto-save? (y or n)
Abort (and dump core)? (y or n)
@end example

@noindent
Answer each one with @kbd{y} or @kbd{n} followed by @key{RET}.

  Saying @kbd{y} to @samp{Auto-save?} causes immediate auto-saving of all
modified buffers in which auto-saving is enabled.

  Saying @kbd{y} to @samp{Abort (and dump core)?} causes an illegal instruction to be
executed, dumping core.  This is to enable a wizard to figure out why Emacs
was failing to quit in the first place.  Execution does not continue
after a core dump.  If you answer @kbd{n}, execution does continue.  With
luck, GNU Emacs will ultimately check @code{quit-flag} and quit normally.
If not, and you type another @kbd{C-g}, it is suspended again.

  If Emacs is not really hung, just slow, you may invoke the double
@kbd{C-g} feature without really meaning to.  Then just resume and answer
@kbd{n} to both questions, and you will arrive at your former state.
Presumably the quit you requested will happen soon.

  The double-@kbd{C-g} feature is turned off when Emacs is running under
the X Window System, since you can use the window manager to kill Emacs
or to create another window and run another program.

  On MS-DOS and compatible systems, the emergency escape feature is
sometimes unavailable, even if you press @kbd{C-@key{BREAK}} twice, when
some system call (MS-DOS or BIOS) hangs, or when Emacs is stuck in a
very tight endless loop (in C code, @strong{not} in Lisp code).

@node Total Frustration
@subsection Help for Total Frustration
@cindex Eliza
@cindex doctor

  If using Emacs (or something else) becomes terribly frustrating and none
of the techniques described above solve the problem, Emacs can still help
you.

  First, if the Emacs you are using is not responding to commands, type
@kbd{C-g C-g} to get out of it and then start a new one.

@findex doctor
  Second, type @kbd{M-x doctor @key{RET}}.

  The doctor will help you feel better.  Each time you say something to
the doctor, you must end it by typing @key{RET} @key{RET}.  This lets
the doctor know you are finished.

@node Bugs, Contributing, Lossage, Top
@section Reporting Bugs

@cindex bugs
  Sometimes you will encounter a bug in Emacs.  Although we cannot
promise we can or will fix the bug, and we might not even agree that it
is a bug, we want to hear about problems you encounter.  Often we agree
they are bugs and want to fix them.

  To make it possible for us to fix a bug, you must report it.  In order
to do so effectively, you must know when and how to do it.

@menu
* Criteria:  Bug Criteria.       Have you really found a bug?
* Understanding Bug Reporting::  How to report a bug effectively.
* Checklist::                    Steps to follow for a good bug report.
* Sending Patches::              How to send a patch for GNU Emacs.
@end menu

@node Bug Criteria
@subsection When Is There a Bug

  If Emacs executes an illegal instruction, or dies with an operating
system error message that indicates a problem in the program (as opposed to
something like ``disk full''), then it is certainly a bug.

  If Emacs updates the display in a way that does not correspond to what is
in the buffer, then it is certainly a bug.  If a command seems to do the
wrong thing but the problem corrects itself if you type @kbd{C-l}, it is a
case of incorrect display updating.

  Taking forever to complete a command can be a bug, but you must make
certain that it was really Emacs's fault.  Some commands simply take a
long time.  Type @kbd{C-g} (@kbd{C-@key{BREAK}} on MS-DOS) and then @kbd{C-h l}
to see whether the input Emacs received was what you intended to type;
if the input was such that you @emph{know} it should have been processed
quickly, report a bug.  If you don't know whether the command should
take a long time, find out by looking in the manual or by asking for
assistance.

  If a command you are familiar with causes an Emacs error message in a
case where its usual definition ought to be reasonable, it is probably a
bug.

  If a command does the wrong thing, that is a bug.  But be sure you know
for certain what it ought to have done.  If you aren't familiar with the
command, or don't know for certain how the command is supposed to work,
then it might actually be working right.  Rather than jumping to
conclusions, show the problem to someone who knows for certain.

  Finally, a command's intended definition may not be best for editing
with.  This is a very important sort of problem, but it is also a matter of
judgment.  Also, it is easy to come to such a conclusion out of ignorance
of some of the existing features.  It is probably best not to complain
about such a problem until you have checked the documentation in the usual
ways, feel confident that you understand it, and know for certain that what
you want is not available.  If you are not sure what the command is
supposed to do after a careful reading of the manual, check the index and
glossary for any terms that may be unclear.

  If after careful rereading of the manual you still do not understand
what the command should do, that indicates a bug in the manual, which
you should report.  The manual's job is to make everything clear to
people who are not Emacs experts---including you.  It is just as
important to report documentation bugs as program bugs.

  If the on-line documentation string of a function or variable disagrees
with the manual, one of them must be wrong; that is a bug.

@node Understanding Bug Reporting
@subsection Understanding Bug Reporting

@findex emacs-version
  When you decide that there is a bug, it is important to report it and to
report it in a way which is useful.  What is most useful is an exact
description of what commands you type, starting with the shell command to
run Emacs, until the problem happens.

  The most important principle in reporting a bug is to report
@emph{facts}.  Hypotheses and verbal descriptions are no substitute for
the detailed raw data.  Reporting the facts is straightforward, but many
people strain to posit explanations and report them instead of the
facts.  If the explanations are based on guesses about how Emacs is
implemented, they will be useless; meanwhile, lacking the facts, we will
have no real information about the bug.

  For example, suppose that you type @kbd{C-x C-f /glorp/baz.ugh
@key{RET}}, visiting a file which (you know) happens to be rather large,
and Emacs displayed @samp{I feel pretty today}.  The best way to report
the bug is with a sentence like the preceding one, because it gives all
the facts.

  A bad way would be to assume that the problem is due to the size of
the file and say, ``I visited a large file, and Emacs displayed @samp{I
feel pretty today}.''  This is what we mean by ``guessing
explanations.''  The problem is just as likely to be due to the fact
that there is a @samp{z} in the file name.  If this is so, then when we
got your report, we would try out the problem with some ``large file,''
probably with no @samp{z} in its name, and not see any problem.  There
is no way in the world that we could guess that we should try visiting a
file with a @samp{z} in its name.

  Alternatively, the problem might be due to the fact that the file starts
with exactly 25 spaces.  For this reason, you should make sure that you
inform us of the exact contents of any file that is needed to reproduce the
bug.  What if the problem only occurs when you have typed the @kbd{C-x C-a}
command previously?  This is why we ask you to give the exact sequence of
characters you typed since starting the Emacs session.

  You should not even say ``visit a file'' instead of @kbd{C-x C-f} unless
you @emph{know} that it makes no difference which visiting command is used.
Similarly, rather than saying ``if I have three characters on the line,''
say ``after I type @kbd{@key{RET} A B C @key{RET} C-p},'' if that is
the way you entered the text.@refill

  So please don't guess any explanations when you report a bug.  If you
want to actually @emph{debug} the problem, and report explanations that
are more than guesses, that is useful---but please include the facts as
well.

@node Checklist
@subsection Checklist for Bug Reports

@cindex reporting bugs
  The best way to send a bug report is to mail it electronically to the
Emacs maintainers at @samp{bug-gnu-emacs@@gnu.org}, or to
@samp{emacs-pretest-bug@@gnu.org} if you are pretesting an Emacs beta
release.  (If you want to suggest a change as an improvement, use the
same address.)

  If you'd like to read the bug reports, you can find them on the
newsgroup @samp{gnu.emacs.bug}; keep in mind, however, that as a
spectator you should not criticize anything about what you see there.
The purpose of bug reports is to give information to the Emacs
maintainers.  Spectators are welcome only as long as they do not
interfere with this.  In particular, some bug reports contain large
amounts of data; spectators should not complain about this.

  Please do not post bug reports using netnews; mail is more reliable
than netnews about reporting your correct address, which we may need in
order to ask you for more information.

  If you can't send electronic mail, then mail the bug report on paper
or machine-readable media to this address:

@format
GNU Emacs Bugs
Free Software Foundation
59 Temple Place, Suite 330
Boston, MA 02111-1307 USA
@end format

  We do not promise to fix the bug; but if the bug is serious,
or ugly, or easy to fix, chances are we will want to.

@findex report-emacs-bug
  A convenient way to send a bug report for Emacs is to use the command
@kbd{M-x report-emacs-bug}.  This sets up a mail buffer (@pxref{Sending
Mail}) and automatically inserts @emph{some} of the essential
information.  However, it cannot supply all the necessary information;
you should still read and follow the guidelines below, so you can enter
the other crucial information by hand before you send the message.

  To enable maintainers to investigate a bug, your report
should include all these things:

@itemize @bullet
@item
The version number of Emacs.  Without this, we won't know whether there
is any point in looking for the bug in the current version of GNU
Emacs.

You can get the version number by typing @kbd{M-x emacs-version
@key{RET}}.  If that command does not work, you probably have something
other than GNU Emacs, so you will have to report the bug somewhere
else.

@item
The type of machine you are using, and the operating system name and
version number.  @kbd{M-x emacs-version @key{RET}} provides this
information too.  Copy its output from the @samp{*Messages*} buffer, so
that you get it all and get it accurately.

@item
The operands given to the @code{configure} command when Emacs was
installed.

@item
A complete list of any modifications you have made to the Emacs source.
(We may not have time to investigate the bug unless it happens in an
unmodified Emacs.  But if you've made modifications and you don't tell
us, you are sending us on a wild goose chase.)

Be precise about these changes.  A description in English is not
enough---send a context diff for them.

Adding files of your own, or porting to another machine, is a
modification of the source.

@item
Details of any other deviations from the standard procedure for installing
GNU Emacs.

@item
The complete text of any files needed to reproduce the bug.

  If you can tell us a way to cause the problem without visiting any files,
please do so.  This makes it much easier to debug.  If you do need files,
make sure you arrange for us to see their exact contents.  For example, it
can often matter whether there are spaces at the ends of lines, or a
newline after the last line in the buffer (nothing ought to care whether
the last line is terminated, but try telling the bugs that).

@item
The precise commands we need to type to reproduce the bug.

@findex open-dribble-file
@cindex dribble file
  The easy way to record the input to Emacs precisely is to write a
dribble file.  To start the file, execute the Lisp expression

@example
(open-dribble-file "~/dribble")
@end example

@noindent
using @kbd{M-:} or from the @samp{*scratch*} buffer just after
starting Emacs.  From then on, Emacs copies all your input to the
specified dribble file until the Emacs process is killed.

@item
@findex open-termscript
@cindex termscript file
@cindex @env{TERM} environment variable
For possible display bugs, the terminal type (the value of environment
variable @env{TERM}), the complete termcap entry for the terminal from
@file{/etc/termcap} (since that file is not identical on all machines),
and the output that Emacs actually sent to the terminal.

The way to collect the terminal output is to execute the Lisp expression

@example
(open-termscript "~/termscript")
@end example

@noindent
using @kbd{M-:} or from the @samp{*scratch*} buffer just after
starting Emacs.  From then on, Emacs copies all terminal output to the
specified termscript file as well, until the Emacs process is killed.
If the problem happens when Emacs starts up, put this expression into
your @file{.emacs} file so that the termscript file will be open when
Emacs displays the screen for the first time.

Be warned: it is often difficult, and sometimes impossible, to fix a
terminal-dependent bug without access to a terminal of the type that
stimulates the bug.@refill

@item
If non-ASCII text or internationalization is relevant, the locale that
was current when you started Emacs.  You can use this shell command to
view the relevant values:

@example
echo LC_ALL=$LC_ALL LC_CTYPE=$LC_CTYPE LANG=$LANG
@end example

You can use the @kbd{M-!} command to execute the shell command from
Emacs, and then copy the output from the @samp{*Messages*} buffer into
the bug report.

@item
A description of what behavior you observe that you believe is
incorrect.  For example, ``The Emacs process gets a fatal signal,'' or,
``The resulting text is as follows, which I think is wrong.''

Of course, if the bug is that Emacs gets a fatal signal, then one can't
miss it.  But if the bug is incorrect text, the maintainer might fail to
notice what is wrong.  Why leave it to chance?

Even if the problem you experience is a fatal signal, you should still
say so explicitly.  Suppose something strange is going on, such as, your
copy of the source is out of sync, or you have encountered a bug in the
C library on your system.  (This has happened!)  Your copy might crash
and the copy here might not.  If you @emph{said} to expect a crash, then
when Emacs here fails to crash, we would know that the bug was not
happening.  If you don't say to expect a crash, then we would not know
whether the bug was happening---we would not be able to draw any
conclusion from our observations.

@item
If the manifestation of the bug is an Emacs error message, it is
important to report the precise text of the error message, and a
backtrace showing how the Lisp program in Emacs arrived at the error.

To get the error message text accurately, copy it from the
@samp{*Messages*} buffer into the bug report.  Copy all of it, not just
part.

To make a backtrace for the error, evaluate the Lisp expression
@code{(setq @w{debug-on-error t})} before the error happens (that is to
say, you must execute that expression and then make the bug happen).
This causes the error to run the Lisp debugger, which shows you a
backtrace.  Copy the text of the debugger's backtrace into the bug
report.

This use of the debugger is possible only if you know how to make the
bug happen again.  If you can't make it happen again, at least copy
the whole error message.

@item
Check whether any programs you have loaded into the Lisp world,
including your @file{.emacs} file, set any variables that may affect the
functioning of Emacs.  Also, see whether the problem happens in a
freshly started Emacs without loading your @file{.emacs} file (start
Emacs with the @code{-q} switch to prevent loading the init file).  If
the problem does @emph{not} occur then, you must report the precise
contents of any programs that you must load into the Lisp world in order
to cause the problem to occur.

@item
If the problem does depend on an init file or other Lisp programs that
are not part of the standard Emacs system, then you should make sure it
is not a bug in those programs by complaining to their maintainers
first.  After they verify that they are using Emacs in a way that is
supposed to work, they should report the bug.

@item
If you wish to mention something in the GNU Emacs source, show the line
of code with a few lines of context.  Don't just give a line number.

The line numbers in the development sources don't match those in your
sources.  It would take extra work for the maintainers to determine what
code is in your version at a given line number, and we could not be
certain.

@item
Additional information from a C debugger such as GDB might enable
someone to find a problem on a machine which he does not have available.
If you don't know how to use GDB, please read the GDB manual---it is not
very long, and using GDB is easy.  You can find the GDB distribution,
including the GDB manual in online form, in most of the same places you
can find the Emacs distribution.  To run Emacs under GDB, you should
switch to the @file{src} subdirectory in which Emacs was compiled, then
do @samp{gdb emacs}.  It is important for the directory @file{src} to be
current so that GDB will read the @file{.gdbinit} file in this
directory.

However, you need to think when you collect the additional information
if you want it to show what causes the bug.

@cindex backtrace for bug reports
For example, many people send just a backtrace, but that is not very
useful by itself.  A simple backtrace with arguments often conveys
little about what is happening inside GNU Emacs, because most of the
arguments listed in the backtrace are pointers to Lisp objects.  The
numeric values of these pointers have no significance whatever; all that
matters is the contents of the objects they point to (and most of the
contents are themselves pointers).

@findex debug_print
To provide useful information, you need to show the values of Lisp
objects in Lisp notation.  Do this for each variable which is a Lisp
object, in several stack frames near the bottom of the stack.  Look at
the source to see which variables are Lisp objects, because the debugger
thinks of them as integers.

To show a variable's value in Lisp syntax, first print its value, then
use the user-defined GDB command @code{pr} to print the Lisp object in
Lisp syntax.  (If you must use another debugger, call the function
@code{debug_print} with the object as an argument.)  The @code{pr}
command is defined by the file @file{.gdbinit}, and it works only if you
are debugging a running process (not with a core dump).

To make Lisp errors stop Emacs and return to GDB, put a breakpoint at
@code{Fsignal}.

For a short listing of Lisp functions running, type the GDB
command @code{xbacktrace}.  

If you want to examine Lisp function arguments, move up the stack, and
each time you get to a frame for the function @code{Ffuncall}, type
these GDB commands:

@example
p *args
pr
@end example

@noindent
To print the first argument that the function received, use these
commands:

@example
p args[1]
pr
@end example

@noindent
You can print the other arguments likewise.  The argument @code{nargs}
of @code{Ffuncall} says how many arguments @code{Ffuncall} received;
these include the Lisp function itself and the arguments for that
function.

The file @file{.gdbinit} defines several other commands that are useful
for examining the data types and contents of Lisp objects.  Their names
begin with @samp{x}.  These commands work at a lower level than
@code{pr}, and are less convenient, but they may work even when
@code{pr} does not, such as when debugging a core dump or when Emacs has
had a fatal signal.

@item
If the symptom of the bug is that Emacs fails to respond, don't assume
Emacs is ``hung''---it may instead be in an infinite loop.  To find out
which, make the problem happen under GDB and stop Emacs once it is not
responding.  (If Emacs is using X directly, you can stop Emacs by typing
@kbd{C-z} at the GDB job.)  Then try stepping with @samp{step}.  If
Emacs is hung, the @samp{step} command won't return.  If it is looping,
@samp{step} will return.

If this shows Emacs is hung in a system call, stop it again and examine
the arguments of the call.  In your bug report, state exactly where in
the source the system call is, and what the arguments are.

If Emacs is in an infinite loop, please determine where the loop starts
and ends.  The easiest way to do this is to use the GDB command
@samp{finish}.  Each time you use it, Emacs resumes execution until it
exits one stack frame.  Keep typing @samp{finish} until it doesn't
return---that means the infinite loop is in the stack frame which you
just tried to finish.

Stop Emacs again, and use @samp{finish} repeatedly again until you get
@emph{back to} that frame.  Then use @samp{next} to step through that
frame.  By stepping, you will see where the loop starts and ends.  Also
please examine the data being used in the loop and try to determine why
the loop does not exit when it should.  Include all of this information
in your bug report.
@end itemize

Here are some things that are not necessary in a bug report:

@itemize @bullet
@item
A description of the envelope of the bug---this is not necessary for a
reproducible bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.

This is often time-consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger with
breakpoints, not by pure deduction from a series of examples.  You might
as well save time by not searching for additional examples.

Of course, if you can find a simpler example to report @emph{instead} of
the original one, that is a convenience.  Errors in the output will be
easier to spot, running under the debugger will take less time, etc.

However, simplification is not vital; if you can't do this or don't have
time to try, please report the bug with your original test case.

@item
A system-call trace of Emacs execution.

System-call traces are very useful for certain special kinds of
debugging, but in most cases they give little useful information.  It is
therefore strange that many people seem to think that @emph{the} way to
report information about a crash is to send a system-call trace.  Perhaps
this is a habit formed from experience debugging programs that don't
have source code or debugging symbols.

In most programs, a backtrace is normally far, far more informative than
a system-call trace.  Even in Emacs, a simple backtrace is generally
more informative, though to give full information you should supplement
the backtrace by displaying variable values and printing them as Lisp
objects with @code{pr} (see above).

@item
A patch for the bug.

A patch for the bug is useful if it is a good one.  But don't omit the
other information that a bug report needs, such as the test case, on the
assumption that a patch is sufficient.  We might see problems with your
patch and decide to fix the problem another way, or we might not
understand it at all.  And if we can't understand what bug you are
trying to fix, or why your patch should be an improvement, we mustn't
install it.

@ifinfo
@xref{Sending Patches}, for guidelines on how to make it easy for us to
understand and install your patches.
@end ifinfo

@item
A guess about what the bug is or what it depends on.

Such guesses are usually wrong.  Even experts can't guess right about
such things without first using the debugger to find the facts.
@end itemize

@node Sending Patches
@subsection Sending Patches for GNU Emacs

@cindex sending patches for GNU Emacs
@cindex patches, sending
  If you would like to write bug fixes or improvements for GNU Emacs,
that is very helpful.  When you send your changes, please follow these
guidelines to make it easy for the maintainers to use them.  If you
don't follow these guidelines, your information might still be useful,
but using it will take extra work.  Maintaining GNU Emacs is a lot of
work in the best of circumstances, and we can't keep up unless you do
your best to help.

@itemize @bullet
@item
Send an explanation with your changes of what problem they fix or what
improvement they bring about.  For a bug fix, just include a copy of the
bug report, and explain why the change fixes the bug.

(Referring to a bug report is not as good as including it, because then
we will have to look it up, and we have probably already deleted it if
we've already fixed the bug.)

@item
Always include a proper bug report for the problem you think you have
fixed.  We need to convince ourselves that the change is right before
installing it.  Even if it is correct, we might have trouble
understanding it if we don't have a way to reproduce the problem.

@item
Include all the comments that are appropriate to help people reading the
source in the future understand why this change was needed.

@item
Don't mix together changes made for different reasons.
Send them @emph{individually}.

If you make two changes for separate reasons, then we might not want to
install them both.  We might want to install just one.  If you send them
all jumbled together in a single set of diffs, we have to do extra work
to disentangle them---to figure out which parts of the change serve
which purpose.  If we don't have time for this, we might have to ignore
your changes entirely.

If you send each change as soon as you have written it, with its own
explanation, then two changes never get tangled up, and we can consider
each one properly without any extra work to disentangle them.

@item
Send each change as soon as that change is finished.  Sometimes people
think they are helping us by accumulating many changes to send them all
together.  As explained above, this is absolutely the worst thing you
could do.

Since you should send each change separately, you might as well send it
right away.  That gives us the option of installing it immediately if it
is important.

@item
Use @samp{diff -c} to make your diffs.  Diffs without context are hard
to install reliably.  More than that, they are hard to study; we must
always study a patch to decide whether we want to install it.  Unidiff
format is better than contextless diffs, but not as easy to read as
@samp{-c} format.

If you have GNU diff, use @samp{diff -c -F'^[_a-zA-Z0-9$]+ *('} when
making diffs of C code.  This shows the name of the function that each
change occurs in.

@item
Avoid any ambiguity as to which is the old version and which is the new.
Please make the old version the first argument to diff, and the new
version the second argument.  And please give one version or the other a
name that indicates whether it is the old version or your new changed
one.

@item
Write the change log entries for your changes.  This is both to save us
the extra work of writing them, and to help explain your changes so we
can understand them.

The purpose of the change log is to show people where to find what was
changed.  So you need to be specific about what functions you changed;
in large functions, it's often helpful to indicate where within the
function the change was.

On the other hand, once you have shown people where to find the change,
you need not explain its purpose in the change log.  Thus, if you add a
new function, all you need to say about it is that it is new.  If you
feel that the purpose needs explaining, it probably does---but put the
explanation in comments in the code.  It will be more useful there.

Please read the @file{ChangeLog} files in the @file{src} and @file{lisp}
directories to see what sorts of information to put in, and to learn the
style that we use.  If you would like your name to appear in the header
line, showing who made the change, send us the header line.
@xref{Change Log}.

@item
When you write the fix, keep in mind that we can't install a change that
would break other systems.  Please think about what effect your change
will have if compiled on another type of system.

Sometimes people send fixes that @emph{might} be an improvement in
general---but it is hard to be sure of this.  It's hard to install
such changes because we have to study them very carefully.  Of course,
a good explanation of the reasoning by which you concluded the change
was correct can help convince us.

The safest changes are changes to the configuration files for a
particular machine.  These are safe because they can't create new bugs
on other machines.

Please help us keep up with the workload by designing the patch in a
form that is clearly safe to install.
@end itemize

@node Contributing, Service, Bugs, Top
@section Contributing to Emacs Development

If you would like to help pretest Emacs releases to assure they work
well, or if you would like to work on improving Emacs, please contact
the maintainers at @email{bug-gnu-emacs@@gnu.org}.  A pretester
should be prepared to investigate bugs as well as report them.  If you'd
like to work on improving Emacs, please ask for suggested projects or
suggest your own ideas.

If you have already written an improvement, please tell us about it.  If
you have not yet started work, it is useful to contact
@email{bug-gnu-emacs@@gnu.org} before you start; it might be
possible to suggest ways to make your extension fit in better with the
rest of Emacs.

@node Service, Command Arguments, Contributing, Top
@section How To Get Help with GNU Emacs

If you need help installing, using or changing GNU Emacs, there are two
ways to find it:

@itemize @bullet
@item
Send a message to the mailing list
@email{help-gnu-emacs@@gnu.org}, or post your request on
newsgroup @code{gnu.emacs.help}.  (This mailing list and newsgroup
interconnect, so it does not matter which one you use.)

@item
Look in the service directory for someone who might help you for a fee.
The service directory is found in the file named @file{etc/SERVICE} in the
Emacs distribution.
@end itemize