Before reporting a bug, first try to see if the problem has already been reported (see Known Problems).
If you are able to, try the latest release of Emacs to see if the problem has already been fixed. Even better is to try the latest development version. We recognize that this is not easy for some people, so do not feel that you absolutely must do this before making a report.
The best way to write a bug report for Emacs is to use the command M-x report-emacs-bug. This sets up a mail buffer (see Sending Mail) and automatically inserts 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. You may feel that some of the information inserted by M-x report-emacs-bug is not relevant, but unless you are absolutely sure it is best to leave it, so that the developers can decide for themselves.
When you have finished writing your report, type C-c C-c and it will be sent to the Emacs maintainers at email@example.com. (If you want to suggest an improvement or new feature, use the same address.) If you cannot send mail from inside Emacs, you can copy the text of your report to your normal mail client (if your system supports it, you can type C-c M-i to have Emacs do this for you) and send it to that address. Or you can simply send an email to that address describing the problem.
Your report will be sent to the ‘bug-gnu-emacs’ mailing list, and stored in the GNU Bug Tracker at http://debbugs.gnu.org. Please include a valid reply email address, in case we need to ask you for more information about your report. Submissions are moderated, so there may be a delay before your report appears.
You do not need to know how the Gnu Bug Tracker works in order to report a bug, but if you want to, you can read the tracker's online documentation to see the various features you can use.
All mail sent to the ‘bug-gnu-emacs’ mailing list is also gatewayed to the ‘gnu.emacs.bug’ newsgroup. The reverse is also true, but we ask you not to post bug reports (or replies) via the newsgroup. It can make it much harder to contact you if we need to ask for more information, and it does not integrate well with the bug tracker.
If your data is more than 500,000 bytes, please don't include it directly in the bug report; instead, offer to send it on request, or make it available by ftp and say where.
To enable maintainers to investigate a bug, your report should include all these things:
M-x report-emacs-bug includes this information automatically, but if you are not using that command for your report you can get the version number by typing M-x emacs-version <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.
configurecommand when Emacs was installed (automatically included by M-x report-emacs-bug).
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.
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 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).
One way to record the input to Emacs precisely is to write a dribble file. To start the file, use the M-x open-dribble-file <RET> command. From then on, Emacs copies all your input to the specified dribble file until the Emacs process is killed.
The way to collect the terminal output is to execute the Lisp expression
using M-: or from the *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 Emacs initialization 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.
echo LC_ALL=$LC_ALL LC_COLLATE=$LC_COLLATE LC_CTYPE=$LC_CTYPE \ LC_MESSAGES=$LC_MESSAGES LC_TIME=$LC_TIME LANG=$LANG
Alternatively, use the locale command, if your system has it, to display your locale settings.
You can use the M-! command to execute these commands from
Emacs, and then copy the output from the *Messages* buffer into
the bug report. Alternatively, M-x getenv <RET> LC_ALL
<RET> will display the value of
LC_ALL in the echo area, and
you can copy its output from the *Messages* buffer.
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 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.
To get the error message text accurately, copy it from the *Messages* buffer into the bug report. Copy all of it, not just part.
To make a backtrace for the error, use M-x toggle-debug-on-error before the error happens (that is to say, you must give that command and then make the bug happen). This causes the error to start the Lisp debugger, which shows you a backtrace. Copy the text of the debugger's backtrace into the bug report. See The Lisp Debugger, for information on debugging Emacs Lisp programs with the Edebug package.
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.
If Emacs appears to be stuck in an infinite loop or in a very long
operation, typing C-g with the variable
nil will start the Lisp debugger and show a backtrace.
This backtrace is useful for debugging such long loops, so if you can
produce it, copy it into the bug report.
If you cannot get Emacs to respond to C-g (e.g., because
inhibit-quit is set), then you can try sending the signal
debug-on-event (default SIGUSR2) from outside
Emacs to cause it to enter the debugger.
-Qswitch to prevent loading the init files). If the problem does 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.
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.
However, you need to think when you collect the additional information if you want it to show what causes the bug.
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).
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
pr to print the Lisp object in
Lisp syntax. (If you must use another debugger, call the function
debug_print with the object as an argument.) The
command is defined by the 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
For a short listing of Lisp functions running, type the GDB
The file .gdbinit defines several other commands that are useful
for examining the data types and contents of Lisp objects. Their names
begin with ‘x’. These commands work at a lower level than
pr, and are less convenient, but they may work even when
pr does not, such as when debugging a core dump or when Emacs has
had a fatal signal.
More detailed advice and other useful techniques for debugging Emacs are available in the file etc/DEBUG in the Emacs distribution. That file also includes instructions for investigating problems whereby Emacs stops responding (many people assume that Emacs is “hung”, whereas in fact it might be in an infinite loop).
To find the file etc/DEBUG in your Emacs installation, use the
directory name stored in the variable
Here are some things that are not necessary in a bug report:
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. It is better to send the bug report right away, go back to editing, and find another bug to report.
Of course, if you can find a simpler example to report 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.
Debugging the core dump might be useful, but it can only be done on your machine, with your Emacs executable. Therefore, sending the core dump file to the Emacs maintainers won't be useful. Above all, don't include the core file in an email bug report! Such a large message can be extremely inconvenient.
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 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
pr (see above).
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.
See Sending Patches, for guidelines on how to make it easy for us to understand and install your patches.
Such guesses are usually wrong. Even experts can't guess right about such things without first using the debugger to find the facts.