Understanding Bug Reporting Bugs Sending Patches
The best way to send a bug report is to mail it electronically to the
Emacs maintainers at `
firstname.lastname@example.org'. (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
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:
GNU Emacs Bugs Free Software Foundation 59 Temple Place, Suite 330 Boston, MA 02111-1307 USA
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.
A convenient way to send 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.
To enable maintainers to investigate a bug, your report should include all these things:
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
RETprovides this information too. Copy its output from the `
*Messages*' buffer, so that you get it all and get it accurately.
configurecommand when Emacs was installed.
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 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).
The easy way to record the input to Emacs precisely is to write a dribble file. To start the file, execute the Lisp expression
using M-: or from the `
*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.
TERM), the complete termcap entry for the terminal from `
/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
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
.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.
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
To make a backtrace for the error, evaluate the Lisp expression
(setq 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
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.
.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 `
.emacs' file (start Emacs with the
-qswitch to prevent loading the init file). 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.
src' subdirectory in which Emacs was compiled, then do `
gdb emacs'. It is important for the directory `
src' to be current so that GDB will read the `
.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.
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
To find out which Lisp functions are running, using GDB, move up the
stack, and each time you get to a frame for the function
Ffuncall, type these GDB commands:
p *args pr
To print the first argument that the function received, use these commands:
p args pr
You can print the other arguments likewise. The argument
Ffuncall says how many arguments
these include the Lisp function itself and the arguments for that
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.
step'. If Emacs is hung, the `
step' command won't return. If it is looping, `
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
finish'. Each time you use it, Emacs resumes execution until it
exits one stack frame. Keep typing `
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 `
finish' repeatedly again until you get
back to that frame. Then use `
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.
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.
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.
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.