Understanding Bug Reporting Bugs Sending Patches

28.10.3: Checklist for Bug Reports

The best way to send a bug report is to mail it electronically to the Emacs maintainers at `bug-gnu-emacs@gnu.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 newsgroup `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:

• 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 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.

• The type of machine you are using, and the operating system name and version number. M-x emacs-version RET provides this information too. Copy its output from the `*Messages*' buffer, so that you get it all and get it accurately.
• The operands given to the configure command when Emacs was installed.
• 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.

• Details of any other deviations from the standard procedure for installing GNU Emacs.
• 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).

• The precise commands we need to type to reproduce the bug.

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

  (open-dribble-file "~/dribble")
  

  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.

• For possible display bugs, the terminal type (the value of environment variable 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

  (open-termscript "~/termscript")
  

  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' 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.

• 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 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.

• 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 `*Messages*' buffer into the bug report. Copy all of it, not just part.

  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 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.

• Check whether any programs you have loaded into the Lisp world, including your `.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 -q switch 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.
• 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.
• 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.

• 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 `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 pr 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 Fsignal.

  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[1]
  pr
  

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

  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.

• 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 Windows directly, you can stop Emacs by typing C-z at the GDB job.) Then try stepping with `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:

• 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 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.

• 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 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 pr (see above).

• 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.

  See Sending Patches, for guidelines on how to make it easy for us to understand and install your patches.

• 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.

Understanding Bug Reporting

Bugs

Sending Patches