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144 lines
5.5 KiB
Plaintext
144 lines
5.5 KiB
Plaintext
Debugging GNU Emacs
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Copyright (c) 1985 Richard M. Stallman.
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Permission is granted to anyone to make or distribute verbatim copies
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of this document as received, in any medium, provided that the
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copyright notice and permission notice are preserved,
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and that the distributor grants the recipient permission
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for further redistribution as permitted by this notice.
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Permission is granted to distribute modified versions
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of this document, or of portions of it,
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under the above conditions, provided also that they
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carry prominent notices stating who last changed them.
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On 4.2 you will probably find that dbx does not work for
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debugging GNU Emacs. For one thing, dbx does not keep the
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inferior process's terminal modes separate from its own.
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For another, dbx does not put the inferior in a separate
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process group, which makes trouble when an inferior uses
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interrupt input, which GNU Emacs must do on 4.2.
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dbx has also been observed to have other problems,
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such as getting incorrect values for register variables
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in stack frames other than the innermost one.
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The Emacs distribution now contains GDB, the new source-level
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debugger for the GNU system. GDB works for debugging Emacs.
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GDB currently runs on vaxes under 4.2 and on Sun 2 and Sun 3
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systems.
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** Some useful techniques
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`Fsignal' is a very useful place to stop in.
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All Lisp errors go through there.
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It is useful, when debugging, to have a guaranteed way
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to return to the debugger at any time. If you are using
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interrupt-driven input, which is the default, then Emacs is using
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RAW mode and the only way you can do it is to store
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the code for some character into the variable stop_character:
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set stop_character = 29
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makes Control-] (decimal code 29) the stop character.
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Typing Control-] will cause immediate stop. You cannot
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use the set command until the inferior process has been started.
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Put a breakpoint early in `main', or suspend the Emacs,
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to get an opportunity to do the set command.
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If you are using cbreak input (see the Lisp function set-input-mode),
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then typing Control-g will cause a SIGINT, which will return control
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to the debugger immediately unless you have done
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ignore 3 (in dbx)
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or handle 3 nostop noprint (in gdb)
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You will note that most of GNU Emacs is written to avoid
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declaring a local variable in an inner block, even in
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cases where using one would be the cleanest thing to do.
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This is because dbx cannot access any of the variables
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in a function which has even one variable defined in an
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inner block. A few functions in GNU Emacs do have variables
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in inner blocks, only because I wrote them before realizing
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that dbx had this problem and never rewrote them to avoid it.
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I believe that GDB does not have such a problem.
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** Examining Lisp object values.
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When you have a live process to debug, and it has not encountered a
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fatal error, you can use the GDB command `pr'. First print the value
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in the ordinary way, with the `p' command. Then type `pr' with no
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arguments. This calls a subroutine which uses the Lisp printer.
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If you can't use this command, either because the process can't run
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a subroutine or because the data is invalid, you can fall back on
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lower-level commands.
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Use the `xtype' command to print out the data type of the last data
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value. Once you know the data type, use the command that corresponds
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to that type. Here are these commands:
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xint xptr xwindow xmarker xoverlay xmiscfree xintfwd xboolfwd xobjfwd
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xbufobjfwd xkbobjfwd xbuflocal xbuffer xsymbol xstring xvector xframe
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xwinconfig xcompiled xcons xcar xcdr xsubr xprocess xfloat xscrollbar
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Each one of them applies to a certain type or class of types.
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(Some of these types are not visible in Lisp, because they exist only
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internally.)
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Each x... command prints some information about the value, and
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produces a GDB value (subsequently available in $) through which you
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can get at the rest of the contents.
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In general, most of the rest of the contents will be addition Lisp
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objects which you can examine in turn with the x... commands.
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** If GDB does not run and your debuggers can't load Emacs.
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On some systems, no debugger can load Emacs with a symbol table,
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perhaps because they all have fixed limits on the number of symbols
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and Emacs exceeds the limits. Here is a method that can be used
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in such an extremity. Do
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nm -n temacs > nmout
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strip temacs
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adb temacs
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0xd:i
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0xe:i
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14:i
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17:i
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:r -l loadup (or whatever)
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It is necessary to refer to the file `nmout' to convert
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numeric addresses into symbols and vice versa.
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It is useful to be running under a window system.
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Then, if Emacs becomes hopelessly wedged, you can create
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another window to do kill -9 in. kill -ILL is often
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useful too, since that may make Emacs dump core or return
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to adb.
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** Debugging incorrect screen updating.
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To debug Emacs problems that update the screen wrong, it is useful
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to have a record of what input you typed and what Emacs sent to the
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screen. To make these records, do
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(open-dribble-file "~/.dribble")
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(open-termscript "~/.termscript")
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The dribble file contains all characters read by Emacs from the
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terminal, and the termscript file contains all characters it sent to
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the terminal. The use of the directory `~/' prevents interference
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with any other user.
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If you have irreproducible display problems, put those two expressions
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in your ~/.emacs file. When the problem happens, exit the Emacs that
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you were running, kill it, and rename the two files. Then you can start
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another Emacs without clobbering those files, and use it to examine them.
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