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1.1 root 1: Debugging GNU Emacs
2: Copyright (c) 1985 Richard M. Stallman.
3:
4: Permission is granted to anyone to make or distribute verbatim copies
5: of this document as received, in any medium, provided that the
6: copyright notice and permission notice are preserved,
7: and that the distributor grants the recipient permission
8: for further redistribution as permitted by this notice.
9:
10: Permission is granted to distribute modified versions
11: of this document, or of portions of it,
12: under the above conditions, provided also that they
13: carry prominent notices stating who last changed them.
14:
15: On 4.2 you will probably find that dbx does not work for
16: debugging GNU Emacs. For one thing, dbx does not keep the
17: inferior process's terminal modes separate from its own.
18: For another, dbx does not put the inferior in a separate
19: process group, which makes trouble when an inferior uses
20: interrupt input, which GNU Emacs must do on 4.2.
21:
22: dbx has also been observed to have other problems,
23: such as getting incorrect values for register variables
24: in stack frames other than the innermost one.
25:
26: The Emacs distribution now contains GDB, the new source-level
27: debugger for the GNU system. GDB works for debugging Emacs.
28: GDB currently runs on vaxes under 4.2 and on Sun 2 and Sun 3
29: systems.
30:
31:
32: ** Some useful techniques
33:
34: `Fsignal' is a very useful place to stop in.
35: All Lisp errors go through there.
36:
37: It is useful, when debugging, to have a guaranteed way
38: to return to the debugger at any time. If you are using
39: interrupt-drived input, which is the default, then Emacs is using
40: RAW mode and the only way you can do it is to store
41: the code for some character into the variable stop_character:
42:
43: set stop_character = 29
44:
45: makes Control-] (decimal code 29) the stop character.
46: Typing Control-] will cause immediate stop. You cannot
47: use the set command until the inferior process has been started.
48: Put a breakpoint early in `main', or suspend the Emacs,
49: to get an opportunity to do the set command.
50:
51: If you are using cbreak input (see the Lisp function set-input-mode),
52: then typing Control-g will cause a SIGINT, which will return control
53: to the debugger immediately unless you have done
54:
55: ignore 3 (in dbx)
56: or handle 3 nostop noprint (in gdb)
57:
58: You will note that most of GNU Emacs is written to avoid
59: declaring a local variable in an inner block, even in
60: cases where using one would be the cleanest thing to do.
61: This is because dbx cannot access any of the variables
62: in a function which has even one variable defined in an
63: inner block. A few functions in GNU Emacs do have variables
64: in inner blocks, only because I wrote them before realizing
65: that dbx had this problem and never rewrote them to avoid it.
66:
67: I believe that GDB does not have such a problem.
68:
69:
70: ** If GDB does not run and your debuggers can't load Emacs.
71:
72: On some systems, no debugger can load Emacs with a symbol table,
73: perhaps because they all have fixed limits on the number of symbols
74: and Emacs exceeds the limits. Here is a method that can be used
75: in such an extremity. Do
76:
77: nm -n temacs > nmout
78: strip temacs
79: adb temacs
80: 0xd:i
81: 0xe:i
82: 14:i
83: 17:i
84: :r -l loadup (or whatever)
85:
86: It is necessary to refer to the file `nmout' to convert
87: numeric addresses into symbols and vice versa.
88:
89: It is useful to be running under a window system.
90: Then, if Emacs becomes hopelessly wedged, you can create
91: another window to do kill -9 in. kill -ILL is often
92: useful too, since that may make Emacs dump core or return
93: to adb.
94:
95:
96: ** Debugging incorrect screen updating.
97:
98: To debug Emacs problems that update the screen wrong, it is useful
99: to have a record of what input you typed and what Emacs sent to the
100: screen. To make these records, do
101:
102: (open-dribble-file "~/.dribble")
103: (open-termscript "~/.termscript")
104:
105: The dribble file contains all characters read by Emacs from the
106: terminal, and the termscript file contains all characters it sent to
107: the terminal. The use of the directory `~/' prevents interference
108: with any other user.
109:
110: If you have unreproduceable display problems, put those two expressions
111: in your ~/.emacs file. When the problem happens, exit the Emacs that
112: you were running, kill it, and rename the two files. Then you can start
113: another Emacs without clobbering those files, and use it to examine them.
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