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1.1 root 1: .\" @(#)dbx.ms 6.4 (Berkeley) 5/10/86
2: .\"
3: .\" modified by mark seiden in cosmetic ways.
4: .\" examples VAXinated by Kevin Dunlap
5: .\" dtbl | ditroff -ms
6: .OH 'Debugging with dbx''PS1:11-%'
7: .EH 'PS1:11-%''Debugging with dbx'
8: .de BE
9: .DS
10: .ft CW
11: .ps -1
12: ..
13: .de EE
14: .ft P
15: .ps +1
16: .DE
17: ..
18: .de UL
19: \f(CW\s-1\\$1\fP\s0
20: ..
21: .TL
22: Debugging with dbx
23: .AU
24: Bill Tuthill
25: .AI
26: Sun Microsystems, Inc.
27: 2550 Garcia Avenue
28: .AU
29: Kevin J. Dunlap
30: .AI
31: Computer Systems Research Group
32: University of California
33: Berkeley, CA 94720
34: .SH
35: Introduction
36: .PP
37: This short paper discusses
38: .I dbx ,
39: a symbolic debugger that is vastly superior to
40: .I adb .
41: It may be as good as the debuggers you remember from those
42: non-
43: .UX
44: systems you worked on before.
45: The advantage of symbolic debuggers is that they allow you
46: to work with the same names (symbols) as in your source code.
47: .PP
48: Like
49: .I adb ,
50: .I dbx
51: is interactive and line-oriented, but
52: .I dbx
53: is a source-level rather than an assembly-level debugger.
54: It allows you to determine where a program crashed,
55: to view the values of variables and expressions,
56: to set breakpoints in the code, and to run and trace a program.
57: Source code may be in C, Fortran, or Pascal.
58: .PP
59: Mark Linton wrote
60: .I dbx
61: as his master's thesis at UC Berkeley.
62: Along with Eric Schmidt's Berknet,
63: .I dbx
64: is among the most successful master's theses done on UNIX. Since
65: .I dbx
66: required changes to the symbol tables
67: generated by the various compilers,
68: you need to compile programs for debugging with the
69: .I \-g
70: flag. For example,
71: C programs should be compiled as follows:
72: .DS
73: % cc \-g \fIprogram\fP.c \-o \fIprogram\fP
74: .DE
75: Programs compiled with the
76: .I \-g
77: option have good symbol tables,
78: while programs compiled without
79: .I \-g
80: have old-style symbol tables intended for
81: .I adb .
82: Stripped programs have no symbol tables at all.
83: Invoke the debugger as follows, where
84: .I program
85: is the pathname of the executable file that dumped core:
86: .DS
87: % dbx \fIprogram\fP
88: .DE
89: The core image should be in the working directory;
90: if it isn't, specify its pathname in the argument after the program name.
91: Among the great advances of
92: .I dbx
93: is that it has a help facility; type the
94: .I help
95: request to see a list of possible requests.
96: You can obtain help on any
97: .I dbx
98: request by giving its name as an argument to
99: .I help .
100: .bp
101: .SH
102: Examining Core Dumps
103: .PP
104: Much of the time, programmers use
105: .I dbx
106: to find out why a program dumped core.
107: As an example, consider the following program
108: .I dumpcore.c ,
109: which dereferences a NULL pointer.
110: This is a legal operation on VAX/UNIX,
111: but not on VAX/VMS or on MC68000-based UNIX systems, on one of
112: which this example was run:
113: .BE
114: #include <stdio.h>
115: .sp.5
116: #define LIMIT 5
117: .sp.5
118: main() /* print messages and die */
119: {
120: int i;
121: .sp.5
122: for (i = 1; i <= 10 ; i++) {
123: printf("Goodbye world! (%d)\en", i);
124: dumpcore(i);
125: }
126: exit(0);
127: }
128: .sp.5
129: int *ip;
130: .sp.5
131: dumpcore(lim) /* dereference NULL pointer */
132: int lim;
133: {
134: if (lim >= LIMIT)
135: *ip = lim;
136: }
137: .EE
138: The program core dumps because of a
139: segmentation violation or memory fault \(em
140: on most machines it is illegal to assign to address zero.
141: Once the program has produced a core dump,
142: here's how you can find out why the program died:
143: .DS
144: %\c
145: .UL " dbx dumpcore"
146: dbx version 3.17 of 4/24/86 15:04 (monet.Berkeley.EDU).
147: Type 'help' for help.
148: reading symbolic information ...
149: [using memory image in core]
150: (dbx)\c
151: .UL " where"
152: dumpcore.dumpcore(lim = 5), line 22 in "dumpcore.c"
153: main(0x1, 0x7fffe904, 0x7fffe90c), line 11 in "dumpcore.c"
154: .DE
155: The
156: .I where
157: request yields a stack trace.
158: As you can see, the
159: .I dumpcore()
160: routine was called from line 11 of the program, with the argument
161: .I lim
162: equal to 5.
163: You can look at the
164: .I dumpcore()
165: procedure by invoking the
166: .I list
167: request as follows:
168: .DS
169: (dbx)\c
170: .UL " list dumpcore"
171: 18 dumpcore(lim) /* dereference NULL pointer */
172: 19 int lim;
173: 20 {
174: 21 if (lim >= LIMIT)
175: 22 *ip = lim;
176: 23 }
177: .DE
178: We immediately suspect that the program's failure had something to do with
179: .I *ip ,
180: so we use the
181: .I print
182: request to retrieve the value of the pointer and what it points to:
183: .DS
184: (dbx)\c
185: .UL " print *ip"
186: reference through nil pointer
187: (dbx)\c
188: .UL " print ip"
189: (nil)
190: .DE
191: This tells us the program has dereferenced a null pointer.
192: It is possible to run the program again from inside the debugger.
193: The first line tells you name of the running program,
194: and successive lines give output from the program:
195: .DS
196: (dbx)\c
197: .UL " run"
198: Goodbye world! (1)
199: Goodbye world! (2)
200: Goodbye world! (3)
201: Goodbye world! (4)
202: Goodbye world! (5)
203: .sp.5
204: Bus error in dumpcore.dumpcore at line 22
205: 22 *ip = lim;
206: (dbx)\c
207: .UL " quit"
208: .DE
209: In this example the program dies with a Bus error at line 22.
210: This method of running the program
211: does not produce a core dump, but the
212: .I where
213: request will still behave properly,
214: because the debugger is in the same state
215: as if it had just read the core file.
216: .SH
217: Setting Breakpoints
218: .PP
219: With
220: .I dbx
221: you can set breakpoints before each line of a program,
222: not just at function and procedure boundaries, as with
223: .I adb .
224: The
225: .I stop
226: request sets a breakpoint.
227: After setting a breakpoint, use the
228: .I run
229: request to execute the program. The
230: .I cont
231: request continues execution from the current stopping point
232: until the program finishes or another breakpoint is encountered. The
233: .I step
234: request executes one source statement,
235: following any function calls. The
236: .I next
237: request executes one source statement,
238: but does not stop inside any function calls. The
239: .I status
240: request lists active breakpoints, while the
241: .I delete
242: request removes them if required.
243: .PP
244: The
245: .I stop
246: request can take a conditional expression
247: to avoid needless single-stepping.
248: We will use a conditional in our example to make things simpler.
249: Of course you can use
250: .I print
251: and
252: .I list
253: requests at any time during statement stepping
254: if you want to print the value of variables
255: or list lines of source code.
256: This sample session shows a mixture of requests
257: as we verify that the program fails when it tries to assign to
258: .I *ip :
259: .DS
260: (dbx)\c
261: .UL " stop at 10 if (i == 5)"
262: [1] if i = 5 { stop } at 10
263: (dbx)\c
264: .UL " run"
265: Goodbye world! (1)
266: Goodbye world! (2)
267: Goodbye world! (3)
268: Goodbye world! (4)
269: [1] stopped in main at line 10
270: 10 printf("Goodbye world! (%d)\en", i);
271: (dbx)\c
272: .UL " next"
273: Goodbye world! (5)
274: stopped in main at line 11
275: 11 dumpcore(i);
276: (dbx)\c
277: .UL " step"
278: stopped in dumpcore at line 21
279: 21 if (lim >= LIMIT)
280: (dbx)\c
281: .UL " step"
282: stopped in dumpcore at line 22
283: 22 *ip = lim;
284: (dbx)\c
285: .UL " step"
286: Bus error in dumpcore.dumpcore at line 22
287: 22 *ip = lim;
288: .DE
289: Running the program with breakpoints assures us
290: that our intuition was correct.
291: We shouldn't be assigning anything to a null pointer \(em
292: .I ip
293: should have been initialized to point at an object of the proper type.
294: To exit from the debugger, use the
295: .I quit
296: request.
297: .PP
298: It is possible to set variables from inside
299: .I dbx .
300: The previous breakpoint session, for example,
301: could have gone like this:
302: .DS
303: %\c
304: .UL " dbx dumpcore"
305: dbx version 3.17 of 4/24/86 15:04 (monet.Berkeley.EDU).
306: Type 'help' for help.
307: reading symbolic information ...
308: [using memory image in core]
309: (dbx)\c
310: .UL " stop at 10"
311: [1] stop at 10
312: (dbx)\c
313: .UL " run"
314: Running: dumpcore
315: stopped in main at line 10
316: 10 printf("Goodbye world! (%d)\en", i);
317: (dbx)\c
318: .UL " assign i = 5"
319: (dbx)\c
320: .UL " next"
321: Goodbye world! (5)
322: stopped in main at line 11
323: 11 dumpcore(i);
324: (dbx)\c
325: .UL " next"
326: Bus error in dumpcore.dumpcore at line 22
327: 22 *ip = lim;
328: .DE
329: It is often useful to assign new values to variables
330: to draw conclusions about alternative conditions.
331: We can't fix the bug in this program, however,
332: because there is no declared variable to which
333: .I ip
334: should point.
335: .SH
336: Conclusion
337: .PP
338: Expressions in
339: .I dbx
340: are similar to those in C,
341: except that there is a distinction between
342: .I /
343: (floating-point division) and
344: .I div
345: (integer division), as in Pascal.
346: The table on the following page shows
347: .I dbx
348: requests organized by function:
349: .PP
350: Like
351: .I adb ,
352: .I dbx
353: can disassemble object code.
354: It can also examine object files
355: and print output in various formats; but
356: .I dbx
357: requires the proper symbol tables, so
358: .I adb
359: is more useful to examine arbitrary binary files.
360: The most important thing
361: .I adb
362: can do that
363: .I dbx
364: cannot is to patch binary files \(em
365: .I dbx
366: has no write option.
367: Despite these shortcomings,
368: .I dbx
369: is much easier to use than
370: .I adb ,
371: so it contributes much more to individual programmer productivity.
372: .SH
373: Acknowledgements
374: .PP
375: Material presented in this document was first presented in
376: ``C Advisor'', \fIUnix Review 4\fP, 1, pp 78\-85.
377: The Regents of the University California expresses their
378: gratitude to Unix Review
379: for allowing them to reprint this document.
380: .PP
381: This document is a good starting point for a more thorough tutorial.
382: Those with the ambition to expand on this document are encouraged
383: to contact the Computer Systems Research Group at ``[email protected].''
384: .KF
385: .TS
386: center box;
387: cf s.
388: .sp.2
389: \s+2Groups of \&\fIdbx\fP Requests\s-2
390: .sp.2
391: _
392: .T&
393: l lfI
394: lp-1fCW l.
395: execution and tracing
396: _
397: run execute object file
398: cont continue execution from where it stopped
399: trace display tracing information at specified place
400: stop stop execution at specified place
401: status display active \&\fItrace\fP and \&\fIstop\fP requests
402: delete delete specific \&\fItrace\fP or \&\fIstop\fP requests
403: catch start trapping specified signals
404: ignore stop trapping specified signals
405: step execute the next source line, stepping into functions
406: next execute the next source line, even if it's a function
407: .T&
408: l lfI
409: lp-1fCW l.
410: _
411: displaying data
412: _
413: print print the value of an expression
414: whatis print the declaration of a given identifier or type
415: which print outer block associated with identifier
416: whereis print all symbols matching identifier
417: assign set the value of a variable
418: .T&
419: l lfI
420: lp-1fCW l.
421: _
422: function and procedure handling
423: _
424: where display active procedures and functions on stack
425: down move down the stack towards stopping point
426: up move up the stack towards \&\fImain\fP
427: call call the named function or procedure
428: dump display names and values of all local variables
429: .T&
430: l lfI
431: lp-1fCW l.
432: _
433: accessing source files and directories
434: _
435: edit invoke an editor on current source file
436: file change current source file
437: func change the current function or procedure
438: list display lines of source code
439: use set directory list to search for source files
440: /.../ search down in file to match regular expression
441: ?...? search up in file to match regular expression
442: .T&
443: l lfI
444: lp-1fCW l.
445: _
446: miscellaneous commands
447: _
448: sh pass command line to the shell
449: alias change \&\fIdbx\fP command name
450: help explain commands
451: source read commands from external file
452: quit exit the debugger
453: .TE
454: .KE
455: .bp
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