Program Design: Using the Debugger

Transcription

1 rogram Design, February 2, Program Design: Using the Debugger A debugger is an alternative to putting print (printf in C) statements in your program, recompiling and trying to find out what values are set incorrectly or where the program terminates prematurely.

2 rogram Design, February 2, A debugger allows you to run your program multiple times and inspect multiple variable values without recompiling. It will indicate what line a program was executing (or what system function was called) in the case that a program terminates prematurely and unexpectedly.

3 rogram Design, February 2, To use the debugger you must compile your program with the -g option. So a compile will look like gcc -g -o exec filename or just gcc -g filename (if you want to use a.out as the executable).

4 rogram Design, February 2, To invoke the debugger type gdb a.out or gdb exec

5 rogram Design, February 2, In the debugger you can use help to find out what commands do. The primary commands you will need are 1. list this list the function you are currently in. You can list specific line numbers or specific functions.

6 rogram Design, February 2, break lnumber break lnumber sets a breakpoint at line lnumber. Then you can examine the state of your running program. 3. cont continue execution of the program. 4. step execute a single statement. 5. next execute up to the next statement (useful for skipping functions). 6. print variable prints the value of variable. 7. quit gets you out of the debugger. Now some examples of debugging modifications (errors introduced) to the program in Fig 3.8 in your book.

7 rogram Design, February 2, waterfall: gcc -g -o debug example.c 202 waterfall: gdb debug GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB 4.16 (sparc-sun-solaris2.6), Copyright 1996 Free Software Foundation, Inc... (gdb) help List of classes of commands: running -- Running the program stack -- Examining the stack data -- Examining data breakpoints -- Making program stop at certain points files -- Specifying and examining files status -- Status inquiries support -- Support facilities user-defined -- User-defined commands aliases -- Aliases of other commands obscure -- Obscure features internals -- Maintenance commands Type "help" followed by a class name for a list of commands in that class.

8 rogram Design, February 2, Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) help breakpoints Making program stop at certain points. List of commands: awatch -- Set a watchpoint for an expression rwatch -- Set a read watchpoint for an expression watch -- Set a watchpoint for an expression catch -- Set breakpoints to catch exceptions that are raised break -- Set breakpoint at specified line or function clear -- Clear breakpoint at specified line or function delete -- Delete some breakpoints or auto-display expressions disable -- Disable some breakpoints enable -- Enable some breakpoints thbreak -- Set a temporary hardware assisted breakpoint hbreak -- Set a hardware assisted breakpoint tbreak -- Set a temporary breakpoint

9 rogram Design, February 2, condition -- Specify breakpoint number N to break only if COND is true commands -- Set commands to be executed when a breakpoint is hit ignore -- Set ignore-count of breakpoint number N to COUNT Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) help List of classes of commands: running -- Running the program stack -- Examining the stack data -- Examining data breakpoints -- Making program stop at certain points files -- Specifying and examining files status -- Status inquiries support -- Support facilities user-defined -- User-defined commands aliases -- Aliases of other commands obscure -- Obscure features

10 rogram Design, February 2, internals -- Maintenance commands Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) help files Specifying and examining files. List of commands: show gnutarget -- Show the current BFD target cd -- Set working directory to DIR for debugger and program being debugged pwd -- Print working directory core-file -- Use FILE as core dump for examining memory and registers section -- Change the base address of section SECTION of the exec file to ADDR exec-file -- Use FILE as program for getting contents of pure memory file -- Use FILE as program to be debugged sharedlibrary -- Load shared object library

11 rogram Design, February 2, symbols for files matching REGEXP path -- Add directory DIR(s) to beginning of search path for object files load -- Dynamically load FILE into the running program add-shared-symbol-files -- Load the symbols from shared objects in the dynamic linker s link map add-symbol-file -- Usage: add-symbol-file FILE ADDR symbol-file -- Load symbol table from executable file FILE set gnutarget -- Set the current BFD target list -- List specified function or line reverse-search -- Search backward for regular expression (see regex(3)) from last line listed search -- Search for regular expression (see regex(3)) from last line listed ---Type <return> to continue, or q <return> to quit--- forward-search -- Search for regular expression (see regex(3)) from last line listed directory -- Add directory DIR to beginning of search path for source files

12 rogram Design, February 2, Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) list 2 sentinel-controlled repetition */ 3 #include <stdio.h> 4 5 main() 6 { 7 float average; /* new data type */ 8 int counter, grade, total; 9 10 /* initialization phase */ 11 total = 0; (gdb) list 12 counter = 0; /* processing phase */ 15 printf("enter grade, -1 to end: "); 16 scanf("%d", grade); /* Oops did not use &grade */ while (grade!= -1) { 19 total = total + grade; 20 counter = counter + 1;

13 rogram Design, February 2, printf("enter grade, -1 to end: "); (gdb) break 18 Breakpoint 1 at 0x107c4: file FIG3_8.C, line 18. (gdb) run GNU gdb 5.0 Copyright 2000 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "sparc-sun-solaris2.8"... (gdb) run Starting program: /home/csee2/hall/tex/progdes/ notes/debug Enter grade, -1 to end: 100 Breakpoint 1, main () at FIG3_8.C:18 18 while (grade!= -1) { (gdb) print grade $1 = (gdb) cont Continuing. Enter grade, -1 to end: -1

14 rogram Design, February 2, Breakpoint 1, main () at FIG3_8.C:18 18 while (grade!= -1) { (gdb) print grade $2 = -1 (gdb) quit The program is running. Quit anyway (and kill it)? (y or n) y 203 waterfall: exit exit

15 rogram Design, February 2, waterfall: gcc -g -o debug FIG3_8.C 202 waterfall: debug Enter grade, -1 to end: 77 Enter grade, -1 to end: 88 Enter grade, -1 to end: -1 Segmentation fault 203 waterfall: gdb debug GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB 4.16 (sparc-sun-solaris2.6), Copyright 1996 Free Software Foundation, Inc... (gdb) run Starting program: /home/cs2/faculty/hall/ tex/progdes/notes/debug Enter grade, -1 to end: 77 Enter grade, -1 to end: 99 Enter grade, -1 to end: -1 Program received signal SIGSEGV, Segmentation fault.

16 rogram Design, February 2, x10854 in main () at FIG3_8.C:28 28 average[2000] = (float) total / counter; (gdb) print counter $1 = 2 (gdb) print average[2000] Cannot access memory at address 0xf00010e8. (gdb) print average[0] $2 = 0 (gdb) list 10 5 main() 6 { 7 float average[9]; /* new data type */ 8 int counter, grade, total; 9 10 /* initialization phase */ 11 total = 0; 12 counter = 0; /* processing phase */ (gdb) list 15 printf("enter grade, -1 to end: "); 16 scanf("%d", &grade); while (grade!= -1) { 19 total = total + grade; 20 counter = counter + 1;

17 rogram Design, February 2, printf("enter grade, -1 to end: "); 22 scanf("%d", &grade); 23 } 24 (gdb) list 25 /* termination phase */ 26 if (counter!= 0) { average[2000] = (float) total / counter; 29 printf("class average is %.2f", average[0]); 30 } 31 else 32 printf("no grades were entered\n"); return 0; /* indicate program ended successfully */ (gdb) quit The program is running. Quit anyway (and kill it)? (y or n) y 204 waterfall: exit exit