Tutorial of gcc and gdb

Frequently Used Options

• Compile your code with debugging information:
  • gcc -g -o garbage garbage.c
• Compile your code with optimizations:
  • gcc -On -o garbage garbage.c
  Notice: n is usually a number through 1 to 3. The larger the number, the more optimizations are performed while compiling the code. The optimization options may differ in each platform. For example, the gcc under Mac OS also supports -Os and -Oz to allow optimization for code size.
• For other optimization/debug options, you may use
  • man gcc
  under any UNIX-like system.

gdb

gcc is a debugger by GNU project. Gdb can step through your source code line-by-line or even instruction by instruction. You may also watch the value of any variable at run-time. In additon, it also helps to identify the place and the reason making the program crash.

Basic Usage

All program to be debugged in gdb must be compiled by gcc with the option "-g" turning on. Continue with the "garbage" example, if we want to debug the program "garbage", we can simply start gdb by:

• gdb ./garbage

GNU gdb Red Hat Linux (6.3.0.0-1.162.el4rh)
Copyright 2004 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 "i386-redhat-linux-gnu"..."/home/h1tseng/garbage":
not in executable format: File format not recognized

(gdb)

To start running and debugging the program, we can simply type the "run" command after the (gdb) prompt as below:

(gdb) run

(gdb) run arg_1 arg_2

Breaking Program

• Break by line: to break the program at the beginning of a certain line, we can use the command "break source_filename:line_number". For example, if we want to break at the beginning of main function in garbage.c, we can do as below:

  (gdb) break garbage.c:8
  Breakpoint 1 at 0x1f7b: file garbage.c, line 8.
  

• Break by function: to break the program at the beginning of a certain function, we can use the command "break source_filename:function_name()". For example, if we want to break at the beginning of main function in garbage.c, we can also try below:

  (gdb) break garbage.c:main
  Breakpoint 1 at 0x1f7b: file garbage.c, line 8.
  

• Break by instruction: to break the program at the beginning of a certain machine instruction, we can use the command "break *PC". For example, if we want to break at the beginning of main function in garbage.c, we can also try below:

  (gdb) break *0x1f7b
  Breakpoint 1 at 0x1f7b: file garbage.c, line 8.
  

To show the current breakpoints we have, we may use the "info breakpoint" command as:

(gdb) info breakpoint
Num Type           Disp Enb Address    What
1   breakpoint     keep y   0x00001f7b in main at garbage.c:8

To disable a breakpoint we have, we may use the "disable breakpoint_number". To re-enable disabled breakpoint, we can turn it on by "enable breakpoint_number". To remove a breakpoint, we can use "delete breakpoint_number" or replace the "break" with "clear" command as we create these breakpoints.

(gdb) clear garbage.c:8
Deleted breakpoint 1

To resume the exeution of the interrupted program, we can use the "continue" or "c" command.

Stepping through program

• s: the debugger will step to the next line in the source code. For example, using the s command, the program will step through line 9 from line 8 after the program interrupted by breakpoint 1.

  (gdb) run
  Starting program: /Users/bunny/Desktop/cse141/garbage 
  Reading symbols for shared libraries ++. done
  
  Breakpoint 1, main (argc=1, argv=0xbffff2bc) at garbage.c:8
  8         a = atoi(argv[1]);
  (gdb) s
  9         b = atoi(argv[2]);
  

• si: the debugger will step to the next instruction in the compiled code. For example, using the si command, the program will step through PC 0x00001f91 from 0x00001f8e.

  (gdb) si
  0x00001f8e      9         b = atoi(argv[1]);
  (gdb) si
  0x00001f91      9         b = atoi(argv[1]);
  

• n: the debugger will step to the next source line. Each function call will be treat as a single source code line.

Inspect variables/register value

Once a running program is interrupted in gdb, we can also inspect the value of a variable using the "print" command.

If we are interested about the current value of variable a, we can simply "print variable_name". For example, after line 8 is executed, we can inspect if the atoi function correctly translate the characters to integer as below:

Starting program: /Users/bunny/Desktop/cse141/garbage 88 2

Breakpoint 1, main (argc=3, argv=0xbffff2ac) at garbage.c:8
8         a = atoi(argv[1]);
(gdb) n
9         b = atoi(argv[2]);
(gdb) print a
$10 = 88

Similiarly, if we are interested about the current value of a register, we can simply "print $register_name". For example, after line 8 is executed, we can inspect $eax as:

(gdb) print $eax
$11 = 88

If we are interested about all the register values, we can use "info registers" command to display the value in all registers.

(gdb) info registers
eax            0x58     88
ecx            0x0      0
edx            0xccccccc        214748364
ebx            0x1f76   8054
esp            0xbffff240       0xbffff240
ebp            0xbffff288       0xbffff288
esi            0x0      0
edi            0x0      0
eip            0x1f8a   0x1f8a 
eflags         0x282    642
cs             0x17     23
ss             0x1f     31
ds             0x1f     31
es             0x1f     31
fs             0x0      0
gs             0x37     55