start execution

• start debug: gdb executeble_name
• run: (gdb) run arg1 "arg2"
• restart program running in the debugger: (gdb) kill
• exit debugger: (gdb) quit / q

jump around

• line by line, jump into call function: (gdb) step
• line by line, not jump into call function: (gbd) next
• return from calling function: (gdb) return/ finish
• examine variable: (gdb) print x
• modify variable: (gdb) set x = 3
• call function: (gdb) call abort()

stack trace

• show stack trace: (gdb) backtrace/ bt
• change into other frame: (gdb) frame frame_num
• show frame info: (gdb) info frame
• show stack local variables: (gdb) info locals
• show function arguments: (gdb) info args

break point

• show breakpoint info: (gdb) info break
• place a breakpoint at specified function: (gdb) break function_name
• place a breakpoint at specified line: (gdb) break line_num
• place a breakpoint at the specified line within the specified source file: (gdb) file_name:line_num
• disable breakpoint identified by breaknum: (gdb) disable breaknum
• enable breakpoint identified by breaknum: (gdb) enable breaknum
• skip a breakpoint a certain number of times: (gdb) ingore breaknum times_to_skip, eg: (gdb) ingore 2 5 will ignore next 5 crossings of breakpoint 2.

advanced

examine memory

Use the x command to examine memory. The syntax for the x command is x/FMT ADDRESS. The FMT field is a count followed by a format letter and a size letter. There are many options here, use the help command ‘help x’ to see them all. The ADDRESS argument can either be a symbol name, such as a variable, or a memory address.

If we have char *s = "Hello World\n", some uses of the x command could be:

Examine the variable as a string:

``

(gdb) x/s s
0x8048434 <_IO_stdin_used+4>:    "Hello World\n"

Examine the variable as a character:

``

(gdb) x/c s
0x8048434 <_IO_stdin_used+4>:   72 'H'

Examine the variable as 4 characters:

``

(gdb) x/4c s
0x8048434 <_IO_stdin_used+4>:   72 'H'  101 'e' 108 'l' 108 'l'

Examine the first 32 bits of the variable:

``

(gdb) x/t s
0x8048434 <_IO_stdin_used+4>:   01101100011011000110010101001000

Examine the first 24 bytes of the variable in hex:

``

(gdb) x/3x s
0x8048434 <_IO_stdin_used+4>:   0x6c6c6548      0x6f57206f      0x0a646c72

Use the list command to have gdb print out the lines of code above and below the line the program is stopped at. In the example below, the breakpoint is on line 8.

``

(gdb) list
3       int main(int argc, char **argv)
4       {
5         int x = 30;
6         int y = 10;
7       
8         x = y;
9       
10        return 0;
11      }

http://www.unknownroad.com/rtfm/gdbtut/gdbtoc.html