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Summary of GDB Hints

posted Nov 3, 2009, 10:11 AM by Likai Liu
Some GDB usage tips collected from messages posted on the mailing list.
  • Prevent bomb explosion:
    (gdb) b explode_bomb

    then when the program is stopped by the debugger, check if the function has been triggered.

  • Tracing execution at assembly level. At the beginning of each gdb session, it would help to enter this command first:
    (gdb) display /i $pc

    then gdb will show the upcoming instruction each time your program steps.

    (gdb) nexti    # next instruction, skips over function calls.
    (gdb) ni       # shorthand for nexti
    (gdb) stepi    # step instruction, steps into function calls.
    (gdb) si       # shorthand for stepi
    To see the content of registers,
    (gdb) info registers
    (gdb) i r      # shorthand for info registers. Note the space.
  • To disassemble a function without running it,
    (gdb) disassemble addr
    (gdb) disas addr    # shorthand for disassemble

    The address "addr" can be a symbol name (e.g. phase_1) or address (e.g. 0x08048ea6) or a register (e.g. $pc).

  • To examine memory content,
    (gdb) x /fmt addr
    where /fmt specifies the format at the memory location "addr". Some examples:
    (gdb) x /s 0x8049890   # shows the string at address 0x8049890
    (gdb) x /16bc $esi     # shows 16 bytes of characters at $esi
    (gdb) x /4wx &node1    # shows 4 words of hex at symbol name node1
    (gdb) x /6wx $ebp - 0x20  # shows 6 words of hex at address $ebp - 0x20.

    Notice that &node1 is the address of that symbol. If you omit the &, it would try to read a word value at that memory location, and then use the value as the address to show for the x command.

  • If you see a constant that seems to refer to a memory location, and if you want to see if there is a symbol associated with that address, you can lookup the symbol name like this:
    (gdb) info symbol 0x08048cfb
    phase_2 in section .text
    (gdb) info symbol 0x804a5fc
    node1 in section .data

    Sometimes the symbol name reveals intent of the program.

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