document updated 12 years ago, on Oct 8, 2012

What if you need to debug a binary that doesn't have any symbols?

Assembly-level debugging in GDB

To break at the very first instruction:

(gdb) info files
    ...
    Entry point: 0x804ded8
    ...
(gdb) break *0x804ded8
(gdb) run

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). If you get the "No function contains specified address" error, you have to indicate the number of bytes to disassemble:
```
(gdb) disas $pc, +60
```
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.

disassembling a binary

To show the assembly code for a binary:

In Linux:
```
objdump -d
```
In Solaris:
```
dis
```