How to disassemble a memory range with GDB?

Gdb Problem Overview

I'm trying to disassemble a program to see a syscall assembly instruction (the INT instruction, I believe) and the handler with GDB and have written a little program (see below) for it that opens and closes a file.

I was able to follow the call to fopen with GDB until it executed a call.

When I tried to tell GDB "disassemble 0x...." (address of call) it responded with 'No function contains specified address.'

Is it possible to force GDB to disassemble (or display it in assembler as good as possible) that memory address? If so, how?

#include <stdio.h>
#include <stdlib.h>

int main() {
	FILE* f;
	f = fopen("main.c", "r");
	if (!f) { 
	  perror("open");
	  return -1;
	}
    fclose(f);
    return 0;
}

Gdb Solutions

Solution 1 - Gdb

Yeah, disassemble is not the best command to use here. The command you want is "x/i" (examine as instructions):

(gdb) x/i 0xdeadbeef

Solution 2 - Gdb

Do you only want to disassemble your actual main? If so try this:

(gdb) info line main 
(gdb) disas STARTADDRESS ENDADDRESS

Like so:

USER@MACHINE /cygdrive/c/prog/dsa
$ gcc-3.exe -g main.c

USER@MACHINE /cygdrive/c/prog/dsa
$ gdb a.exe
GNU gdb 6.8.0.20080328-cvs (cygwin-special)
...
(gdb) info line main
Line 3 of "main.c" starts at address 0x401050 <main> and ends at 0x401075 <main+
(gdb) disas 0x401050 0x401075
Dump of assembler code from 0x401050 to 0x401075:
0x00401050 <main+0>:    push   %ebp
0x00401051 <main+1>:    mov    %esp,%ebp
0x00401053 <main+3>:    sub    $0x18,%esp
0x00401056 <main+6>:    and    $0xfffffff0,%esp
0x00401059 <main+9>:    mov    $0x0,%eax
0x0040105e <main+14>:   add    $0xf,%eax
0x00401061 <main+17>:   add    $0xf,%eax
0x00401064 <main+20>:   shr    $0x4,%eax
0x00401067 <main+23>:   shl    $0x4,%eax
0x0040106a <main+26>:   mov    %eax,-0xc(%ebp)
0x0040106d <main+29>:   mov    -0xc(%ebp),%eax
0x00401070 <main+32>:   call   0x4010c4 <_alloca>
End of assembler dump.

I don't see your system interrupt call however. (its been a while since I last tried to make a system call in assembly. INT 21h though, last I recall

Solution 3 - Gdb

This isn't the direct answer to your question, but since you seem to just want to disassemble the binary, perhaps you could just use objdump:

objdump -d program

This should give you its dissassembly. You can add -S if you want it source-annotated.

Solution 4 - Gdb

You can force gcc to output directly to assembly code by adding the -S switch

gcc -S hello.c

Solution 5 - Gdb

fopen() is a C library function and so you won't see any syscall instructions in your code, just a regular function call. At some point, it does call open(2), but it does that via a trampoline. There is simply a jump to the VDSO page, which is provided by the kernel to every process. The VDSO then provides code to make the system call. On modern processors, the SYSCALL or SYSENTER instructions will be used, but you can also use INT 80h on x86 processors.

Solution 6 - Gdb

If all that you want is to see the disassembly with the INTC call, use objdump -d as someone mentioned but use the -static option when compiling. Otherwise the fopen function is not compiled into the elf and is linked at runtime.

Solution 7 - Gdb

gdb disassemble has a /m to include source code alongside the instructions. This is equivalent of objdump -S, with the extra benefit of confining to just the one function (or address-range) of interest.

Solution 8 - Gdb

You don't have to use gdb. GCC will do it.

 gcc -S foo.c

This will create foo.s which is the assembly.

gcc -m32 -c -g -Wa,-a,-ad foo.c > foo.lst

The above version will create a listing file that has both the C and the assembly generated by it. GCC FAQ

Solution 9 - Gdb

full example for disassembling a memory range to C

/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/arm-none-eabi-gdb

(gdb)file /root/ncs/zephyr/samples/hello_world/build_nrf9160dk_nrf9160ns/zephyr/zephyr.elf
(gdb) directory /root/ncs/zephyr/samples/hello_world/src
#here you want 1
(gdb) info line* 0x000328C0
#here you want 2, -0x04 ~ +0x04 is your range size
(gdb) disassemble /m 0x000328C0-0x04, 0x000328C0+0x04
#here with binary code
(gdb) disassemble /r 0x000328C0-0x04, 0x000328C0+0x04
(gdb) info thread
(gdb) interpreter-exec mi -thread-info

Content Type	Original Author	Original Content on Stackoverflow
Question	Patrick	View Question on Stackoverflow
Solution 1 - Gdb	Michael Snyder	View Answer on Stackoverflow
Solution 2 - Gdb	nont	View Answer on Stackoverflow
Solution 3 - Gdb	Falaina	View Answer on Stackoverflow
Solution 4 - Gdb	askjhdf	View Answer on Stackoverflow
Solution 5 - Gdb	Joel	View Answer on Stackoverflow
Solution 6 - Gdb	Al.	View Answer on Stackoverflow
Solution 7 - Gdb	Lakshman Kumar	View Answer on Stackoverflow
Solution 8 - Gdb	Milhous	View Answer on Stackoverflow
Solution 9 - Gdb	YU Chen Shih	View Answer on Stackoverflow