How to disassemble a memory range with GDB?

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;
}

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

(gdb) x/i 0xdeadbeef

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

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.

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

gcc -S hello.c

How to disassemble a memory range with GDB?

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