Question

I’m seeing what I think is strange behaviour from object files output by the Microsoft Visual Studio 2003 tools. The file utility tells me:

asmfile.obj: 80386 COFF executable not stripped - version 30821

For objects created by the assembler, but for objects coming from C files, I get just:

cfile.obj: data

Using Microsoft’s dumpbin utility and the objdump I got from cygwin, I can disassemble the assembly-built file, but I get no useful results from either utility for the C-built files.

I have a couple of questions related to this difference:

1. What is the object file format generated by the MSVC2003 compiler?
2. How can I disassemble that object file?

I am particularly interested in getting the disassembly in AT&T syntax – I’m doing a port of a large source base to make it work with GCC, and I would like to use this method as a shortcut for some of the inline assembly routines in the project.

Edit: Adding some more information.

When I run dumpbin on one of these files gives me no results:

C:\> dumpbin /disasm Func.obj
Microsoft (R) COFF/PE Dumper Version 7.10.6030    
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file Func.obj

FileType: ANONYMOUS OBJECT

With objdump, it gives:

$ objdump -d Func.obj
objdump: Func.obj: File truncated

On the files built from assembly, I get reasonable results.

Edit again: Adding command line information.

The assembly files are built with a command line resembling the following:

ml -nologo -W3 -WX -c -coff -FoAssemblyFile.obj -Zi -Cx AssemblyFile.asm

ml when executed by itself says:

Microsoft (R) Macro Assembler Version 6.15.8803
Copyright (C) Microsoft Corp 1981-2000.  All rights reserved.

The C files are built with the following command:

cl -nologo -W4 -WX -Gs32768 -GX -Gy -c -FdCFile.pdb -FoCFile.obj -Zi 
   -Gm -O1 -Oy- -Gy -GL -X CFile.c

There are some -I and -D options passed to ml and to cl, but I’ve omitted them for brevity here. The cl options are described here.

Answer 1

Edit based on the cl command line options being added to the question:

I think the problem is the use of the /GL option, which specifies that link-time code generation optimization will be done. from a doc page on that option:

obj files produced with /GL will not be available to such linker utilities as EDITBIN and DUMPBIN.

Using this option causes the compiler to generate .obj files that the linker can perform program-wide optimization on – apparently the file format is proprietary (maybe it’s documented somewhere, but I suspect not).

The docs for /GL (also known as “whole program optimization”, “link-time code generation”, or LTCG) contain several warnings about interoperability of the .obj files or libraries containing such objects files.

---

Original answer:

What exactly is in the C source for the .obj file you’re trying to disassemble? I get the following using dumpbin /disasm test.obj for a simple ‘hello world’ program:

Microsoft (R) COFF/PE Dumper Version 8.00.50727.42
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file test.obj

File Type: COFF OBJECT

_main:
  00000000: 55                 push        ebp
  00000001: 8B EC              mov         ebp,esp
  00000003: 6A 01              push        1
  00000005: 68 00 00 00 00     push        offset $SG4665
  0000000A: E8 00 00 00 00     call        _printf
  0000000F: 83 C4 08           add         esp,8
  00000012: 33 C0              xor         eax,eax
  00000014: 3B EC              cmp         ebp,esp
  00000016: E8 00 00 00 00     call        __RTC_CheckEsp
  0000001B: 5D                 pop         ebp
  0000001C: C3                 ret

  Summary

         7AC .debug$S
          30 .debug$T
          2F .drectve
           4 .rdata
           4 .rtc$IMZ
           4 .rtc$TMZ
          1D .text

Note: this is using an .obj file compiled by and a dumpbin provided by VS2005, but I can’t imagine this stuff would have changed much from VS2003.