How to determine the major compiler version from .obj files compiled with /GL?

Question

I'm trying to determine Visual Studio version (2002/2003, 2005, 2008, 2010, 2012, 2013, 2015) from the .obj file generated with the link time code generation option.

The file I have, generated with MSVC2012, has following COFF header contents:

                File Header
+0  00 00       Machine - Unknown Machine
+2  FF FF       NumberOfSections
+4  01 00 4C 01 TimeDateStamp
+8  70 94 F9 55 PointerToSymbolTable
+12 38 FE B3 0C NumberOfSymbols
+16 A5 D9       SizeOfOptionalHeader
+18 AB 4D       Characteristics
                Optional Header
+20 AC 9B       Magic
+22 D6 B6       Linker Version Major/Minor

It seems that the initial 4 bytes being 00,00,FF,FF mark it as a LTCG object, and what follows is proprietary. None of the usual file header members make "sense" (maybe the timestamp is OK, I didn't check).

Does anyone know offhand if any part of this header is compiler-specific? All I need to determine is the MSVC major version used to compile the object...

It appears that there is a version, coded as <MAJOR:16:LE> 0x80 <MINOR:16:LE>, stored shortly after the header. E.g.:

17.00.61030 -> 0x11.0xEE66 -> 11 00 80 66 EE
19.00.23026 -> 0x13.0x59F2 -> 13 00 80 F2 59

What's needed is to figure out how to get to it reliably by offsets from preceding data.

This is a related question, with no resolution...

/LTCG is a linker option. But an .obj file is created by the compiler, the linker never got close to it. So that doesn't make sense. You are probably talking about the /GL option. And yes, it is special, the .obj file doesn't contain machine code at all. It contains IL, the intermediate language used between the front and back end of the compiler. Used by the linker to restart the back-end for /LTCG. As you'd expect, it is completely undocumented and subject to change without notice. — Hans Passant, Sep 17 '15 at 21:56
@HansPassant For what it's worth, released MSVC versions will not retroactively change their output. So while "change without notice" applies to future releases, it's not that much of a hindrance in practice. — Kuba hasn't forgotten Monica, Sep 17 '15 at 22:23

score 1 · Answer 1 · edited Nov 29 '15 at 11:54

TL,DR :
You can't get the compiler version with this file format, I guess ...

Complete answer :

It looks like some variation of the "anonymous file format", described in the "winnth.h" by various ANON_OBJECT_HEADER_XXX structures (replace XXX by V2 or BIGOBJ). Here is a copy of the ANON_OBJECT_HEADER_BIGOBJ found in winnt.h :

typedef struct ANON_OBJECT_HEADER_BIGOBJ {
    /* same as ANON_OBJECT_HEADER_V2 */
    WORD    Sig1;            // Must be IMAGE_FILE_MACHINE_UNKNOWN
    WORD    Sig2;            // Must be 0xffff
    WORD    Version;         // >= 2 (implies the Flags field is present)
    WORD    Machine;         // Actual machine - IMAGE_FILE_MACHINE_xxx
    DWORD   TimeDateStamp;
    CLSID   ClassID;         // CLSID is a 16 bytes struct  (not original comment)
    DWORD   SizeOfData;      // Size of data that follows the header
    DWORD   Flags;           // 0x1 -> contains metadata
    DWORD   MetaDataSize;    // Size of CLR metadata
    DWORD   MetaDataOffset;  // Offset of CLR metadata
    /* bigobj specifics */
    DWORD   NumberOfSections; // extended from WORD
    DWORD   PointerToSymbolTable;
    DWORD   NumberOfSymbols;
} ANON_OBJECT_HEADER_BIGOBJ;</code>

The description match:

Sig1 :    00 00
Sig2 :    FF FF
Version : >=2
Machine : 0x14c`

The other header structures (i.e, ANON_OBJECT_HEADER and ANON_OBJECT_HEADER_V2) are basically the same, but with less fields.
For the Version field, I found some information here : http://www.geoffchappell.com/studies/msvc/link/dump/infiles/obj.htm

Looks like the Version field is "1" for anonymous files, and it seems like the anonymous files and the so called "import files" shared the same characteristics, only that Version = 0 for import file format (I do not really know what it is admittedly).

But yeah, by just looking at the header, it seems that we have no information on what compiler version was used. And even then, when looking at .obj files generated with the /GL switch, they do not exactly follow this format and I didn't find much information about them. I'll be glad that someone prove me wrong.

How to determine the major compiler version from .obj files compiled with /GL?

1 Answers1

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