What does number part of mangled name for anonymous namespace variable generated in Visual Studio means?

Question

I am recompiling a C++ binary using the same sources. OS is Windows Server 2008 R2, compiler is Microsoft Visual Studio 2005.

After that I am running dumpbin.exe /ALL /DISASM /RAWDATA:NONE and comparing outputs. There is no difference in disassembly but there is difference in how some names from anonymous namespaces are mangled.

For example for the following definition in .cpp file:

namespace {
    TCHAR l_pszSlashes[] = _T("\\/");
}

I can have the following variations:

?l_pszSlashes@?A0x79ec3b0d@@3PA_WA (wchar_t * `anonymous namespace'::l_pszSlashes)
?l_pszSlashes@?A0xa9cc3791@@3PA_WA (wchar_t * `anonymous namespace'::l_pszSlashes)

The difference is presented only when I am comparing freshly built dll and the old one from another machine. If I am compiling this dll on the same PC I do not have this difference.

So the questions are: What does this hex number part in mangled name means? Why are this hex number part is different in some cases? How can I compile this project to have these name the same.

One strategy for anonymous namespaces is for the compiler to synthesize a random name. This is what MSVC (CL.EXE) uses. So it will change for each compile as it generates a new translation unit. — Eljay, Jun 18 '19 at 13:32
Why do you ask? Why do you need to know names of **anonymous** namespaces? — Yksisarvinen, Jun 18 '19 at 13:33
@Eljay Please go ahead and hover your mouse over the "add a comment" link — Lightness Races in Orbit, Jun 18 '19 at 13:37

score 1 · Answer 1 · answered Jun 18 '19 at 13:32

Anonymous namespaces are still unique. Each one's different. So, internally, they still need a name.

This appears to be a way of automatically generating one, presumably with a random number (in hex notation).

You will have to stop relying on these names being deterministic. You could look out for the ?A then ignore the subsequent hex number when comparing the decorated names of anonymous namespaces.

However, this may result in false positives, depending what you're doing, because the two namespaces may literally be in different places in the original source. There's no real way to know.

What does number part of mangled name for anonymous namespace variable generated in Visual Studio means?

1 Answers1