Are libfoo.a and foo.lib compatabile formats?

Question

Some build scripts (such as the one in numpy) simply does the following to make a gcc-compiled library archive work with the Visual Studio linker:

copy libfoo.a foo.lib

Surprisingly it seems to work. Does anyone know why?

Answer 1

While I do not know any details for the lib format, I can give some information on the lib*.a format. This is just a archive of *.o object files, and can be manipulated with the ar program (part of binutils).

prompt>ar t /usr/lib64/libc.a | head
init-first.o
libc-start.o
sysdep.o
version.o
check_fds.o
libc-tls.o
elf-init.o
dso_handle.o
errno.o
errno-loc.o
prompt>ar t /usr/lib64/libc.a | wc
   1447    1447   16904
prompt>

I assume then that *.lib also is an archive of object files with either a identical or compatible content index.

Answer 2

Depending on several factors, it may or may not work - and for a couple reasons. I take it you mean full-on, reversible compatibility.

1. For implibs that are used to bind DLLs to executables, the answer is no.

   I once tried to link an MSVC++ implib with a gcc-produced dll. If the formats were compatible, it would have worked when I renamed the library libfoo.a. To get around this, there is a utility called reimp that can produce a suitable gcc implib from a DLL. To reverse the process, microsoft's `lib` tool can create a static implib from a .def file, which gcc can produce with a DLL should it be given the correct flags. [Search "reimp" at the mingw website for more info]

2. C++-compiled object files are also incompatible because of Name Mangling.

   Different C++ compilers mangle variable and object names differently, resulting in "[object] code ... that is not usually linkable"

3. For a static library compiled as plain old C? Yes

   I'd need a bit more information as to what type of project you are concerned with, but if it is C and the `copy libfoo.a foo.lib` hack worked, this may be the case. Try to see if this works in reverse with Mingw or Dev-C++. Edit: Actually, this will only work when the library is compiled with gcc on a Windows platform (and it works both ways!). the only exceptions to this AFAIK are cross-compiled libraries - MSVC refuses to take them, at least with Ubuntu-provided "i586-mingw32msvc-ar".

Another alternative explanation is that MSVC linker is compatible with the .a format for historical reasons. IIRC, it has been around since the days of UNIX. Again, though I could only see this working for pure C, and not C++.
EDIT: It is actually the other way around. The Windows version of the GCC toolkit creates static libraries that are compatible with Microsoft's COFF format.