COFF_TYPE on x86_64

Steven Robbins steve at
Fri Oct 2 18:09:13 UTC 2020

On Friday, October 2, 2020 12:59:10 P.M. CDT Torbjörn Granlund wrote:
> Jeremy Drake <gmp at> writes:
>   On msys2's MINGW-packages, we recently hit an issue[1] which I eventually
>   tracked down to the very issue documented in
> This is a link to a huge file.  Many things are mentioned there.

But, to be fair, it is a link to a specific line -- 2128 -- which documents one 
specific thing:

dnl  On MINGW, recent versions of the linker have an automatic import scheme
dnl  for data in a DLL which is referenced by a mainline but without
dnl  __declspec (__dllimport__) on the prototype.  It seems functions
dnl  without type information are treated as data, or something, and calls
dnl  to them from the mainline will crash.  gcc puts type information on the
dnl  C functions it generates, we need to do the same for assembler
dnl  functions.
dnl  This applies only to functions without __declspec(__dllimport__),
dnl  ie. without __GMP_DECLSPEC in the case of libgmp, so it also works just
dnl  to ensure all assembler functions used from outside libgmp have
dnl  __GMP_DECLSPEC on their prototypes.  But this isn't an ideal situation,
dnl  since we don't want perfectly valid calls going wrong just because
dnl  there wasn't a prototype in scope.
dnl  When an auto-import takes place, the following warning is given by the
dnl  linker.  This shouldn't be seen for any functions.
dnl      Info: resolving _foo by linking to __imp__foo (auto-import)
dnl  COFF type directives look like the following
dnl      .def    _foo
dnl      .scl    2
dnl      .type   32
dnl      .endef
dnl  _foo is the symbol with GSYM_PREFIX (_).  .scl is the storage class, 2
dnl  for external, 3 for static.  .type is the object type, 32 for a
dnl  function.
dnl  On an ELF system, this is (correctly) rejected due to .def, .endef and
−dnl  .scl being invalid, and .type not having enough arguments.

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