I would like to generate a .def file for building a DLL with the MSVC compiler. I used LLVM to search through the compiled .obj files to find all definitions of C++ functions or variables. I would like to distinguish between normal definitions and inline definitions to only export the normal definitions in the DLL. I used this simple .cpp file as an example:
test.cpp:
#include <string>
static std::string static_test() {
return "test.cpp";
}
std::string test() { return static_test(); }
The compiler generates the object file test.cpp.obj
that contain several functions defined by the standard library. One of these functions is the inline constructor public: __cdecl std::exception::exception(char const *const, int)
. The COFF symbol class is IMAGE_SYM_CLASS_EXTERNAL
for both std::string test()
and std::exception::exception(char const*, int)
. Is there a way to distiguish these functions?
I found a solution myself, the information is contained in the COMDAT section definition, see https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#comdat-sections-object-only.
This is my code using LLVM to detect definitions:
void process_obj(const llvm::object::COFFObjectFile &object) {
std::vector<bool> section_contains_definition(object.getNumberOfSections() + 1);// sections are 1-based, +1 to avoid index arithm
for (const auto &sym : object.symbols()) {
const auto &coff_symbol = object.getCOFFSymbol(sym);
if (coff_symbol.isSectionDefinition()) {
const llvm::object::coff_section *section = nullptr;
uint32_t section_number = coff_symbol.getSectionNumber();
object.getSection(section_number, section);
if (!section)
continue;
// this is true for some sections containing global variables
if (!(section->Characteristics & llvm::COFF::IMAGE_SCN_LNK_COMDAT)) {
section_contains_definition[section_number] = true;
} else {
auto *def = coff_symbol.getSectionDefinition();
section_contains_definition[section_number] =
def->Selection == llvm::COFF::IMAGE_COMDAT_SELECT_NODUPLICATES;
}
}
}
for (const auto &sym : object.symbols()) {
const auto &coff_symbol = object.getCOFFSymbol(sym);
if (!coff_symbol.isExternal() || coff_symbol.isAnyUndefined() || !section_contains_definition[coff_symbol.getSectionNumber()])
continue;
... this coff_symbol is a definition ...
}
}