Embedded resources are missing when res_embed target is linked as a static library

[dmikushin]

Summary

When res_embed() is used on a target that is later consumed as a static library, embedded resources may be missing at runtime even though the build succeeds and the generated resource objects are present in the archive.

This happens because the generated <resource>.cpp registers the resource through a static initializer:

static AddFile_<hash> addFile_<hash>;

For a static archive, the linker only extracts object files that satisfy unresolved symbols. A static initializer inside an archive member does not create an unresolved symbol by itself, so the generated resource object is not pulled into the final executable. As a result, res::embed::get("payload") returns nullptr.

Minimal reproducer

cmake_minimum_required(VERSION 3.5)
project(res_embed_static_repro C CXX ASM)

option(REPRO_SHARED "Build repro_lib as shared" OFF)
option(BUILD_EXAMPLE "" OFF)
add_subdirectory(res_embed EXCLUDE_FROM_ALL)

if(REPRO_SHARED)
  add_library(repro_lib SHARED lib.cpp)
else()
  add_library(repro_lib STATIC lib.cpp)
endif()
target_include_directories(repro_lib PRIVATE res_embed/include)
res_embed(TARGET repro_lib NAME payload PATH ${CMAKE_CURRENT_SOURCE_DIR}/payload.txt KEYWORD)

add_executable(repro main.cpp)
target_link_libraries(repro PRIVATE repro_lib)

payload.txt:

hello from res_embed

lib.cpp:

#include <res_embed.h>
#include <cstddef>

extern "C" const char* repro_get(size_t* size) {
    return res::embed::get("payload", size);
}

main.cpp:

#include <cstddef>
#include <cstdio>
#include <cstring>

extern "C" const char* repro_get(size_t* size);

int main() {
    size_t size = 0;
    const char* data = repro_get(&size);
    if (!data) {
        std::puts("MISSING");
        return 2;
    }
    std::printf("FOUND %zu %.*s", size, (int)size, data);
    return std::memcmp(data, "hello from res_embed\n", size) == 0 ? 0 : 3;
}

Observed behavior

With repro_lib as STATIC:

MISSING

With repro_lib as SHARED:

FOUND 22 hello from res_embed

Confirmed workarounds

Both of these make the static-library case work:

1. Link the archive with --whole-archive:

target_link_libraries(repro PRIVATE
  -Wl,--whole-archive repro_lib -Wl,--no-whole-archive)

2. Explicitly call the generated initializer so the linker has an unresolved symbol to pull from the archive:

namespace res { namespace embed { namespace init { void payload(); } } }
res::embed::init::payload();

Expected behavior

res_embed() should either support static-library consumers without requiring --whole-archive/manual init::<name>() calls, or document/generate an explicit initialization mechanism for static-library use cases.