Merging dedicated ELF sections from libraries into application dedicated ELF section - c

Question

Here is my test. I have a main application composed of the sources main.c and misc.c and a static library made of lib.c.
GOAL: I want to declare all my struct module declarations in an ELF section .modules.
ISSUE: I can only see the struct module declarations from the main application. Here is the output I can see with the following code:
Hello World
- module:module_a
- module:module_b
If I call my_lib() into main() then I see:
Hello World
MyLib
- module:module_a
- module:module_b
- module:module_lib
But I am not interested to directly call module's function into my main application.
CMakeLists.txt
add_executable(main main.c misc.c)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffunction-sections -fdata-sections")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--gc-sections")
set(LINKER_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/linker.ld")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -T ${LINKER_SCRIPT}")
add_library(static_lib STATIC lib.c)
target_link_libraries(main static_lib)
main.c
#include "module.h"
extern const struct module modules_start[];
extern const struct module modules_end[];
struct module __attribute__ ((section (".modules"))) module_a = {
.name = "module_a",
};
int main(void) {
puts("Hello World");
const struct module *m = modules_start;
while (m < modules_end) {
printf("- module:%s\n", m->name);
m++;
}
return 0;
}
misc.c
#include "module.h"
struct module __attribute__ ((section (".modules"))) module_b = {
.name = "module_b",
};
module.h
#include <stdio.h>
struct module {
const char* name;
};
lib.c
#include "module.h"
struct module __attribute__ ((section (".modules"))) __attribute__ ((used)) module_lib = {
.name = "module_lib",
};
int my_lib(void) {
puts("MyLib");
return 0;
}
linker.ld
SECTIONS
{
.modules : {
modules_start = .;
KEEP(*(.modules))
modules_end = .;
}
}
INSERT AFTER .rodata;
Here are some ELF information:
$ readelf --sections libstatic_lib.a | grep -A 1 modules
[ 5] .modules PROGBITS 0000000000000000 00000058
0000000000000008 0000000000000000 WA 0 0 8
[ 6] .rela.modules RELA 0000000000000000 00000278
0000000000000018 0000000000000018 I 13 5 8
$ readelf --sections main | grep -A 1 modules
[17] .modules PROGBITS 00000000000009c0 000009c0
0000000000000010 0000000000000000 WA 0 0 8
$ nm libstatic_lib.a | grep module
0000000000000000 D module_lib
$ nm main | grep module
00000000000009c0 D module_a
00000000000009c8 D module_b
00000000000009d0 D modules_end
00000000000009c0 D modules_start

Answer 1

If there are no references to an object file in a static library, by default that object file is not included in the link. With the binutils linker, you can disable this optimization using the --whole-archive option.