How integrate gnatmake/gnatbind/gnatlink in CMake files for C/Ada code?

Question

I wrote a code in a few languages (C, C++, Fortran77, Fortran90) and I can compile it without any sort of problem by using CMake. It works out perfectly.

Now, I would like to add in the main(), which is written in C, some Ada function and I want to compile it by CMake. Given that I am not able to link my Ada function to the main one by using CMake, I get

main.c:(.text.startup+0x16a): undefined reference to adainit
main.c:(.text.startup+0x179): undefined reference to adafunction
main.c:(.text.startup+0x190): undefined reference to adafinal

I did another simplified test by using the main function (written in C) calling the only Ada function, which I coded, and I compiled it by using

gcc -c main.c
gnatmake -c lib_ada.ali
gnatbind -n lib_ada.ali
gnatlink lib_ada.ali main.o -o exe

and it works out. Do you know how I can integrate this approach in a CMakeList.txt?

Note: I think (maybe I mistake) I cannot use the only gnatlink because I need to link all other functions I already have.

Here is reported a minimal reproducible example.

--- main.c ---

#include <stdio.h>

extern int adainit();
extern int adafinal();
extern int Add(int,int);

int main()
{
    adainit();
    printf ("Sum of 3 and 4 is: %d\n", Add (3,4));
    adafinal();

    return 0;
}

--- lib_test.adb ---

package body Lib_Test is
    function Ada_Add (A, B : Integer) return Integer is
    begin
        return A + B;
    end Ada_Add;
end Lib_Test;

--- lib_test.ads ---

package Lib_Test is
   function Ada_Add (A, B : Integer) return Integer;
   pragma Export (C, Ada_Add, "Add");
end Lib_Test;

1° test: if you compile by using the following commands:

gcc -c main.c
gnatmake -c lib_test.adb
gnatbind -n lib_test.ali
gnatlink lib_test.ali main.o -o exe

and run ./exe you get Sum of 3 and 4 is: 7.

2° test: I tried to use the following CMake file (CMakeLists.txt) linking the *.a

cmake_minimum_required(VERSION 2.6)
project(Ada2C)

enable_language(C)

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake")
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_VERBOSE_MAKEFILE ON)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O3 -m64")

find_library(TEST_lib lib_test.a PATHS ${CMAKE_CURRENT_SOURCE_DIR})
message(STATUS "Finding library: ${TEST_lib}")
add_executable(TEST_release ${CMAKE_CURRENT_SOURCE_DIR}/main.c)
target_link_libraries(TEST_release ${TEST_lib})

I generate library lib_test.a for the Ada function

gnatmake lib_test.adb
ar rc lib_test.a

I run the cmake and make and I get

main.c:(.text.startup+0x16a): undefined reference to adainit
main.c:(.text.startup+0x179): undefined reference to adafunction
main.c:(.text.startup+0x190): undefined reference to adafinal

Answer 1

More of a comment than an answer, but too long for a comment, so here goes:

Compiling Ada code into your binary means that your binary needs access to the GNAT runtime. This is one thing gnatlink does when you use it to link the final executable. The other thing is the b~<something>.ad{s,b} source gnatbind generates which you need to compile and link against as others mentioned.

The cleanest way to embed Ada in C I've seen so far is to create an encapsulated library. This probably does not make sense if your actual problem is with only one Ada function, but it does with larger chunks of Ada. The encapsulated library will be a shared library that has GNAT's runtime baked in. Being a shared library enables it to implicitly handle initialization during library loading so you don't need adainit() / adafinal() anymore.

The easiest way to create an encapsulated library is to use a ada_code.gpr file:

project ada_code is
   for Library_Name use "mylib";
   for Library_Dir use "lib";
   for Library_Kind use "relocatable";
   for Library_Standalone use "encapsulated";
   for Library_Auto_Init use "true";
   for Library_Interface use ("All", "Packages", "In.Your", "Ada.Code");
   for Source_Dirs use ("adasrc");
end ada_code;

In CMake, you can then do:

# tell CMake how to call `gprbuild` on the `.gpr` file.
# you may need to replace `gprbuild` with the absolute path to it
# or write code that finds it on your system.
add_custom_target(compile_mylib
        COMMAND gprbuild -P ada_code.gpr)

# copy the library file generated by gprbuild to CMake's build tree
# (you may skip this and just link against the file in the source tree)
add_custom_command(
        OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/mylib.so
        DEPENDS compile_mylib
        COMMAND ${CMAKE_COMMAND} -E copy
                ${CMAKE_SOURCE_DIR}/lib/mylib.so
                ${CMAKE_CURRENT_BINARY_DIR}/mylib.so)

# ... snip ...

# link to the copied library
# I am not 100% sure this adds the correct dependency to the custom command.
# You may need to experiment a bit yourself
target_link_libraries(TEST_release ${CMAKE_CURRENT_BINARY_DIR}/mylib.so)

In your C file, you can then delete everything related to adainit() and adafinal().