Replace individual function definition based on definitions in toolchain file

Question

I am making a library which will define a number of functions. There will be a default version of each function, and then the user may provide a file containing alternative definitions of each function, optimized for a certain hardware target.

I have a header file eg operations.h:

//operations.h

#ifndef OPERATIONS_H
#define OPERATIONS_H

#define BASE_OPERATION_1
int operation_1(int a, int b);
#define BASE_OPERATION_2
int operation_2(int a, int b);

#endif

Then the default implementations in a source file:

//base/operations.c

#ifdef BASE_OPERATION_1
int op_1(int a, int b)
{
    return a + b;
}
#endif 

#ifdef BASE_OPERATION_2
int op_2(int a, int b)
{
    return 2 * (a + b);
}
#endif 

I then have custom implementations in a separate source file. There may not be a replacement for every function.

// custom/operations.c

#ifdef CUSTOM_OPERATION_2
#undef BASE_OPERATION_2
int op_2(int a, int b)
{
    return 3 * (a + b);
}
#endif 

Then in a toolchain file I can do

set(CUSTOM_SOURCES custom/operations.c)
add_definitions( -DCUSTOM_OPERATION_2 )

---

The above solution results in a "duplicate symbol" error. I think because the preprocessor does not see both .c files at the same time, and so the undef does not achieve the behaviour I want. They are in different translation units so the undef is never seen when compiling the base file.

Can anyone tell me a way to get this behavior?

Answer 1

To solve your problem, remove all preprocessor directives concerning BASE_OPERATION_N and rewrite source files like so:

//operations.h

#ifndef OPERATIONS_H
#define OPERATIONS_H

int op_1(int a, int b);
int op_2(int a, int b);

#endif

//base/operations.c

#include <operations.h>

#ifndef CUSTOM_OP_1
int op_1(int a, int b)
{
    return a + b;
}
#endif 

#ifndef CUSTOM_OP_2
int op_2(int a, int b)
{
    return 2 * (a + b);
}
#endif 

// custom/operations.c

#ifdef CUSTOM_OP_2
int op_2(int a, int b)
{
    return 3 * (a + b);
}
#endif

With this, -DCUSTOM_OP_2 suffices to disable compilation of base op_2 and enable the compilation of custom op_2. Conversely the absence of -DCUSTOM_OP_2 suffices to disable the compilation of custom op_2 and enable the compilation of base op_2.

Here's a demo. My CMake project is:

$ tree
.
├── base
│   └── operations.c
├── CMakeLists.txt
├── config.cmake
├── custom
│   └── operations.c
├── operations.h
└── test.c

cmake.config is my toolchain file. It doesn't need to exist for the default build. I'm building a test program as well as the library to prove this works. For the same purpose I'm putting some printfs in the sources:

$ cat base/operations.c 
//base/operations.c

#include <stdio.h>
#include <operations.h>

#ifndef CUSTOM_OP_1
int op_1(int a, int b)
{
    printf("%s_%s\n","Base",__func__);
    return a + b;
}
#endif 

#ifndef CUSTOM_OP_2
int op_2(int a, int b)
{
    printf("%s_%s\n","Base",__func__);
    return 2 * (a + b);
}
#endif

$ cat custom/operations.c 
// custom/operations.c
#include <stdio.h>

#ifdef CUSTOM_OP_1
int op_1(int a, int b)
{
    printf("%s_%s\n","Custom",__func__);
    return 3 * (a + b);
}
#endif

#ifdef CUSTOM_OP_2
int op_2(int a, int b)
{
    printf("%s_%s\n","Custom",__func__);
    return 3 * (a + b);
}
#endif

My CMakeLists.txt is:

$ cat CMakeLists.txt 
cmake_minimum_required(VERSION 3.14)
project(operations)

set(BASE_SOURCES ./base/operations.c)
set(SOURCES ${BASE_SOURCES} ${CUSTOM_SOURCES})
include_directories(./)

add_library(
  operations
  ${SOURCES}
)

add_executable(
    test
    test.c)
    
target_link_libraries(test operations)

The default build goes:

$ mkdir build
$ cd build
$ cmake ..
-- The C compiler identification is GNU 13.2.0
-- The CXX compiler identification is GNU 13.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: /home/imk/develop/so/cmake_prob/build
$ make
[ 25%] Building C object CMakeFiles/operations.dir/base/operations.c.o
[ 50%] Linking C static library liboperations.a
[ 50%] Built target operations
[ 75%] Building C object CMakeFiles/test.dir/test.c.o
[100%] Linking C executable test
[100%] Built target test

And the program runs:

$ ./test
Base_op_1
Base_op_2

Now here's my toolchain file v. 1:

$ cd ..
$ cat config.cmake 
set(CUSTOM_SOURCES ./custom/operations.c)
add_compile_options(-DCUSTOM_OP_2)

I'm disabling base op_2, enabling custom op_2

Zap and regenerate the build system:

$ cd build
$ rm -fr *
$ cmake -DCMAKE_TOOLCHAIN_FILE=config.cmake ..
-- The C compiler identification is GNU 13.2.0
-- The CXX compiler identification is GNU 13.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: /home/imk/develop/so/cmake_prob/build

Rebuild and test:

$ make && ./test
[ 20%] Building C object CMakeFiles/operations.dir/base/operations.c.o
[ 40%] Building C object CMakeFiles/operations.dir/custom/operations.c.o
[ 60%] Linking C static library liboperations.a
[ 60%] Built target operations
[ 80%] Building C object CMakeFiles/test.dir/test.c.o
[100%] Linking C executable test
[100%] Built target test
Base_op_1
Custom_op_2

There's the custom op_2.

Now I'd like to revert to base op_2 and enable custom op_1. The tool chain file for that is v.2:

$ cd ..
$ cat config.cmake 
set(CUSTOM_SOURCES ./custom/operations.c)
add_compile_options(-DCUSTOM_OP_1)

Don't need to regenerate the build system now. I can do:

$ cd build
$ make
-- Configuring done (0.0s)
-- Generating done (0.0s)
-- Build files have been written to: /home/imk/develop/so/cmake_prob/build
[ 20%] Building C object CMakeFiles/operations.dir/base/operations.c.o
[ 40%] Building C object CMakeFiles/operations.dir/custom/operations.c.o
[ 60%] Linking C static library liboperations.a
[ 60%] Built target operations
[ 80%] Building C object CMakeFiles/test.dir/test.c.o
[100%] Linking C executable test
[100%] Built target test
$ ./test
Custom_op_1
Base_op_2

There's the custom op_1.