# Shared library in CMake
**URL:** https://discourse.cmake.org/t/shared-library-in-cmake/8773
**Category:** Code
**Tags:** os:linux
**Created:** [August 18, 2023, 3:13pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773 "2023-08-18T15:13:07Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Matheus\_Diogenes\_And](https://discourse.cmake.org/u/Matheus_Diogenes_And)
#### Post date: [August 18, 2023, 3:13pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773/1 "2023-08-18T15:13:08Z")
Hello everyone.
**Summary**
I am working in a C++20 project that makes use of a shared library. So far, I successfully managed to call the shared library when compiling and running the script, however, when using CMake to do the job, the things are not going that well.
**Pre-CMake**
I have a code `main.cpp` with a structure similar to the one below:
```cpp
extern "C"
{
void c_custom(char *param);
}
int main() {
char *param = ...
c_custom(param);
return 0;
}
```
The above script has no imports or includes whatsoever.
For compiling, I simply type:
```sh
g++ -O3 main.cpp -L/opt/custom/lib -std=c++20 -lcustom -lgfortran -lm -o main
```
Where the folder `/opt/custom/lib` contains a file named `libcustom.a`.
The compilation and execution work perfectly well.
**Pos-CMake**
For the CMake project, I created the following folder structure:
- includes
- src
- build
With the following `CMakeLists.txt`:
```cmake
cmake_minimum_required(VERSION 3.16)
project(Main)
##### Include Dirs
include_directories(SYSTEM includes "${PROJECT_BINARY_DIR}")
##### Compile options
add_compile_options(
-std=c++20
# CUSTOM Lib
-L/opt/custom/lib
-lcustom
-lgfortran
-lm
)
file(GLOB_RECURSE SOURCES "src/*.cpp")
add_executable(main ${SOURCES})
target_link_libraries(main)
```
With the following codes:
- `includes/main.hpp`:
```cpp
#ifndef MAIN_HPP
#define MAIN_HPP
#ifdef __cplusplus
extern "C" {
#endif
void c_custom(char *param);
#ifdef __cplusplus
}
#endif
#endif
```
- `src/main.cpp`:
```cpp
#include "main.hpp"
int main() {
char *param = ...
c_custom(param);
return 0;
}
```
When running the commands:
```sh
cd build
cmake ..
```
I obtain the following output:
```sh
-- The C compiler identification is GNU 11.4.0
-- The CXX compiler identification is GNU 11.4.0
-- 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
-- Generating done
-- Build files have been written to: project/build
```
Which seems to be fine. However, when running `make`, we obtain these:
```sh
[50%] Building CXX object CMakeFiles/main.dir/src/main.cpp.o
[100%] Linking CXX executable main
/usr/bin/ld: CMakeFiles/main.dir/src/main.cpp.o: in the function `main':
main.cpp:(.text+0x812): reference to `c_custom' not defined
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/main.dir/build.make:97: main] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/main.dir/all] Error 2
make: *** [Makefile:91: all] Error 2
```
I took some hours to play with the commands `add_library`, `set_target_properties`, `target_include_directories`, and even setting the `LANGUAGE` to `C CXX Fortran` in the `project` statement, but none worked.
I would like to know if someone ever did something similar before.
Thanks and regards.
---
### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [August 18, 2023, 3:36pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773/2 "2023-08-18T15:36:35Z")
> [@Matheus\_Diogenes\_And](#):
>
> ```auto
> add_compile_options(
> -std=c++20
> # CUSTOM Lib
> -L/opt/custom/lib
> -lcustom
> -lgfortran
> -lm
> )
>
> ```
You’re giving linker options as compile options. You should use `add_link_options()` for those. Better would be to use `IMPORTED` targets, but this should at least get you _a_ working build.
---
### Author:  [@Matheus\_Diogenes\_And](https://discourse.cmake.org/u/Matheus_Diogenes_And)
#### Post date: [August 18, 2023, 3:57pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773/3 "2023-08-18T15:57:50Z")
> [@ben.boeckel](#):
>
> add\_link\_options
Thank you very much. I tried to follow the recommendation, ending-up with the following script:
```cmake
cmake_minimum_required(VERSION 3.16)
project(Main)
##### Include Dirs
include_directories(SYSTEM includes "${PROJECT_BINARY_DIR}")
##### Compile options
# CUSTOM Lib
add_link_options(-L/opt/custom/lib)
add_compile_options(
-std=c++20
# CUSTOM Lib
-lcustom
-lgfortran
-lm
)
file(GLOB_RECURSE SOURCES "src/*.cpp")
add_executable(main ${SOURCES})
target_link_libraries(main)
```
But the same error persists. What do you mean by `IMPORTED` targets? I am sorry, but could you post some code snippet? I am newbie in CMake.
Regards.
---
### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [August 18, 2023, 5:17pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773/4 "2023-08-18T17:17:35Z")
The `-l` flags are also linker options.
For an `IMPORTED` target, it’s something like:
```cmake
add_library(custom STATIC IMPORTED)
set_target_properties(custom PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "/abs/path/to/custom/includes/dir"
IMPORTED_LOCATION "/opt/custom/lib/libcustom.a"
INTERFACE_LINK_LIBRARIES "gfortran;m"
)
```
You then use it by `target_link_libraries(main PRIVATE custom)`.
---
### Author:  [@Matheus\_Diogenes\_And](https://discourse.cmake.org/u/Matheus_Diogenes_And)
#### Post date: [August 18, 2023, 7:02pm UTC](https://discourse.cmake.org/t/shared-library-in-cmake/8773/5 "2023-08-18T19:02:49Z")
> [@ben.boeckel](#):
>
> ```auto
> add_library(custom STATIC IMPORTED)
> set_target_properties(custom PROPERTIES
> INTERFACE_INCLUDE_DIRECTORIES "/abs/path/to/custom/includes/dir"
> IMPORTED_LOCATION "/opt/custom/lib/libcustom.a"
> INTERFACE_LINK_LIBRARIES "gfortran;m"
> )
>
> ```
Thank you very much, it works prefectly well.