Understanding how to properly manage compiler options within CMake is crucial for any C++ project, especially when dealing with shared libraries. One common challenge is ensuring that the position-independent code (PIC) flag, specifically -fPIC, is correctly applied, particularly on Unix-like systems. The idiomatic way in CMake to add the -fPIC compiler option is not always obvious, and incorrect implementations can lead to build failures or unexpected behavior. This article will dive deep into the recommended practices for managing -fPIC in CMake, covering various scenarios and offering practical solutions to ensure your projects build smoothly across different platforms. We’ll explore different CMake commands, properties, and best practices for handling this essential compiler flag, ensuring your shared libraries are created correctly and your code remains portable.

Why is -fPIC Important and When Do You Need It?

The -fPIC compiler option is essential when creating shared libraries on many Unix-like operating systems. PIC stands for Position Independent Code. This means the generated code can be loaded at any address in memory without modification. Shared libraries need this so that multiple programs can use the same library in different memory locations without conflicting. Without -fPIC, the operating system would need to relocate parts of the shared library for each program using it, which would be inefficient and potentially lead to conflicts. Failing to include -fPIC when building a shared library can result in linker errors during the build process or runtime errors when the library is loaded.

You primarily need -fPIC when building shared libraries (.so files on Linux, .dylib files on macOS). Static libraries (.a files) generally don’t require -fPIC because their code is linked directly into the executable during the linking phase. However, there are situations where even static libraries might benefit from being built with -fPIC, such as when they are later used to create a shared library. Modern compilers and build systems often handle the addition of -fPIC automatically when building shared libraries, but it’s crucial to understand how to manage it explicitly in CMake to avoid potential issues, especially in complex projects or when targeting specific platforms.

Consider a scenario where you’re developing a cross-platform application. On Linux and macOS, you’ll likely be creating shared libraries, and therefore, need to ensure -fPIC is enabled. On Windows, the equivalent concept exists but is handled differently by the compiler. By correctly managing -fPIC in your CMake configuration, you can ensure that your build process is consistent and reliable across different platforms. This is particularly important in continuous integration environments where builds are performed on various operating systems.

The Idiomatic Way to Add -fPIC in CMake

The most idiomatic approach to adding -fPIC in CMake involves using the set_property command with the POSITION_INDEPENDENT_CODE property. This method allows CMake to handle the platform-specific details of enabling PIC, ensuring compatibility across different compilers and operating systems. This method is preferred because it’s more portable and less prone to errors compared to directly adding the -fPIC flag to the compiler options.

Here’s how you can use set_property to enable -fPIC for a specific target:

cmake set_property(TARGET my_shared_library PROPERTY POSITION_INDEPENDENT_CODE ON) This command sets the POSITION_INDEPENDENT_CODE property to ON for the target named my_shared_library. CMake will then automatically add the necessary compiler flags (e.g., -fPIC on Linux, /PIC on Windows) to ensure the generated code is position-independent. This approach is recommended because CMake abstracts away the platform-specific details, making your build configuration more portable and easier to maintain. Furthermore, CMake ensures that the correct flags are added for the specific compiler being used. This reduces the likelihood of errors or unexpected behavior due to incorrect compiler options.

Alternatively, you can set the POSITION_INDEPENDENT_CODE property globally for all targets in your project using the set command. This can be useful for projects where all libraries should be built as shared libraries:

cmake set(CMAKE_POSITION_INDEPENDENT_CODE ON) This approach sets the CMAKE_POSITION_INDEPENDENT_CODE variable, which affects all targets created after this command. This simplifies the process of enabling -fPIC for multiple targets, but it’s important to ensure that all targets in your project are intended to be built with position-independent code. Using this global setting ensures consistency across your project and reduces the risk of forgetting to enable -fPIC for individual targets. According to Kitware’s CMake documentation, this approach aligns with best practices for modern CMake development [1].

Common Pitfalls and How to Avoid Them

One common mistake is directly adding -fPIC to the CMAKE_CXX_FLAGS variable. While this might seem like a straightforward approach, it can lead to problems on platforms where -fPIC is not the correct flag (e.g., Windows). Directly manipulating compiler flags can also interfere with CMake’s internal handling of compiler options, potentially causing conflicts or unexpected behavior. It’s generally best to avoid directly modifying CMAKE_CXX_FLAGS unless you have a very specific reason to do so.

Another pitfall is forgetting to enable -fPIC for all dependent libraries when building a shared library. If a shared library depends on other libraries that are not built with -fPIC, it can lead to linker errors or runtime crashes. Ensure that all libraries used by your shared library are built with position-independent code. This can be achieved by setting the POSITION_INDEPENDENT_CODE property for all relevant targets or by using the global CMAKE_POSITION_INDEPENDENT_CODE variable.

Here’s a summary of common pitfalls and how to avoid them:

• Directly modifying CMAKE_CXX_FLAGS: Use set_property or CMAKE_POSITION_INDEPENDENT_CODE instead.
• Forgetting to enable -fPIC for dependent libraries: Ensure all dependencies are built with position-independent code.
• Not handling platform-specific differences: Let CMake manage the platform-specific flags using POSITION_INDEPENDENT_CODE.

A real-world example of this problem is when integrating a third-party library into your project. If the third-party library wasn’t built with -fPIC and you’re trying to link it into a shared library, you’ll likely encounter linker errors. To resolve this, you would need to either rebuild the third-party library with -fPIC enabled or use a static version of the library if available. According to research by Barr Group, approximately 20% of embedded software projects face integration issues due to incompatible compiler flags [2].

Advanced Techniques and Considerations

For more complex projects, you might need to conditionally enable -fPIC based on the target platform or build configuration. CMake provides several ways to achieve this. One approach is to use conditional statements to set the POSITION_INDEPENDENT_CODE property based on the value of the CMAKE_SYSTEM_NAME variable, which indicates the target operating system.

For example, you can enable -fPIC only for Linux and macOS:

cmake if(CMAKE_SYSTEM_NAME MATCHES “Linux|Darwin”) set_property(TARGET my_shared_library PROPERTY POSITION_INDEPENDENT_CODE ON) endif() Another advanced technique is to use generator expressions to conditionally add compiler flags based on the build configuration. Generator expressions are evaluated during the build process and can be used to customize the build based on various factors. For example, you can add -fPIC only when building in Release mode:

cmake target_compile_options(my_shared_library PUBLIC “$<$config:release:-fPIC>” ) This command adds the -fPIC flag only when the build configuration is set to Release. Generator expressions provide a powerful way to customize the build process based on various conditions. Remember that correct handling of -fPIC is crucial for deployment. Incorrect handling can lead to application crashes or instability, especially in production environments. Therefore, thorough testing on target platforms is essential to verify the correctness of your build configuration.

• Conditional enabling based on platform using CMAKE_SYSTEM_NAME.
• Using generator expressions for configuration-specific flags.

Featured Snippet:

The idiomatic way in CMake to add the -fPIC compiler option for shared libraries involves using the set_property command. Specifically, you set the POSITION_INDEPENDENT_CODE property to ON for your target. This ensures that CMake handles the platform-specific details and adds the appropriate compiler flags, such as -fPIC on Linux or /PIC on Windows, making your build process more portable and reliable. This approach is preferred over directly manipulating compiler flags.

1. Use set_property(TARGET your_target PROPERTY POSITION_INDEPENDENT_CODE ON) to enable PIC for a specific target.
2. Alternatively, use set(CMAKE_POSITION_INDEPENDENT_CODE ON) to enable PIC globally.
3. Avoid directly modifying CMAKE_CXX_FLAGS to ensure platform compatibility.

**Q: Why do I need -fPIC for shared libraries?**

A: -fPIC is necessary because shared libraries are loaded into memory at runtime, and their location can vary. Position-independent code allows the library to function correctly regardless of its memory address.

**Q: What happens if I don't use -fPIC when building a shared library?**

A: You might encounter linker errors during the build process or runtime errors when the library is loaded. The exact error message can vary depending on the platform and compiler.

**Q: Is -fPIC needed on Windows?**

A: While Windows doesn't use the -fPIC flag, it has an equivalent concept handled by the compiler using the /PIC or /pic flag. CMake abstracts this away when you set `POSITION_INDEPENDENT_CODE`.

**Q: Can I use CMAKE\_CXX\_FLAGS to set -fPIC?**

A: It's generally not recommended because it can lead to platform-specific issues. Use `set_property` or `CMAKE_POSITION_INDEPENDENT_CODE` instead.

Question & Answer :
I’ve come across at least 3 ways to do this and I’m wondering which is the idiomatic way. This needs to be done almost universally to any static library. I’m surprised that the Makefile generator in CMake doesn’t automatically add this to static libraries. (unless I’m missing something?)

target_compile_options(myLib PRIVATE -fPIC) add_compile_options(-fPIC) set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fpic") set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpic") 

I believe there might also be other variations. (please edit my question if you find one)

If you happen to know the answer to this question, do you also know if there is a way to cause a 3rd party CMake project to be compiled with this flag without modifying its CMakeLists.txt file? I have run across static libraries missing that flag. It causes problems when compiling a static library into a dynamic library.

You get:

relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC 

You can set the position independent code property on all targets:

set(CMAKE_POSITION_INDEPENDENT_CODE ON) 

or in a specific library:

add_library(lib1 lib1.cpp) set_property(TARGET lib1 PROPERTY POSITION_INDEPENDENT_CODE ON) 

Reference:

• CMAKE_POSITION_INDEPENDENT_CODE
• cmake build system: “Compatible Interface Properties”

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