Understanding IMPORTED_SONAME in CMake for Linking with External Shared Libraries

What is IMPORTED_SONAME?

• An imported shared library refers to one that exists outside your project's build tree and is not built by CMake itself.
• A shared library is a reusable collection of code that can be linked with multiple executables.
• In CMake, IMPORTED_SONAME is a target property specifically used with imported shared libraries.

Purpose of IMPORTED_SONAME

• This information is crucial for the linker to find the appropriate library file when running your program.
• Setting IMPORTED_SONAME tells CMake the SONAME of the imported shared library.
• Shared libraries often have a SONAME (Shared Object Name), which is a versioned identifier used by the dynamic linker (loader) to locate the correct library at runtime.

When to Use IMPORTED_SONAME

• This ensures the linker searches for the library file using its versioned name, promoting backward compatibility.
• You'll typically use IMPORTED_SONAME when your project needs to link against an external shared library that has a SONAME.

How to Set IMPORTED_SONAME

1. add_library(my_external_lib SHARED IMPORTED)
   

2. Set the IMPORTED_LOCATION property
   

   set_target_properties(my_external_lib PROPERTIES IMPORTED_LOCATION /path/to/my_external_lib.so)
   

3. Set the IMPORTED_SONAME property
   

   set_target_properties(my_external_lib PROPERTIES IMPORTED_SONAME "my_external_lib.so.1.0"  # Example SONAME with version
   

Additional Considerations

• If the imported shared library doesn't have a SONAME, you might need to use IMPORTED_NO_SONAME instead, which instructs CMake to link using the bare library file name (e.g., -lfoo).
• IMPORTED_SONAME is primarily relevant on platforms like Linux and macOS that support SONAMEs.

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# Define the path to the external library directory (adjust as needed)
set(EXTERNAL_LIB_DIR "/path/to/external/libs")

# Create an imported library target for the external library
add_library(my_external_lib SHARED IMPORTED)

# Set the location of the library file
set_target_properties(my_external_lib PROPERTIES IMPORTED_LOCATION "${EXTERNAL_LIB_DIR}/libexample.so")

# Set the SONAME of the external library (assuming it has one)
set_target_properties(my_external_lib PROPERTIES IMPORTED_SONAME "libexample.so.2.3"  # Example SONAME with version

# Create your executable that uses the external library
add_executable(my_program main.cpp)

# Link the executable against the imported library
target_link_libraries(my_program my_external_lib)

1. We define the EXTERNAL_LIB_DIR variable to point to the directory containing the external library libexample.so.
2. We use add_library with IMPORTED to create a target named my_external_lib for the external library.
3. set_target_properties is used to specify the location of the library file within EXTERNAL_LIB_DIR using the IMPORTED_LOCATION property.
4. We set the IMPORTED_SONAME property to "libexample.so.2.3", assuming the external library has this SONAME.
5. We create an executable target named my_program using add_executable.
6. Finally, target_link_libraries links the executable to the imported library target my_external_lib.

• If the external library doesn't have a SONAME, remove the set_target_properties line setting IMPORTED_SONAME. In that case, linking might work with just the IMPORTED_LOCATION set.
• Replace /path/to/external/libs and main.cpp with your actual paths.

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IMPORTED_NO_SONAME

• Setting IMPORTED_NO_SONAME to TRUE instructs CMake to link using the bare library file name (e.g., -lfoo) during the linking stage.
• Use this if the external shared library doesn't have a SONAME.

Example

add_library(my_external_lib SHARED IMPORTED)
set_target_properties(my_external_lib PROPERTIES IMPORTED_LOCATION /path/to/my_external_lib.so IMPORTED_NO_SONAME TRUE)

Manual Link Flags

• If you know the exact flags needed to link with the library (e.g., -L/path/to/libs -lfoo), you can manually provide them using target_link_libraries with the PUBLIC or PRIVATE keyword:

target_link_libraries(my_program PUBLIC -L/path/to/libs -lfoo)

• This approach is less portable and might require modifications across different platforms.

Package Managers (if applicable)

• These packages might handle SONAMEs or linking details internally, simplifying the CMake configuration.
• If you're using a package manager like apt-get (Linux) or brew (macOS), you can often find pre-built packages for external libraries.

• Package managers offer a convenient option if available for the specific libraries you need.
• Manual link flags can be used for specific linking requirements, but consider portability.
• If you're unsure or the library doesn't have a SONAME, use IMPORTED_NO_SONAME for a more standard approach.
• If the external library has a SONAME, IMPORTED_SONAME is the preferred method for versioned management.