fvisibility

Fvisibility is a compiler feature implemented by GCC and Clang that controls the visibility of symbols in shared libraries. It determines which functions and variables are exposed to the dynamic linker and to external binaries, thereby shaping the library’s public interface. By default, compilers may export many symbols, which can increase the dynamic symbol table and the potential for ABI disputes. fvisibility provides a way to limit or specify symbol exposure.

The feature is used through the -fvisibility option and related flags. Common values are default, hidden, and internal. -fvisibility=hidden makes non-explicitly annotated symbols hidden by default, reducing what is exported. -fvisibility=default makes symbols public by default. -fvisibility=internal is a more aggressive variant that is less commonly used. In addition, the visibility attribute __attribute__((visibility("visibility-name"))) can be applied to individual declarations to override the default for a specific symbol. The same concept is available in other forms for certain compilers, such as __declspec, depending on the platform.

There is also -fvisibility-inlines-hidden, which hides inline functions from being emitted as exported symbols, further reducing the library’s public surface and enabling certain optimizations.

Typical usage involves compiling a library with -fvisibility=hidden and explicitly marking the library’s API with visibility("default"). This minimizes the dynamic symbol table, can improve load times, and reduces ABI fragility, but it requires careful management to ensure all intended API symbols remain accessible to users of the library.

Limitations and scope: fvisibility primarily affects symbols in Unix-like shared libraries. Windows uses a separate export mechanism. Some language features and toolchains may require additional considerations to maintain correct visibility for all required interfaces.