Inline function visibility under -fvisibility=hidden in GCC

Visibility of Inline Functions in GCC 13

In GCC 13, when -fvisibility=hidden is specified, any header-defined inline function that is emitted as a standalone symbol (out-of-line) will have hidden visibility by default. GCC 13 does not introduce a specific exception that elevates inline functions to default visibility if they lack an explicit attribute.

The Role of -fvisibility-inlines-hidden

The -fvisibility-inlines-hidden flag does not provide a consistent result across GCC 10, 12, and 13 because its behavior depends on the language being compiled:

• C++: This flag is designed for inline member functions and free functions. In many GCC versions, -fvisibility=hidden already implies this behavior for C++ inlines.
• C: This flag has no effect on standard C inline functions. Visibility for C inlines is governed by -fvisibility=hidden and the linkage rules of the chosen C standard (e.g., C11 vs GNU89).

Adding this flag will not resolve visibility leaks in pure C libraries across these versions.

Portable Enforcement of Hidden Visibility

Because compiler flags can be overridden or behave differently across languages, the most portable way to force hidden visibility for internal inline functions is to use a guarded attribute macro. This ensures the symbol remains hidden regardless of the global visibility setting.

#if defined(__GNUC__) || defined(__clang__)
  #define INTERNAL_HIDDEN __attribute__((visibility("hidden")))
#else
  #define INTERNAL_HIDDEN
#endif

// Usage in header
INTERNAL_HIDDEN inline void internal_helper(void) {
    // implementation
}

Verification and Diagnostics

To confirm that your inline functions are not leaking into the public API, do not rely on nm alone, as it may show symbols in the standard symbol table (.symtab) that are not exported. Instead, check the dynamic symbol table:

readelf --dyn-syms libyourlibrary.so | grep internal_helper

If the function is correctly hidden, it will either be absent from this output or marked as LOCAL. If it appears as GLOBAL with DEFAULT visibility, it is being exported.

Missing Diagnostic Detail: To provide a more precise recommendation, please specify if you are compiling as C or C++ and which -std flag (e.g., -std=c11 or -std=gnu89) is in use, as C inline linkage varies significantly between these standards.