GCC Bugzilla – Bug 48568
Missing documentation for __attribute__((visibility ("protected"))) on variables.
Last modified: 2011-04-12 10:37:38 UTC
I discovered that __attribute__((visibility ("protected"))) is supported on variables, not just on functions. This should be documented in the node "Variable Attributes" in the GCC manual
This feature is useful in particular for accessing read-only tables (allocated in the .rodata section) from PIC code. By default, access to such data in other compilation units is done via the GOT table. Using the above attribute (and assuming that the definition is linked into in the same shared library file), the GOT lookup is omitted, and instead one gets a direct access via pc-relative addressing. I was working on GNU/linux x86_64, where this corresponds to
lea table(%rip), %rax # To get the address of the table
rather than the default
mov table@GOTPCREL(%rip), %rax
in assembler input. I admit I don't fully understand the .protected pseudo op, but it's appearantly essential for getting the linker to do the right thing when linking the shared library (gcc -shared table-def.o table-use.o -o lib.so).