GCC and Visibility flag

-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified option—all symbols will be marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, default always means public ie; available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option will be hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public—this causes the same behavior as previous versions of GCC.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by “How To Write Shared Libraries” by Ulrich Drepper (which can be found at http://people.redhat.com/~drepper/)—however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLL's on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.