Because nobody cares about performance. As far as I remember on Linux every C symbol not explicitly inline or static defaults to visible and dynamically linked.

Because nobody cares about performance. As far as I remember on Linux every C symbol not explicitly inline or static defaults to visible and dynamically linked. You can try to tell gcc to treat every symbol as hidden by default but at that point every third party header would have to explicitly declare its exported symbols as visible. That whole dllexport/import mess Windows binaries require? That is the implicit default on Linux.