Symbol Lookup

When the runtime linker needs to look up a symbol, by default it does so by searching in each object. The runtime linker starts with the dynamic executable, and progresses through each shared object in the same order that the objects are loaded. In many instances, the shared object that requires a symbolic relocation turns out to be the provider of the symbol definition.

In this situation, if the symbol used for this relocation is not required as part of the shared object's interface, then this symbol is a strong candidate for conversion to a static or automatic variable. A symbol reduction can also be applied to removed symbols from a shared objects interface. See Reducing Symbol Scope for more details. By making these conversions, the link-editor incurs the expense of processing any symbolic relocation against these symbols during the shared object's creation.

The only global data items that should be visible from a shared object are those that contribute to its user interface. Historically this has been a hard goal to accomplish, because global data are often defined to allow reference from two or more functions located in different source files. By applying symbol reduction, unnecessary global symbols can be removed. See Reducing Symbol Scope. Any reduction in the number of global symbols exported from a shared object results in lower relocation costs and an overall performance improvement.

The use of direct bindings can also significantly reduce the symbol lookup overhead within a dynamic process that has many symbolic relocations and many dependencies. See Appendix D, Direct Bindings.