Function Calls Between Shared Objects

When a function in one shared object calls a function in another shared object, the execution is more complicated than in a simple call to a function within the program. Each shared object contains a Program Linkage Table, or PLT, which contains entries for every function external to that shared object that is referenced from it. Initially the address for each external function in the PLT is actually an address within ld.so, the dynamic linker. The first time such a function is called, control is transferred to the dynamic linker, which resolves the call to the real external function and patches the PLT address for subsequent calls.

If a profiling event occurs during the execution of one of the three PLT instructions, the PLT PCs are deleted, and exclusive time is attributed to the call instruction. If a profiling event occurs during the first call through a PLT entry, but the leaf PC is not one of the PLT instructions, any PCs that arise from the PLT and code in ld.so are replaced by a call to an artificial function, @plt, which accumulates inclusive time. There is one such artificial function for each shared object. If the program uses the LD_AUDIT interface, the PLT entries might never be patched, and non-leaf PCs from @plt can occur more frequently.