Comment by mort96

Passing a pointer within one thread requires putting the pointer into a register. Passing a SHM offset within one process requires putting the offset into a register.

Passing a pointer between threads requires going through the memory system and letting cache coherence algorithms sort out the data sharing between cores (with or without a futex lock/unlock depending on implementation). Passing a SHM offset between processes requires going through the memory system and letting cache coherence algorithms sort out the data sharing between cores (with or without a context switch to the kernel depending on implementation).

It's not that different.