typo in definition of atomic_exchange_explicit for Clang on Unix

Summary

stdatomic.h says:

#define atomic_exchange_explicit(obj, desired, order) \
	__c11_atomic_exchange_explicit(obj, expected, order)

This only works if the second argument to that macro is the identifier 'expected'. Otherwise this (hopefully) produces a 'no such local variable' message from the compiler.

Steps to reproduce

Compile queue.c with compiler options that trigger the faulty macro.

The original code in queue.c is:

    item = atomic_exchange(&(lh->items[idx]),
                           (uintptr_t)&queue->taken);

This produces the following line after preprocessing:

  item = __c11_atomic_exchange_explicit(&(lh->items[idx]), expected, memory_order_seq_cst);

This line does not contain queue->taken anymore.

What is the current bug behavior?

Compilation fails because expected is not a defined variable.

What is the expected correct behavior?

Compilation succeeds.

Possible fixes

Write 'desired' instead of 'expected'.