A pool allocator's alloc returns NULL. What does that mean and how do you handle it?

It means the pool is exhausted, all blocks are currently in use, because a pool is a fixed set of blocks with no ability to grow. Unlike desktop malloc returning NULL under genuine memory pressure (rare), pool exhaustion is a normal, expected condition you must design for. Handling depends on the data's semantics: for a packet buffer pool you might drop the incoming packet and increment a dropped-counter (and rely on the protocol's retransmission), or apply backpressure to the source; for a task pool you might reject the spawn and report an error; in a safety-critical path you fail safe to a known state. What you must not do is ignore the NULL and dereference it (an immediate crash) or block forever waiting for a free that may never come in an ISR. The pool size is a design parameter you set from worst-case concurrent usage plus margin, and the NULL path is the safety valve when reality exceeds the estimate.