artdaq

h1. Details on lock-free buffer management

The _artdaq_ shared memory interface uses several flags in the buffer descriptor structure to achieve self-contained, lock-free operation. 

# std::atomic<BufferSemaphoreFlags> sem

** The state of the buffer. One of Empty, Writing, Full, or Reading.

# std::atomic<int16_t> sem_id

** The current owner of the buffer. Each instance of SharedMemoryManager takes an ID number (from ShmStruct::std::atomic<int> next_id), and uses this number to indicate that it is the current owner of the buffer. Buffers without owners have sem_id set to -1.

# std::atomic<uint64_t> last_touch_time

** The last time (microseconds, system clock) the buffer was "touched". Buffers should be touched whenever a modifying operation occurs, such as writes or reads, or before the buffer transitions from an inactive state (Empty, Full) to an active state (Writing, Reading).