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.

1. std::atomic<BufferSemaphoreFlags> sem
   • The state of the buffer. One of Empty, Writing, Full, or Reading.
2. 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.
3. 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).