[8 Notice that they are just invalidated and not freed since it would be too costly to remove the data structures from the perprocess lists of all processes The queue of pending requests

The kernel associates a queue of pending requests with each IPC semaphore to identify processes that are waiting on one (or more) of the semaphores in the array. The queue is a doubly linked list of sem_queue data structures whose fields are shown in Table 19-10. The first and last pending requests in the queue are referenced, respectively, by the sem_pending and sem_pending_last fields of the sem_array structure. This last field allows the list to be handled easily as a FIFO; new pending requests are added to the end of the list so they will be serviced later. The most important fields of a pending request are nsops (which stores the number of primitive semaphores involved in the pending operation) and sops (which points to an array of integer values describing each semaphore operation). The sleeper field stores the descriptor address of the sleeping process that requested the operation.

Continue reading here: Table 1910 The fields in the semqueue data structure

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