The Linux Programming Interface: a linux and unix system Programming Handbook
Algorithm Employed by System V IPC
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- Figure 45-1
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Algorithm Employed by System V IPC get Calls Figure 45-1 shows some of the structures used internally by the kernel to represent information about System V IPC objects (in this case semaphores, but the details are similar for other IPC mechanisms), including the fields used to calculate IPC keys. For each IPC mechanism (shared memory, message queue, or semaphore), 932 Chapter 45 the kernel maintains an associated ipc_ids structure that records various global information about all instances of that IPC mechanism. This information includes a dynamically sized array of pointers, entries, to the associated data structure for each object instance (semid_ds structures in the case of semaphores). The current size of the entries array is recorded in the size field, with the max_id field holding the index of the highest currently in-use element. Figure 45-1: Kernel data structures used to represent System V IPC (semaphore) objects When an IPC get call is made, the algorithm used on Linux (other systems use simi- lar algorithms) is approximately as follows: 1. The list of associated data structures (pointed to by elements of the entries array) is searched for one whose key field matches that specified in the get call. a) If no match is found, and IPC_CREAT was not specified, then the error ENOENT is returned. b) If a match is found, but both IPC_CREAT and IPC_EXCL were specified, then the error EEXIST is returned. c) Otherwise, if a match is found, then the following step is skipped. 2. If no match was found, and IPC_CREAT was specified, then a new mechanism-specific associated data structure (semid_ds in Figure 45-1) is allocated and initialized. This also involves updating various fields of the ipc_ids structure, and may involve resizing the entries array. A pointer to the new structure is placed in the first free element of entries. Two substeps are included as part of this initialization: a) The key value supplied in the get call is copied into the xxx_perm.__key field of the newly allocated structure. b) The current value of the seq field of the ipc_ids structure is copied into the xxx_perm.__seq field of the associated data structure, and the seq field is incremented by one. seq = 10 entries 0 1 2 3 sem_perm.__key = 0x4d0731db sem_perm.__seq = 9 sem_perm.__key = 0x4b079002 sem_perm.__seq = 5 tải về 6.93 Mb. Chia sẻ với bạn bè của bạn:
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