Using this structure is a little complicated, so lets start with the definition of union semun.This union structure is not actually defined in any of the header files, so you will have to definethe structure in your program before you call main: Int semctl(int semaid, int semnum, int cmd, union semun arg) Mainly to remove a semaphore structure from the system, and to set the values of each semaphore in the structure. You will want to see the manpages: man 2 semctl for all the details. The function for doing this is semctl which is a catch-all for alotof different functions. Once you have the semaid, you initialize the structure, by setting the value of everysemaphore in the set. The return value semaid will be used to refer to the semaphore structure in all furtherfunctions. Semaid = semget(IPC_PRIVATE, numsems, IPC_CREAT) The return value semaid can only be shared with related processes Create the structure if it doesn't exist using a private key Semaid = semget( key, numsems, IPC_CREAT | IPC_EXCL) Throws an error if a structure exists with the key Semaid = semget( key, numsems, IPC_CREAT) or use an existing structure with the same key Create the structure if it doesn't exist There are three ways you may want to call semget Int semget(key_t key, int numsems, int flag)ĮEXIST: structure exists and IPC_EXCL set Process 1 (P1)and Process 2 (P2) have a section of code with the following lines: Let this value be named by the variable V. Suppose we have two processes that share a memory segment of four bytes which storesan integer value. ![]() The Critical Section Problem is to design a protocol that the processes can useto coordinate their activities when one wants to enter its critical section of code. ![]() ![]() That part of the code ofeach program, where one process is reading from or writing to a shared memory area, is a critical section of code, because we must ensure that only one process execute a critical sectionof code at a time. Suppose two processes share access to a file, or shared memory segment (or when we discuss threads in a couple ofweeks, we'll see threads share the same memory, so they must synchronize their actions), and at leastone of these processes can modify the data in this shared area of memory. The most synchronization problem confronting cooperating processes, is controling access to shared resource. I assume you have read the tutorial on cooperating processes, and Interprocess Communication (IPC)facilities provided by the Operating System for process cooperation. Finally, the Bounded Buffer Problem, described inthe tutorial on Cooperation, will be tackled. The tutorial will start with thebasics on creating and setting-up semaphores, then tackle the most basic use of semaphores, to protect critical sections of code. The goal of this tutorial is explain how semaphores can be used to solved synchronizationproblems, which arise through cooperation between processes. Synchronization: Semaphores Synchronization: Semaphores Goal
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