68 lines
2.5 KiB
C++
68 lines
2.5 KiB
C++
// semaphore_server.cpp: Illustration of simple semaphore passing
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// Inspired by: http://users.cs.cf.ac.uk/Dave.Marshall/C/node26.html
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/* Each process communicates via a semaphore.
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* The respective process can only do its work (not much here)
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* When it notices that the semaphore track is free when it returns to 0
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* Each process must modify the semaphore accordingly
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*/
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#include <unistd.h>
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#include <stdio.h>
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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int main()
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{ int semid; /* semid of semaphore set */
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key_t key = 1234; /* key to pass to semget() */
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int semflg = IPC_CREAT | 0666; /* semflg to pass to semget() */
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int nsems = 1; /* nsems to pass to semget() */
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int nsops; /* number of operations to do */
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sembuf sops;
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/* ptr to operations to perform */
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/* set up semaphore */
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printf("\nsemget: Setting up seamaphore %d: semget(%#lx, %#o)\n",key, nsems, semflg);
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if((semid = semget(key, nsems, semflg)) == -1)
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{ perror("semget: semget failed");
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return 1;
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}
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printf("semget: semget succeeded: semid =%d\n", semid);
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/* allow for 3 semaphore sets */
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for(int i = 0;i < 3;i++)
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{ nsops = 2;
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/* wait for semaphore to reach zero */
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sops[0].sem_num = 0;
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sops[0].sem_op = 0; /* wait for semaphore flag to become zero */
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sops[0].sem_flg = SEM_UNDO; /* take off semaphore asynchronous */
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sops[1].sem_num = 0;
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sops[1].sem_op = 1; /* increment semaphore -- take control of track */
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sops[1].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore */
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/* Recap the call to be made. */
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printf("\nsemop:Parent Calling semop(%d, &sops, %d) with:", semid, nsops);
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int j = 0;
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for(; j < nsops; j++)
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{ printf("\n\tsops[%d].sem_num = %d, ", j, sops[j].sem_num);
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printf("sem_op = %d, ", sops[j].sem_op);
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printf("sem_flg = %#o\n", sops[j].sem_flg);
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}
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/* Make the semop() call and report the results. */
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if((j = semop(semid, sops, nsops)) == -1)
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{ perror("semop: semop failed");
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continue;
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}
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printf("semop: semop returned %d\n", j);
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printf("Parent Process Taking Control of Track: %d/3 times\n", i+1);
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sleep(5); /* Do nothing for 5 seconds */
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nsops = 1;
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/* wait for semaphore to reach zero */
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sops[0].sem_num = 0;
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sops[0].sem_op = -1; /* Give UP COntrol of track */
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sops[0].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore, asynchronous */
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if((j = semop(semid, sops, nsops)) == -1)
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{ perror("semop: semop failed");
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continue;
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}
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printf("Parent Process Giving up Control of Track: %d/3 times\n", i+1);
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sleep(5); /* halt process to allow child to catch semaphor change first */
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}
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} |