62 lines
1.5 KiB
C
62 lines
1.5 KiB
C
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/* semabinit.c - initialize a semaphore for use by programs sema and semb */
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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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#include <stdio.h>
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#include <stdlib.h>
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/* The semaphore key is an arbitrary long integer which serves as an
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external identifier by which the semaphore is known to any program
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that wishes to use it. */
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#define KEY (1492)
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int main()
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{
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int id; /* Number by which the semaphore is known within a program */
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/* The next thing is an argument to the semctl() function. Semctl()
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does various things to the semaphore depending on which arguments
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are passed. We will use it to make sure that the value of the
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semaphore is initially 0. */
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union semun {
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int val;
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struct semid_ds *buf;
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ushort * array;
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} argument;
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argument.val = 0;
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/* Create the semaphore with external key KEY if it doesn't already
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exists. Give permissions to the world. */
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id = semget(KEY, 1, 0666 | IPC_CREAT);
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/* Always check system returns. */
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if(id < 0)
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{
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fprintf(stderr, "Unable to obtain semaphore.\n");
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exit(0);
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}
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/* What we actually get is an array of semaphores. The second
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argument to semget() was the array dimension - in our case
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1. */
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/* Set the value of the number 0 semaphore in semaphore array
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# id to the value 0. */
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if( semctl(id, 0, SETVAL, argument) < 0)
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{
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fprintf( stderr, "Cannot set semaphore value.\n");
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}
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else
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{
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fprintf(stderr, "Semaphore %d initialized.\n", KEY);
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}
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return 0;
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}
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