130 lines
4.3 KiB
C
130 lines
4.3 KiB
C
// Dining philosophers (swansons) using System V Semaphores
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// Original: http://www.lisha.ufsc.br/~guto/teaching/os/exercise/phil.html
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// Modified by Chris Kauffman
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//
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// This version uses a C array of semaphores. Each System V semaphore
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// itself can be an array but of the 5 distinct semphores in the C array are
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// initialized to contain only a single counter at sem_num=0.
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//
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// Short random delays are added between each philosophers
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// thinking/eating cycle to generate some variance in execution order
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//
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// To see the multiple processes, run the following commands
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// > gcc -o philosophers philosophers.c
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// > philosophers > xxx & watch -d -n 0.1 'ps ux | grep philosophers'
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// Ctrl-c to stop the "watch" command (may screw up the terminal display)
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#include <stdio.h>
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#include <stdlib.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#include <sys/types.h>
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#include <sys/wait.h>
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#include <time.h>
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#include <unistd.h>
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#define N_PHILOSOPHERS 5 // Number of philosophers
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const int MEALS_TO_FINISH =
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10; // Number of iterations before philosophers finish
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const int MAX_DELAY = 50000; // Maximum delay in nanos between meal iterations
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// Semaphore ids
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int utensils[N_PHILOSOPHERS]; // IDs for arrays of IPC semaphores
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int philosopher(int n); // Defined after main()
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int main() {
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setvbuf(stdout, NULL, _IONBF, 0);
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int i, status;
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pid_t phil[N_PHILOSOPHERS];
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printf("The Dining Swansons (Philosophers) Problem\n");
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// Parent process only:
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//
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// Allocate utensils: semaphores which are initially set to value
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// one. Each semaphore is only a single counter with sem_num=0.
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for (i = 0; i < N_PHILOSOPHERS; i++) {
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utensils[i] = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
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semctl(utensils[i], 0, SETVAL, 1);
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}
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// Parent generates child processes
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for (i = 0; i < N_PHILOSOPHERS; i++) {
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int pid = fork();
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if (pid == 0) { // child has pid 0
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int ret = philosopher(i); // child acts as philosopher
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exit(ret); // then exits
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} else { // parent gets pid > 0
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phil[i] = pid; // parent tracks children
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}
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}
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// Parent waits on all children to finish
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for (i = 0; i < N_PHILOSOPHERS; i++) {
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waitpid(phil[i], &status, 0);
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}
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printf("JJ: All Swansons finished, cleaning up the diner\n");
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// Eliminate the utensils and table semaphores
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for (i = 0; i < N_PHILOSOPHERS; i++) {
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semctl(utensils[i], 0, IPC_RMID, 0);
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}
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return 0;
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}
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// Code for dining philosopher child processes
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int philosopher(int n) {
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int i, first, second;
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struct sembuf op; // Used to perform semaphore operations
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op.sem_flg = 0;
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op.sem_num =
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0; // C array of semaphores each with 1, op always on the 0th sem
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srand(time(NULL)); // Seed random number generator
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// Avoid deadlock via slightly different order of utensil requests
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// for last philospher
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// first utensil to get, most go for n first, last philospher goes for 0
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// first
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first = (n != N_PHILOSOPHERS - 1) ? n : 0;
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// second utensil to get, last philopher goes for n second
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second = (n != N_PHILOSOPHERS - 1) ? n + 1 : n;
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printf("Swanson %d wants utensils %d and %d\n", n, first, second);
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printf("Swanson %d at the table\n", n);
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// Main loop of thinking/eating cycles
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for (i = 0; i < MEALS_TO_FINISH; i++) {
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int sleep_time = rand() % MAX_DELAY;
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usleep(sleep_time); // sleep for for a short time
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printf("%2d: Swanson %d is contemplating his awesomeness ...\n", i, n);
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// get first utensil
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op.sem_op = -1;
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semop(utensils[first], &op, 1);
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printf("%2d: Swanson %d got utensil %d\n", i, n, first);
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// get second utensil
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op.sem_op = -1;
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semop(utensils[second], &op, 1);
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printf("%2d: Swanson %d got utensil %d\n", i, n, second);
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printf("%2d: Swanson %d is eating an egg ...\n", i, n);
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usleep(sleep_time); // sleep for for a short time
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// release first utensil
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op.sem_op = +1;
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semop(utensils[first], &op, 1);
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// release second utensil
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op.sem_op = +1;
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semop(utensils[second], &op, 1);
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}
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printf("Swanson %d leaving the diner\n", n);
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exit(n);
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}
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