Ex05-Linux IPC-Semaphores

To Write a C program that implements a producer-consumer system with two processes using Semaphores.

Navigate to any Linux environment installed on the system or installed inside a virtual environment like virtual box/vmware or online linux JSLinux (https://bellard.org/jslinux/vm.html?url=alpine-x86.cfg&mem=192) or docker.

Write the C Program using Linux Process API - Sempahores

Execute the C Program for the desired output.

/*

• sem.c - demonstrates a basic producer-consumer

•                        implementation.              */
  

#include <stdio.h> /* standard I/O routines. / #include <stdlib.h> / rand() and srand() functions / #include <unistd.h> / fork(), etc. / #include <time.h> / nanosleep(), etc. / #include <sys/types.h> / various type definitions. / #include <sys/ipc.h> / general SysV IPC structures / #include <sys/sem.h> / semaphore functions and structs. / #define NUM_LOOPS 20 / number of loops to perform. / #if defined(GNU_LIBRARY) && !defined(_SEM_SEMUN_UNDEFINED) / union semun is defined by including <sys/sem.h> / #else / according to X/OPEN we have to define it ourselves */

union semun {

    int val;                    /* value for SETVAL */   
    struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */   
    unsigned short int *array;  /* array for GETALL, SETALL */   
    struct seminfo *__buf;      /* buffer for IPC_INFO */    

};
#endif
int main(int argc, char* argv[])
{
int sem_set_id; /* ID of the semaphore set. /
union semun sem_val; / semaphore value, for semctl(). /
int child_pid; / PID of our child process. /
int i; / counter for loop operation. /
struct sembuf sem_op; / structure for semaphore ops. /
int rc; / return value of system calls. /
struct timespec delay; / used for wasting time. /
/ create a private semaphore set with one semaphore in it, /
/ with access only to the owner. /
sem_set_id = semget(IPC_PRIVATE, 1, 0600);
if (sem_set_id == -1) {
perror("main: semget");
exit(1);
}
printf("semaphore set created, semaphore set id '%d'.\n", sem_set_id);
/ intialize the first (and single) semaphore in our set to '0'. /
sem_val.val = 0;
rc = semctl(sem_set_id, 0, SETVAL, sem_val);
/ fork-off a child process, and start a producer/consumer job. /
child_pid = fork();
switch (child_pid) {
case -1: / fork() failed /
perror("fork");
exit(1);
case 0: / child process here /
for (i=0; i<NUM_LOOPS; i++) {
/ block on the semaphore, unless it's value is non-negative. /
sem_op.sem_num = 0;
sem_op.sem_op = -1;
sem_op.sem_flg = 0;
semop(sem_set_id, &sem_op, 1);
printf("consumer: '%d'\n", i);
fflush(stdout);
}
break;
default: / parent process here /
for (i=0; i<NUM_LOOPS; i++) {
printf("producer: '%d'\n", i);
fflush(stdout);
/ increase the value of the semaphore by 1. /
sem_op.sem_num = 0;
sem_op.sem_op = 1;
sem_op.sem_flg = 0;
semop(sem_set_id, &sem_op, 1);
/ pause execution for a little bit, to allow the /
/ child process to run and handle some requests. /
/ this is done about 25% of the time. /
if (rand() > 3(RAND_MAX/4)) {
delay.tv_sec = 0;
delay.tv_nsec = 10;
//nanosleep(&delay, NULL);
sleep(10); }
if(NUM_LOOPS>=10) { semctl(sem_set_id, 0, IPC_RMID, sem_val) ;} // Remove the sem_set_id }} break; } return 0; }

$ ./sem.o

$ ipcs

The program is executed successfully.