inter/init.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/shm.h>
#include <time.h>
#include <unistd.h>

#define NUM_PROCS 4
#define SHM_SIZE 1024
#define PATHNAME "."
#define PROJ_ID 65

struct shared_data {
  int ready;
  int priorities[NUM_PROCS];
};

void write_pipes(int pipefds[][2], pid_t pids[]) {
  for (int i = 0; i < NUM_PROCS; i++) {
    // Create pipe (child -> parent)
    if (pipe(pipefds[i]) == -1) {
      perror("[!] Pipe failed");
      exit(EXIT_FAILURE);
    }

    // Create fork
    pids[i] = fork();

    if (pids[i] == -1) {
      perror("[!] Fork failed");
      exit(EXIT_FAILURE);
    }

    // Executing inside the child process
    if (pids[i] == 0) {
      pid_t child_pid = getpid();
      printf("[P%d] PID: %d\n", i + 1, child_pid);

      // Read end of the pipe is closed before writing (sync)
      close(pipefds[i][0]);

      // Random priority in the range 0-19
      srand(time(NULL) ^ getpid());
      int priority = rand() % 20;

      // Write to the pipe and close it afterwards (sync)
      write(pipefds[i][1], &priority, sizeof(priority));
      close(pipefds[i][1]);

      printf("[P%d] Wrote %d into pipe\n", i + 1, priority);
      printf("[P%d] Exiting...\n", i + 1);

      exit(0);
    }
  }
}

void read_pipes(int pipefds[][2], int priorities[]) {
  for (int i = 0; i < NUM_PROCS; i++) {
    // Write end of the pipe is closed before reading (sync)
    close(pipefds[i][1]);

    // Read from the pipe and close it afterwards (sync)
    read(pipefds[i][0], &priorities[i], sizeof(priorities[i]));
    close(pipefds[i][0]);

    printf("[INIT] Read %d from P%d\n", priorities[i], i + 1);
  }
}

void write_mem_segment(int priorities[]) {
  struct shared_data *shm_ptr;
  int shmid;

  key_t key = ftok(PATHNAME, PROJ_ID);

  // Get the segment identifier
  if ((shmid = shmget(key, SHM_SIZE, 0666)) < 0) {
    perror("[!] Shmget failed");
    exit(EXIT_FAILURE);
  }

  // Attach (system chooses a suitable address)
  if ((shm_ptr = (struct shared_data *)shmat(shmid, NULL, 0)) ==
      (struct shared_data *)-1) {
    perror("[!] Shmat failed");
    exit(EXIT_FAILURE);
  }

  printf("[INIT] Connected to shared segment %d (%p)\n", shmid,
         (void *)&shm_ptr);

  // Write
  for (int i = 0; i < NUM_PROCS; i++) {
    shm_ptr->priorities[i] = priorities[i];
  }

  // Signal that data is ready
  shm_ptr->ready = 1;

  printf("[INIT] Wrote priorities to shared memory\n");
  printf("[INIT] Flipped the signal flag to 1\n");

  // Detach
  if (shmdt(shm_ptr) == -1) {
    perror("[!] Shmdt failed");
    exit(EXIT_FAILURE);
  }

  printf("[INIT] Detached from shared memory\n");
}

int main() {
  int pipefds[NUM_PROCS][2];
  pid_t pids[NUM_PROCS];

  // Create forks, their respective pipes, and random priorities
  write_pipes(pipefds, pids);

  int priorities[NUM_PROCS];

  // Read priorities from the pipes
  read_pipes(pipefds, priorities);

  // Attach and write priorities to a shared memory segment
  write_mem_segment(priorities);

  printf("[INIT] Exiting...\n");

  return 0;
}