ipc.c

#include <unistd.h>
#include <semaphore.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define MAX_SIZE 1024
#define FILE_NAME "record.txt"
#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while(0)

int main() {
    sem_t *buf1_full, *buf2_full;
    if((buf1_full=sem_open("sem_buf1", O_CREAT, S_IRUSR|S_IWUSR, 0))==SEM_FAILED)
        handle_error("Open semaphore for buffer1");
    if((buf2_full=sem_open("sem_buf2", O_CREAT, S_IRUSR|S_IWUSR, 0))==SEM_FAILED)
        handle_error("Open semaphore for buffer2");

    char *buf1, *buf2;
    if((buf1=mmap(NULL, MAX_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0))==MAP_FAILED)
        handle_error("Open shared buffer 1");
    if((buf2=mmap(NULL, MAX_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0))==MAP_FAILED)
        handle_error("Open shared buffer 2");

    pid_t p1 = fork();
    if(p1<0)
        handle_error("Fork p1");
    else if (p1==0) {
        // child process 1
        // read from file
        int fd;
        if((fd = open(FILE_NAME, O_RDONLY))==-1) {
            handle_error("Open file");
            exit(EXIT_FAILURE);
        }
        if(read(fd, buf1, MAX_SIZE)==-1) {
            handle_error("Read file");
            exit(EXIT_FAILURE);
        }

        // buffer 1 is full
        sem_post(buf1_full);
        return 0;
    }

    pid_t p2 = fork();
    if(p2<0)
        handle_error("Fork p2");
    else if (p2==0) {
        // child process 2
        // wait until buf1 is filled by process 1
        sem_wait(buf1_full);

        // copy from buffer1 to buffer2
        strncpy(buf2, buf1, MAX_SIZE);

        // buffer2 is full
        sem_post(buf2_full);
        return 0;
    }

    pid_t p3 = fork();
    if(p3<0)
        handle_error("Fork p3");
    else if (p3==0) {
        // child process 3
        // wait until buffer 2 is full
        sem_wait(buf2_full);

        // print the file
        printf("%s", buf2);
        return 0;
    }

    // wait until all the child processes finished
    waitpid(p1, NULL, 0);
    waitpid(p2, NULL, 0);
    waitpid(p3, NULL, 0);
    return 0;
}