#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#include <signal.h>
#include <string.h>
    char DataFile [] = "data.xxx";
    char info     [] = "abcdefghijklmnopqrstuvwxyz";
    #define OFFSET 5
    #define SIZE   12
    #define PAUSE 2
    int trial = 1;
    int fd, pid;
    char buffer[120], myname[20];
    void writeAccess(), readAccess();
    void fcleanup(int nsig){
            unlink(DataFile);
            printf("cleanup:%s\n", myname);
            if(nsig) exit(0);
    }
    int main(){
            int i;
            fd = creat(DataFile, 0644);
            write(fd, info, strlen(info));
            close(fd);
            signal(SIGINT, fcleanup);
            sprintf(myname, fork() ? "B-%06d" : "A-%06d", pid = getpid());
            srand(time(NULL)+pid);
            printf("%s:started\n", myname);
            fd = open(DataFile, O_RDWR|O_EXCL);
            printf("%s:opened %s\n", myname, DataFile);
            for(i=0; i < 30; i++){
                    if(rand()%2)    readAccess();
                    else            writeAccess();
            }
            close(fd);
            printf("%s:finished\n", myname);
            wait(NULL);
            fcleanup(0);
            return 0;
    }
    void writeAccess(){
            printf("Write:%s #%d\n", myname, trial);
            //if(fcntl(fd, F_SETLKW, &lock) <0)
            if(lockf(fd, F_LOCK, 0) <0)
                    perror("SETLOCK");
            printf("\twrite:%s locked\n", myname);
            sprintf(buffer, "%s #%02d", myname, trial);
            printf ("\twrite:%s \"%s\"\n", myname, buffer);
            lseek (fd, (off_t) OFFSET, SEEK_SET);
            write (fd, buffer, SIZE);
            sleep (PAUSE);
            if(lockf(fd, F_ULOCK, 0) <0)
                    perror("F_SETLOCK");
            printf("\twrite:%s unlocked\n", myname);
            trial++;
    }
    void readAccess(){
            printf("Read:%s #%d\n", myname, trial);
            if(lockf(fd, F_LOCK, 0) <0)
                    perror("F_SETLKW");
            printf("\tread:%s locked\n", myname);
            lseek(fd, (off_t) OFFSET, SEEK_SET);
            read (fd, buffer, SIZE);
            printf("\tcontents:%s \"%*.*s\"\n", myname, SIZE, SIZE, buffer);
            sleep (PAUSE);
            if(lockf(fd, F_ULOCK, 0) <0)
                    perror("F_SETLKW");
            printf("\tread:%s unlocked\n", myname);
            trial++;
    }