semaphores.c

/* basic semaphore operations, UNIX-native */

 /* pointer arithmetics:
  *
  * push(pointer, v) INDICATES *(pointer++) = v
  * pop(pointer) INDICATES *(--pointer)
  * (*++v)[0] INDICATES (**++v)
  *           INDICATES first char in v
  *           INDICATES name of string/vector v
  * likewise, *v[0] INDICATES **v
  *       and *v[n] INDICATES **(v + n)
  * returntype (*funct)(args) INDICATES a function funct with arguments args which returns...
  * char **argv INDICATES pointer to char pointer
  * int(*v)[len] INDICATES pointer "v" to a vector of "len" int elements
  * int *v[len] INDICATES vector "v" of "len" pointers to int elements
  * void *funct() INDICATES function "funct" that returns a pointer-to-void
  * void (*funct)() INDICATES pointer to a function "funct" that returns void
  *
  */

 /* useful characters: [] # */

 # include <stdio.h>
 # include <stdlib.h> // for exit
 # include <sys/ipc.h>
 # include <sys/shm.h> // for shmdet
 # include <sys/sem.h>
 # include <fcntl.h> // for O_RDWR

 # define BUFFER 256
 # define RSIZE 20 // reply size
 # define exception(x) { puts(x); exit(1); }

 /* semaphore buffer */
 typedef struct {
   unsigned short num;
   short op;
   short flag;
 } sembuf;

 /* unsigned semaphore */
 struct sem_uns {
   int val;
   unsigned short * array;
 };

 int main(int argc, char * argv[])
    {
     /* type of basic semaphore operations:
      *
      * wait(sem_descr, sem_num)
      *     {
      *     struct sembuf operations[1] = {{ sem_num, -1, 0 }};
      *     return sem_operation(sem_descr, operations, 1);
      *     }
      *
      * signal(...) does the same, but with +1 in operations[].
      */

     /* SEE MEMSHARE.C
      *
      * >> Sample at: pg. 45
      * >> One function for the memory writer, one for the reader, both in a do-while(!quit)
      * >>
      * >> writer() contains shmat(), puts("please input"), scanf() and strcpy() (last two in do-while)
      * >> reader() contains shmat(), printf() (in do-while)
      * >>
      * >> main() contains: shmget(), and then
      * >>  if(fork()) wait(&status); else writer(shm_descriptor);
      * >>  if(fork()) wait(&status); else reader(shm_descriptor);
      * >>
      * >> ...And then shmctl(IPC_RMID).
      *
      * The difference with semaphores:
      * 1) sem_num/op/flag = 0/-1/0 & semop(ID, &op, 1) AFTER the writer() loop (= signal())
      * 2) sem_num/op/flag = 0/0/0 & semop(ID, &op, 1) BEFORE the reader() loop (= wait())
      *
      * The main() uses shmget, semget, semctl (when necessary) and forks just like in MESSAGES.C
      * Then removes the memory (shmctl) and semaphore (semctl) and quits
      */
     return 0;
    }