// File; testSemUNDO.c
// See if the system has SEMUNDO implemented
// SEMUNDO will reverse the effect of a P() (wait) operation if
//   process that successfully issued it crashes.
//   This protects a system from unbounded waiting

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <math.h>
#include <sys/errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#define NumSems 1

void PrintSems(int semid);

// Issues a wait() with SEMUNDO
void P(key_t semID,int semNum);

// Issue a signal()
void V(key_t semID,int semNum);

int main()
{ key_t semKey=getuid();
  int semID;
  if ((semID = semget(semKey, 1, 0))<0) {
    semID = semget(semKey, 1, 0666|IPC_CREAT);
    V(semID,0);
    PrintSems(semID);
  }
  P(semID,0);
  printf("Process %d is in! \n ",getpid());
  // Tried to just have a read but the suspension was a problem
  // I assume a possibkly indefinite suspension is detected
  printf("Will sleep for 10 seconds so you can test SEM_UNDO\n");
  sleep(10);
  V(semID,0);
}

void PrintSems(semid)
int semid;
{int i;
 ushort Values[NumSems];
 semctl(semid,0,GETALL,Values);
 fprintf(stderr,"Semaphore Values ");
 for (i=0;i<NumSems;i++)
  fprintf(stderr,"#%d. %d  | ",i,Values[i]);
 fprintf(stderr,"\n");
}

// Issues a wait() with SEMUNDO
void P(key_t semID,int semNum)
{ struct sembuf semCmd;
  semCmd.sem_num=semNum;
  semCmd.sem_op=-1;
  semCmd.sem_flg=SEM_UNDO;
  // semCmd.sem_flg=0;
  semop(semID,&semCmd,1);
}

// Issue a signal()
void V(key_t semID,int semNum)
{ struct sembuf semCmd;
  semCmd.sem_num=semNum;
  semCmd.sem_op=1;
  semCmd.sem_flg=0;
  semop(semID,&semCmd,1);
}