slide 13 of 19

Compute Pi Using System V SHM

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

volatile struct shared {
  double pi; int lock;
} *shared;

inline extern int xchg(register int reg,
volatile int * volatile obj)
{
  /* Atomic exchange instruction */
__asm__ __volatile__ ("xchgl %1,%0"
                      :"=r" (reg), "=m" (*obj)
                      :"r" (reg), "m" (*obj));
  return(reg);
}

main(int argc,
char **argv)
{
  register double width, localsum;
  register int intervals, i;
  register int shmid;
  register int iproc = 0;;

  /* Allocate System V shared memory */
  shmid = shmget(IPC_PRIVATE,
                 sizeof(struct shared),
                 (IPC_CREAT | 0600));
  shared = ((volatile struct shared *) shmat(shmid, 0, 0));
  shmctl(shmid, IPC_RMID, 0);

  /* Initialize... */
  shared->pi = 0.0;
  shared->lock = 0;

  /* Fork a child */
  if (!fork()) ++iproc;

  /* get the number of intervals */
  intervals = atoi(argv[1]);
  width = 1.0 / intervals;

  /* do the local computations */
  localsum = 0;
  for (i=iproc; i<intervals; i+=2) {
    register double x = (i + 0.5) * width;
    localsum += 4.0 / (1.0 + x * x);
  }
  localsum *= width;

  /* Atomic spin lock... */
  while (xchg((iproc + 1), &(shared->lock))) ;	

  /* Do non-atomic op... */
  shared->pi += localsum;

  /* Now unlock */
  shared->lock = 0;

  /* Terminate child (barrier sync) */
  if (iproc == 0) {
    wait(NULL);
    printf("Estimation of pi is %14.12lf\n", shared->pi);
  }

  /* Check out */
  exit(0);
}