Shared memory example

[Richard Fox]

Thanks for all of the responses to my question regarding shared memory.

Many people asked for me to send them a copy of an example that I received.
Instead of sending to all I thought I would just post a very simple
program derived from a more complex program. If there are still comments
regarding shared memory please send them otherwise thanks again for
the response.


-------------------------------------------------------------


#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>

extern int errno;

#define SHMKEY	75
#define K	1024
int shmid;
char *addr1,*addr2;


main(argc,argv)
int argc;
char **argv;
{
  int i, *pint;
  key_t key;
  extern char *shmat();
  extern cleanup();

  key = IPC_PRIVATE;
  shmid = shmget( key, 1*K,0777|IPC_CREAT);
  printf("shmid %d  errno=%d\n",shmid,errno);
  addr1 = shmat(shmid,0,0);
  printf("addr1 0x%x\n",addr1);

  for(i=0; i<20; i++)
	signal(i,cleanup);
  addr1[K-1]=0;
  if(fork()) {
     writer();
     printf("RETURN WRITER\n");
  } else
  {
     reader();
     printf("RETURN READER\n");
     kill(0,9);
  }
  pause();
}

cleanup()
{
  shmctl(shmid, IPC_RMID,0);
  printf("In clean up\n");
  exit(0);
}

writer()
{
  int i;
  printf("In writer\n");
  for(i=0;i<K;i++){
    /*printf("addr1[%d] = %d\n",i,addr1[i]);*/
    addr1[i]=5;
  }
  while(addr1[K-1]!=0);
}

reader()
{
  int i;
  printf("In reader\n");
  while(addr1[K-1]==0);
  printf("going....reader\n");
  for(i=0;i<K;i++)
	printf(" addr1[%d]=%d\n",i,addr1[i]);
  addr1[K-1]=0;
}