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/*
    PUBLIC DOMAIN

    Fuck GNU, Fuck Creative Commons, Fuck BSD.
    Attribution are for tards.
                                                   */


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

const key_t SHM_KEY = 0x42385;              /* random number known by all processes, somehow */
const int SHM_SIZE = 16*1024*1024;          /* 16MB get allocated.. */
const int SHM_FLAGS = IPC_CREAT | 0666;     /* It does not work unless you supply IPC_CREAT (!?),
                                               0666 are the permissions. */

/* Allocate does not allocate anything, it just returns an *identifier* to be used by attach() */
int allocate()
{
   int shmid;

   if (( shmid = shmget( SHM_KEY, SHM_SIZE, SHM_FLAGS )) == -1 )
   {
      perror("shmget: shmget failed");
      exit(1);
   }
   return shmid;
}


/* Returns a pointer to the shared memory, takes an identifier created by shmget() or allocate() */
void *attach(int shmid)
{
   void *shm;
   if(( shm = shmat( shmid, NULL, 0 )) == (void*) -1 )
   {
      perror("shmat");
      exit(1);
   }
   return shm;
}


/* Child writes "Hello world.\n" to the shared memory, sleeps for one second and the dies. */
void child()
{
   char *str = "Hello world.\n";
   void *shared_mem;

   shared_mem = attach( allocate() );

   strncpy( shared_mem, str, strlen(str)+1 );
   sleep(1);

   printf("The child has written to shared memory. Goodbye ;)\n");
   _exit(0);
}


/* Mother reads from the shared memory after 2 seconds and prints the content. */
void mother()
{
   void *shared_mem;

   shared_mem = attach( allocate() );
   sleep(2);
   printf("Mother reads from shared memory: %s", shared_mem);

   exit(0);
}


/* Creates two processes. */
void forkoff()
{
   if( fork() == 0 )
      child();
   else mother();
}


/* bla bla blaa bla-bla blabla blabla-bla bla */
int main()
{
   forkoff();
   return -1;  /* should never return */
}