set_data_by_mmap(it,fname,len,writable,shared,creat,mode,trunc)
	pdl *it
	char *fname
	int len
	int writable
	int shared
	int creat
	int mode
	int trunc
	CODE:
#ifdef USE_MMAP
       int fd;
       pdl_freedata(it);
       fd = open(fname,(writable && shared ? O_RDWR : O_RDONLY)|
               (creat ? O_CREAT : 0),mode);
       if(fd < 0) {
               croak("Error opening file");
       }
       if(trunc) {
               ftruncate(fd,0);   /* Clear all previous data */
               ftruncate(fd,len); /* And make it long enough */
       }
       if(len) {
		it->data = mmap(0,len,PROT_READ | (writable ?
					PROT_WRITE : 0),
				(shared ? MAP_SHARED : MAP_PRIVATE),
				fd,0);
		if(!it->data)
			croak("Error mmapping!");
       } else {
               /* Special case: zero-length file */
               it->data = NULL;
       }
       PDLDEBUG_f(printf("PDL::MMap: mapped to %d\n",it->data);)
       it->state |= PDL_DONTTOUCHDATA | PDL_ALLOCATED;
       pdl_add_deletedata_magic(it, pdl_delete_mmapped_data, len);
       close(fd);
#else