Fcntl Learning Process

# Include  <  Sys  /  Types. h  >  
# Include  <  Sys  /  Stat. h  >  
# Include  <  Fcntl. h  >  
# Include "  Ourhdr. h  "  
# Include  "  Error. c  "  
# Include  "  F10.17.c  "  
  Static     Void  Lockabyte (  Const    Char     *  ,  Int  , Off_t );

  Int  Lock_reg (  Int  FD,  Int  CMD,  Int  Type, off_t offset,  Int  Whence, off_t Len)
{
 Struct  Flock  Lock  ;

  Lock  . L_type  =  Type;  /*  F_rdlck, f_wrlck, f_unlck  */  
  Lock  . L_start  =  Offset;  /* Byte offset, relative to l_whence  */  
  Lock  . L_whence  =  Whence;  /*  Seek_set, seek_cur, seek_end  */  
  Lock  . L_len  =  Len;  /* Bytes (0 means to EOF)  */  

  Return  (Fcntl (FD, CMD,  &  Lock  ));
}

  Int  Main (  Void  )
{
  Int  FD;
Pid_t PID;

  /* Create a file and write two bytes to it  */  
  If  (FD  =  Creat (  "  Templock  "  , File_mode ))  <     0  )
Err_sys (  " Create Error  "  );

  If  (Write (FD,  "  AB  "  ,  2  )  ! =     2  )
Err_sys (  "  Write Error "  );

Tell_wait ();
  If  (PID  =  Fork ())  <     0  )
Err_sys (  "  Fork Error  "  );
  Else     If (PID  =     0  ){  /*  Child  */  
Lockabyte (  "  Child  "  , FD,  0  );
Tell_parent (getppid ());
Wait_parent ();
Lockabyte ( "  Child  "  , FD,  1  );
}  Else  {
Lockabyte (  "  Parent  "  , FD,  1  );
Tell_child (PID );
Wait_child ();
Lockabyte (  " Parent  "  , FD,  0  );
}
Exit (  0  );
}

  Static     Void  Lockabyte (  Const     Char     *  Name, Int  FD, off_t offset)
{
  If  (Writew_lock (FD, offset, seek_set,  1  )  <     0  )
Err_sys (  "  % S: writew_lock Error  "  , Name );

Printf (  " % S: got the lock, byte % d \ n  "  , Name, offset );
}