Lecture 21 CSCE 510 - IPC

• locking with fcntl

       #include sys/types.h
       #include unistd.h
       #include fcntl.h
   
       int fcntl(int fildes, int cmd, /* arg */ ...);
  

  • F_RDLCK, F_WRLCK, F_UNLCK
• Consider an Airline Reservation System
  • Airline reservation system using memory mapped Files for the data
  • Commands: Reserve, delete, show, query_flights
  • Use cbreak mode with TAB interpreted as completion character
  • data
    • flight numbers 4 digits
    • seat number: [1-40] [A-F]
    • Names: last (18), first (18)
    • phone number optional area code (default 803), optional country code default US
  • signals - catch SIGINT and all others except SIQUIT (and SUREKILL = 9)
• Non-Blocking I/O
  • i open ("a", O_RDONLY | O_NONBLOCK)
• Memory Mapped I/O - mapping file to memory area (section 12.3)
  1. High Speed File access
  2. Executable files - dynamic loading
  3. newmoery allocated from dev/zero (Electric Fence)
  4. New memory - filled with executable statements Just-In-Time Compilers
  5. Files can be treated as memory eliminating read, write, lseek calls
  6. shared memory segments can be saved accross process creation and destruction
• memory mapping functions

  #include sys/mman.h
  void *mmap(void *addr, size_t len, int prot, int flags, int fd, off_t off);
  
  int munmap(void *addr, size_t len);
  
  int msync(void *addr, size_t len, int flags);
  
  

• size_t get pagesize()
• mmap protections: PROT_READ PROT_WRITE PROT_EXEC
• mmap flags
  • MAP_ANONYMOUS - ignore fd
  • MAP_FIXED -fail if address invalid
  • MAP_PRIVATE
  • MAP_SHARED
  • MAP_DENYWRITE
  • MAP_GROWSDOWN
  • MAP_LOCKED
• msync
• locking memory regions
  • int mlock(const void * addr, size_t len);
  • int munlock(const void * addr, size_t len);
  • mlockall, munlockall
• IPC Facility Keys
  • IPC Keys are used by the System V IPC communication routines to uniquely identify an IPC object much in the way a pathname identifies a file.
  • key_t ``key type'' defined in stdio.h
• ftok - non-standard library function that maps path to a key
• The function Ftok
• [Header Files:] sys/types.h sys/ipc.h
• [Parameters:]
  • [] char *pathname;
  • [] char id;
• [USAGE:] keyval = ftok(path, id);
• The function Ftok
• IPC / IO Anology {\bf Notes on FTOK}
  • The id is an additional factor so that there can be more than 1 key assoc iated with a file.
  • returns (key\_t) -1 if the path does not exist
  • if one deletes and then puts back the file key values may be different
  \newpage {\bf IPC / IO Anology} IPC keys are used:
  • to create IPC objects,
  • to gain access to existing IPC objects.
• System V IPC objects:
  1. msg_queues
  2. semaphores
  3. shared memory segments
  4. maintained in system wide table
• Semaphores- Semaphores were introduced by Edgar Dijkstra to provide a mechanism for the synchronization of processes.

  p(sem)  or wait operation
  
  if (sem != 0)
     decrement sem by one
  else
     wait until sem becomes non-zero
  

• v(sem) or signal operation

  if (queue of waiters non-empty)
     restart first process in the queue
  else
     increment sem by one
  

• Critical Sections
• Mutual Exclusion
• msgqueue calls
  • int msgget(key_t key, int msgflg);
  • int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)
  • int msgsnd (msqid, msgp, msgsz, msgflg)
  • int msgctl(int msqid, int cmd, struct msqid_ds, buf)
• Removing an ipc object
  • ipcs - "ipc ls"
  • ipcrm -q fid
  • msgctl(shmid, IPC_RMID, 0, 0)

[Readings:]

Section 12.2 for Memory Mapped I/O

[Assignment:]

Reservation System

[Reminders:]

Test Nov 15