src/ipc/mem/PageStack.h

   1 /*
   2  * $Id$
   3  *
   4  */
   5
   6 #ifndef SQUID_IPC_MEM_PAGE_STACK_H
   7 #define SQUID_IPC_MEM_PAGE_STACK_H
   8
   9 #include "ipc/AtomicWord.h"
  10 #include "ipc/mem/Segment.h"
  11
  12 namespace Ipc {
  13
  14 namespace Mem {
  15
  16 /// Atomic container of "free" page numbers inside a single SharedMemory space.
  17 /// Assumptions: all page numbers are unique, positive, have an known maximum,
  18 /// and can be temporary unavailable as long as they are never trully lost.
  19 class PageStack {
  20 public:
  21     typedef uint32_t Value; ///< stack item type (a free page number)
  22
  23     /// creates a new shared stack that can hold up to capacity items
  24     PageStack(const String &id, const unsigned int capacity);
  25     /// attaches to the identified shared stack
  26     PageStack(const String &id);
  27     /// unlinks shared memory segment
  28     static void Unlink(const String &id);
  29
  30     /// lower bound for the number of free pages
  31     unsigned int size() const { return max(0, shared->theSize.get()); }
  32
  33     /// sets value and returns true unless no free page numbers are found
  34     bool pop(Value &value);
  35     /// makes value available as a free page number to future pop() callers
  36     void push(const Value value);
  37
  38 private:
  39     /// stack index and size type (may temporary go negative)
  40     typedef int Offset;
  41
  42     struct Shared {
  43         Shared(const unsigned int aCapacity);
  44
  45         /// total shared memory size required to share
  46         static size_t MemSize(const unsigned int capacity);
  47
  48         // these help iterate the stack in search of a free spot or a page
  49         Offset next(const Offset idx) const { return (idx + 1) % theCapacity; }
  50         Offset prev(const Offset idx) const { return (theCapacity + idx - 1) % theCapacity; }
  51
  52         const Offset theCapacity; ///< stack capacity, i.e. theItems size
  53         /// lower bound for the number of free pages (may get negative!)
  54         AtomicWordT<Offset> theSize;
  55
  56         /// last readable item index; just a hint, not a guarantee
  57         AtomicWordT<Offset> theLastReadable;
  58         /// first writable item index; just a hint, not a guarantee
  59         AtomicWordT<Offset> theFirstWritable;
  60
  61         typedef AtomicWordT<Value> Item;
  62         Item theItems[]; ///< page number storage
  63     };
  64
  65     Segment shm; ///< shared memory segment to store metadata (and pages)
  66     Shared *shared; ///< our metadata, shared among all stack users
  67 };
  68
  69 } // namespace Mem
  70
  71 } // namespace Ipc
  72
  73 #endif // SQUID_IPC_MEM_PAGE_STACK_H