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ATOMIC(3)                BSD Library Functions Manual                ATOMIC(3)

NAME
     OSAtomicAdd32, OSAtomicAdd32Barrier, OSAtomicIncrement32,
     OSAtomicIncrement32Barrier, OSAtomicDecrement32,
     OSAtomicDecrement32Barrier, OSAtomicOr32, OSAtomicOr32Barrier,
     OSAtomicOr32Orig, OSAtomicOr32OrigBarrier, OSAtomicAnd32,
     OSAtomicAnd32Barrier, OSAtomicAnd32Orig, OSAtomicAnd32OrigBarrier,
     OSAtomicXor32, OSAtomicXor32Barrier, OSAtomicXor32Orig,
     OSAtomicXor32OrigBarrier, OSAtomicAdd64, OSAtomicAdd64Barrier,
     OSAtomicIncrement64, OSAtomicIncrement64Barrier, OSAtomicDecrement64,
     OSAtomicDecrement64Barrier, OSAtomicCompareAndSwapInt,
     OSAtomicCompareAndSwapIntBarrier, OSAtomicCompareAndSwapLong,
     OSAtomicCompareAndSwapLongBarrier, OSAtomicCompareAndSwapPtr,
     OSAtomicCompareAndSwapPtrBarrier, OSAtomicCompareAndSwap32,
     OSAtomicCompareAndSwap32Barrier, OSAtomicCompareAndSwap64,
     OSAtomicCompareAndSwap64Barrier, OSAtomicTestAndSet,
     OSAtomicTestAndSetBarrier, OSAtomicTestAndClear,
     OSAtomicTestAndClearBarrier, OSSpinLockTry, OSSpinLockLock,
     OSSpinLockUnlock, OSAtomicEnqueue, OSAtomicDequeue -- atomic add, incre-ment, increment,
     ment, decrement, or, and, xor, compare and swap, test and set, test and
     clear, spinlocks, and lockless queues

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <libkern/OSAtomic.h>

     int32_t
     OSAtomicAdd32(int32_t theAmount, volatile int32_t *theValue);

     int32_t
     OSAtomicAdd32Barrier(int32_t theAmount, volatile int32_t *theValue);

     int32_t
     OSAtomicIncrement32(volatile int32_t *theValue);

     int32_t
     OSAtomicIncrement32Barrier(volatile int32_t *theValue);

     int32_t
     OSAtomicDecrement32(volatile int32_t *theValue);

     int32_t
     OSAtomicDecrement32Barrier(volatile int32_t *theValue);

     int32_t
     OSAtomicOr32(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicOr32Barrier(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicAnd32(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicAnd32Barrier(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicXor32(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicXor32Barrier(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicOr32Orig(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicOr32OrigBarrier(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicAnd32Orig(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicAnd32OrigBarrier(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicXor32Orig(uint32_t theMask, volatile uint32_t *theValue);

     int32_t
     OSAtomicXor32OrigBarrier(uint32_t theMask, volatile uint32_t *theValue);

     int64_t
     OSAtomicAdd64(int64_t theAmount, volatile int64_t *theValue);

     int64_t
     OSAtomicAdd64Barrier(int64_t theAmount, volatile int64_t *theValue);

     int64_t
     OSAtomicIncrement64(volatile int64_t *theValue);

     int64_t
     OSAtomicIncrement64Barrier(volatile int64_t *theValue);

     int64_t
     OSAtomicDecrement64(volatile int64_t *theValue);

     int64_t
     OSAtomicDecrement64Barrier(volatile int64_t *theValue);

     bool
     OSAtomicCompareAndSwapInt(int oldValue, int newValue,
         volatile int *theValue);

     bool
     OSAtomicCompareAndSwapIntBarrier(int oldValue, int newValue,
         volatile int *theValue);

     bool
     OSAtomicCompareAndSwapLong(long oldValue, long newValue,
         volatile long *theValue);

     bool
     OSAtomicCompareAndSwapLongBarrier(long oldValue, long newValue,
         volatile long *theValue);

     bool
     OSAtomicCompareAndSwapPtr(void* oldValue, void* newValue,
         void* volatile *theValue);

     bool
     OSAtomicCompareAndSwapPtrBarrier(void* oldValue, void* newValue,
         void* volatile *theValue);

     bool
     OSAtomicCompareAndSwap32(int32_t oldValue, int32_t newValue,
         volatile int32_t *theValue);

     bool
     OSAtomicCompareAndSwap32Barrier(int32_t oldValue, int32_t newValue,
         volatile int32_t *theValue);

     bool
     OSAtomicCompareAndSwap64(int64_t oldValue, int64_t newValue,
         volatile int64_t *theValue);

     bool
     OSAtomicCompareAndSwap64Barrier(int64_t oldValue, int64_t newValue,
         volatile int64_t *theValue);

     bool
     OSAtomicTestAndSet(uint32_t n, volatile void *theAddress);

     bool
     OSAtomicTestAndSetBarrier(uint32_t n, volatile void *theAddress);

     bool
     OSAtomicTestAndClear(uint32_t n, volatile void *theAddress);

     bool
     OSAtomicTestAndClearBarrier(uint32_t n, volatile void *theAddress);

     bool
     OSSpinLockTry(OSSpinLock *lock);

     void
     OSSpinLockLock(OSSpinLock *lock);

     void
     OSSpinLockUnlock(OSSpinLock *lock);

     void
     OSAtomicEnqueue(OSQueueHead *list, void *new, size_t offset);

     void*
     OSAtomicDequeue(OSQueueHead *list, size_t offset);

DESCRIPTION
     These functions are thread and multiprocessor safe.  For each function,
     there is a version that does and another that does not incorporate a mem-ory memory
     ory barrier.  Barriers strictly order memory access on a weakly-ordered
     architecture such as PPC.  All loads and stores executed in sequential
     program order before the barrier will complete before any load or store
     executed after the barrier.  On a uniprocessor, the barrier operation is
     typically a nop.  On a multiprocessor, the barrier can be quite expen-sive. expensive.
     sive.

     Most code will want to use the barrier functions to insure that memory
     shared between threads is properly synchronized.  For example, if you
     want to initialize a shared data structure and then atomically increment
     a variable to indicate that the initialization is complete, then you must
     use OSAtomicIncrement32Barrier() to ensure that the stores to your data
     structure complete before the atomic add.  Likewise, the consumer of that
     data structure must use OSAtomicDecrement32Barrier(), in order to ensure
     that their loads of the structure are not executed before the atomic
     decrement.  On the other hand, if you are simply incrementing a global
     counter, then it is safe and potentially much faster to use OSAtomicIn-crement32(). OSAtomicIncrement32().
     crement32().  If you are unsure which version to use, prefer the barrier
     variants as they are safer.

     The logical (and, or, xor) and bit test operations are layered on top of
     the OSAtomicCompareAndSwap() primitives.  There are four versions of each
     logical operation, depending on whether or not there is a barrier, and
     whether the return value is the result of the operation (eg,
     OSAtomicOr32() ) or the original value before the operation (eg,
     OSAtomicOr32Orig() ).

     The memory address theValue must be naturally aligned, ie 32-bit aligned
     for 32-bit operations and 64-bit aligned for 64-bit operations.

     The 64-bit operations are not implemented for 32-bit processes on PPC
     platforms.

     The OSAtomicCompareAndSwap() operations compare oldValue to *theValue,
     and set *theValue to newValue if the comparison is equal.  The comparison
     and assignment occur as one atomic operation.

     OSAtomicTestAndSet() and OSAtomicTestAndClear() operate on bit (0x80 >> (
     n & 7)) of byte ((char*) theAddress + ( n >> 3)).  They set the named bit
     to either 1 or 0, respectively.  theAddress need not be aligned.

     The routines OSAtomicEnqueue() and OSAtomicDequeue() operate on singly
     linked LIFO queues.  Ie, a dequeue operation will return the most
     recently enqueued element, or NULL if the list is empty.  The operations
     are lockless, and barriers are used as necessary to permit thread-safe
     access to the queue element.  offset is the offset in bytes to the link
     field in the queue element.  For example:

                   typedef struct elem {
                           long    data1;
                           struct elem *link;
                           int     data2;
                   } elem_t;

                   elem_t fred, mary, *p;

                   OSQueueHead q = OS_ATOMIC_QUEUE_INIT;

                   OSAtomicEnqueue( &q, &fred, offsetof(elem_t,link) );
                   OSAtomicEnqueue( &q, &mary, offsetof(elem_t,link) );

                   p = OSAtomicDequeue( &q, offsetof(elem_t,link) );

     In this example, the call of OSAtomicDequeue() will return a ptr to mary.

RETURN VALUES
     The arithmetic operations return the new value, after the operation has
     been performed.  The boolean operations come in two styles, one of which
     returns the new value, and one of which (the "Orig" versions) returns the
     old.  The compare-and-swap operations return true if the comparison was
     equal, ie if the swap occured.  The bit test and set/clear operations
     return the original value of the bit.  The dequeue operation returns the
     most recently enqueued element, or NULL if the list in empty.

SEE ALSO
     spinlock(3), barrier(3)

HISTORY
     Most of these functions first appeared in Mac OS 10.4 (Tiger).  The
     "Orig" forms of the boolean operations, the "int", "long" and "ptr" forms
     of compare-and-swap, and lockless enqueue/dequeue first appeared in Mac
     OS 10.5 (Leopard).

Darwin                           May 26, 2004                           Darwin