ATOMIC(3)

NAME
OSAtomicAdd32, OSAtomicAdd32Barrier, OSAtomicIncrement32, OSAtomicIncrement32Barrier, OSAtomicDecrement32, OSAtomicDecrement32Barrier, OSAtomicOr32, OSAtomicOr32Barrier, OSAtomicAnd32, OSAtomicAnd32Barrier, OSAtomicXor32, OSAtomicXor32Barrier, OSAtomicAdd64, OSAtomicAdd64Barrier, OSAtomicIncrement64, OSAtomicIncrement64Barrier, OSAtomicDecrement64, OSAtomicDecrement64Barrier, OSAtomicCompareAndSwap32, OSAtomicCompareAndSwap32Barrier, OSAtomicCompareAndSwap64, OSAtomicCompareAndSwap64Barrier, OSAtomicTestAndSet, OSAtomicTestAndSetBarrier, OSAtomicTestAndClear, OSAtomicTestAndClearBarrier - atomic add, increment, decrement, or, and, xor, compare and swap, test and set, and test and clear

LIBRARY
libc

SYNOPSIS
#include <libkern/OSAtomic.h>
int32_t
OSAtomicAdd32(int32_t theAmount, int32_t *theValue);
int32_t
OSAtomicAdd32Barrier(int32_t theAmount, int32_t *theValue);
int32_t
OSAtomicIncrement32(int32_t *theValue);
int32_t
OSAtomicIncrement32Barrier(int32_t *theValue);
int32_t
OSAtomicDecrement32(int32_t *theValue);
int32_t
OSAtomicDecrement32Barrier(int32_t *theValue);
int32_t
OSAtomicOr32(uint32_t theMask, uint32_t *theValue);
int32_t
OSAtomicOr32Barrier(uint32_t theMask, uint32_t *theValue);
int32_t
OSAtomicAnd32(uint32_t theMask, uint32_t *theValue);
int32_t
OSAtomicAnd32Barrier(uint32_t theMask, uint32_t *theValue);
int32_t
OSAtomicXor32(uint32_t theMask, uint32_t *theValue);
int32_t
OSAtomicXor32Barrier(uint32_t theMask, uint32_t *theValue);
int64_t
OSAtomicAdd64(int64_t theAmount, int64_t *theValue);
int64_t
OSAtomicAdd64Barrier(int64_t theAmount, int64_t *theValue);
int64_t
OSAtomicIncrement64(int64_t *theValue);
int64_t
OSAtomicIncrement64Barrier(int64_t *theValue);
int64_t
OSAtomicDecrement64(int64_t *theValue);
int64_t
OSAtomicDecrement64Barrier(int64_t *theValue);
bool
OSAtomicCompareAndSwap32(int32_t oldValue, int32_t newValue, int32_t *theValue);
bool
OSAtomicCompareAndSwap32Barrier(int32_t oldValue, int32_t newValue, int32_t *theValue);
bool
OSAtomicCompareAndSwap64(int64_t oldValue, int64_t newValue, int64_t *theValue);
bool
OSAtomicCompareAndSwap64Barrier(int64_t oldValue, int64_t newValue, int64_t *theValue);
bool
OSAtomicTestAndSet(uint32_t n, void *theAddress);
bool
OSAtomicTestAndSetBarrier(uint32_t n, void *theAddress);
bool
OSAtomicTestAndClear(uint32_t n, void *theAddress);
bool
OSAtomicTestAndClearBarrier(uint32_t n, void *theAddress);

DESCRIPTION
These functions are thread and multiprocessor safe. For each function, there is a version that does and anoother that does not incorporate a memory 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 expensive.

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 OSAtomicIncrement32(). 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.

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 only implemented for 64-bit processes.

OSAtomicCompareAndSwap32() and OSAtomicCompareAndSwap64() 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.

RETURN VALUES
The arithmetic and logical operations return the new value, after the operation has been performed. 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.

SEE ALSO
atomicqueue(3), spinlock(3), barrier(3)