#include "SDL_stdinc.h"
#include "SDL_platform.h"
#include "begin_code.h"
#include "close_code.h"

Classes
struct	SDL_atomic_t
	A type representing an atomic integer value. It is a struct so people don't accidentally use numeric operations on it. More...

Macros
#define	SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
	Decrement an atomic variable used as a reference count. More...

#define	SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
	Increment an atomic variable used as a reference count. More...

#define	SDL_CompilerBarrier() { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }

#define	SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()

#define	SDL_MemoryBarrierRelease() SDL_CompilerBarrier()

Functions
SDL_FORCE_INLINE int	SDL_AtomicAdd (SDL_atomic_t *a, int v)
	Add to an atomic variable. More...

DECLSPEC SDL_bool SDLCALL	SDL_AtomicCAS (SDL_atomic_t *a, int oldval, int newval)
	Set an atomic variable to a new value if it is currently an old value. More...

DECLSPEC SDL_bool SDLCALL	SDL_AtomicCASPtr (void *a, void oldval, void *newval)
	Set a pointer to a new value if it is currently an old value. More...

SDL_FORCE_INLINE int	SDL_AtomicGet (SDL_atomic_t *a)
	Get the value of an atomic variable. More...

SDL_FORCE_INLINE void *	SDL_AtomicGetPtr (void **a)
	Get the value of a pointer atomically. More...

SDL_FORCE_INLINE int	SDL_AtomicSet (SDL_atomic_t *a, int v)
	Set an atomic variable to a value. More...

SDL_FORCE_INLINE void *	SDL_AtomicSetPtr (void *a, void v)
	Set a pointer to a value atomically. More...

SDL AtomicLock
The atomic locks are efficient spinlocks using CPU instructions, but are vulnerable to starvation and can spin forever if a thread holding a lock has been terminated. For this reason you should minimize the code executed inside an atomic lock and never do expensive things like API or system calls while holding them. The atomic locks are not safe to lock recursively. Porting Note: The spin lock functions and type are required and can not be emulated because they are used in the atomic emulation code.
typedef int	SDL_SpinLock

DECLSPEC SDL_bool SDLCALL	SDL_AtomicTryLock (SDL_SpinLock *lock)
	Try to lock a spin lock by setting it to a non-zero value. More...

DECLSPEC void SDLCALL	SDL_AtomicLock (SDL_SpinLock *lock)
	Lock a spin lock by setting it to a non-zero value. More...

DECLSPEC void SDLCALL	SDL_AtomicUnlock (SDL_SpinLock *lock)
	Unlock a spin lock by setting it to 0. Always returns immediately. More...

Detailed Description

Atomic operations.

IMPORTANT: If you are not an expert in concurrent lockless programming, you should only be using the atomic lock and reference counting functions in this file. In all other cases you should be protecting your data structures with full mutexes.

The list of "safe" functions to use are: SDL_AtomicLock() SDL_AtomicUnlock() SDL_AtomicIncRef() SDL_AtomicDecRef()

Seriously, here be dragons! ^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can find out a little more about lockless programming and the subtle issues that can arise here: http://msdn.microsoft.com/en-us/library/ee418650%28v=vs.85%29.aspx

There's also lots of good information here: http://www.1024cores.net/home/lock-free-algorithms http://preshing.com/

These operations may or may not actually be implemented using processor specific atomic operations. When possible they are implemented as true processor specific atomic operations. When that is not possible the are implemented using locks that do use the available atomic operations.

All of the atomic operations that modify memory are full memory barriers.

Definition in file SDL_atomic.h.

Macro Definition Documentation

#define SDL_AtomicDecRef ( a ) (SDL_AtomicAdd(a, -1) == 1)

Decrement an atomic variable used as a reference count.

Returns: SDL_TRUE if the variable reached zero after decrementing, SDL_FALSE otherwise

Definition at line 307 of file SDL_atomic.h.

#define SDL_AtomicIncRef ( a ) SDL_AtomicAdd(a, 1)

Increment an atomic variable used as a reference count.

Definition at line 297 of file SDL_atomic.h.

Referenced by SDL_AddTimer(), and SDL_TLSCreate().

#define SDL_CompilerBarrier ( ) { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }

The compiler barrier prevents the compiler from reordering reads and writes to globally visible variables across the call.

Definition at line 135 of file SDL_atomic.h.

Referenced by SDL_AtomicGet(), and SDL_AtomicGetPtr().

#define SDL_MemoryBarrierAcquire ( ) SDL_CompilerBarrier()

Definition at line 181 of file SDL_atomic.h.

Referenced by SDL_Generic_GetTLSData(), SDL_GetErrBuf(), and SDL_SYS_GetTLSData().

#define SDL_MemoryBarrierRelease ( ) SDL_CompilerBarrier()

Memory barriers are designed to prevent reads and writes from being reordered by the compiler and being seen out of order on multi-core CPUs.

A typical pattern would be for thread A to write some data and a flag, and for thread B to read the flag and get the data. In this case you would insert a release barrier between writing the data and the flag, guaranteeing that the data write completes no later than the flag is written, and you would insert an acquire barrier between reading the flag and reading the data, to ensure that all the reads associated with the flag have completed.

In this pattern you should always see a release barrier paired with an acquire barrier and you should gate the data reads/writes with a single flag variable.

For more information on these semantics, take a look at the blog post: http://preshing.com/20120913/acquire-and-release-semantics

Definition at line 180 of file SDL_atomic.h.

Referenced by SDL_Generic_GetTLSData(), SDL_GetErrBuf(), and SDL_SYS_GetTLSData().