SDL_atomic.h File Reference

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

Go to the source code of this file.

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_CompilerBarrier()   { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
#define	SDL_MemoryBarrierRelease()   SDL_CompilerBarrier()
#define	SDL_MemoryBarrierAcquire()   SDL_CompilerBarrier()
#define	SDL_AtomicIncRef(a)   SDL_AtomicAdd(a, 1)
	Increment an atomic variable used as a reference count. More...
#define	SDL_AtomicDecRef(a)   (SDL_AtomicAdd(a, -1) == 1)
	Decrement an atomic variable used as a reference count. More...

Functions
DECLSPEC void SDLCALL	SDL_MemoryBarrierReleaseFunction (void)
DECLSPEC void SDLCALL	SDL_MemoryBarrierAcquireFunction (void)
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 int SDLCALL	SDL_AtomicSet (SDL_atomic_t *a, int v)
	Set an atomic variable to a value. More...
DECLSPEC int SDLCALL	SDL_AtomicGet (SDL_atomic_t *a)
	Get the value of an atomic variable. More...
DECLSPEC int SDLCALL	SDL_AtomicAdd (SDL_atomic_t *a, int v)
	Add to an atomic variable. 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...
DECLSPEC void *SDLCALL	SDL_AtomicSetPtr (void **a, void *v)
	Set a pointer to a value atomically. More...
DECLSPEC void *SDLCALL	SDL_AtomicGetPtr (void **a)
	Get the value of a pointer 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

SDL_AtomicDecRef

#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 262 of file SDL_atomic.h.

SDL_AtomicIncRef

#define SDL_AtomicIncRef

(

a

)

SDL_AtomicAdd(a, 1)

Increment an atomic variable used as a reference count.

Definition at line 252 of file SDL_atomic.h.

SDL_CompilerBarrier

#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 132 of file SDL_atomic.h.

SDL_MemoryBarrierAcquire

#define SDL_MemoryBarrierAcquire

(

)

SDL_CompilerBarrier()

Definition at line 208 of file SDL_atomic.h.

SDL_MemoryBarrierRelease

#define SDL_MemoryBarrierRelease

(

)

SDL_CompilerBarrier()

Definition at line 207 of file SDL_atomic.h.

Typedef Documentation

SDL_SpinLock

typedef int SDL_SpinLock

Definition at line 89 of file SDL_atomic.h.

Function Documentation

SDL_AtomicAdd()

DECLSPEC int SDLCALL SDL_AtomicAdd	(	SDL_atomic_t *	a,
		int	v
	)

Add to an atomic variable.

Returns

The previous value of the atomic variable.

Note

This same style can be used for any number operation

SDL_AtomicCAS()

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.

Returns

SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.

Note

If you don't know what this function is for, you shouldn't use it!

SDL_AtomicCASPtr()

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.

Returns

SDL_TRUE if the pointer was set, SDL_FALSE otherwise.

Note

If you don't know what this function is for, you shouldn't use it!

SDL_AtomicGet()

DECLSPEC int SDLCALL SDL_AtomicGet

(

SDL_atomic_t *

a

)

Get the value of an atomic variable.

SDL_AtomicGetPtr()

DECLSPEC void* SDLCALL SDL_AtomicGetPtr

(

void **

a

)

Get the value of a pointer atomically.

SDL_AtomicLock()

DECLSPEC void SDLCALL SDL_AtomicLock

(

SDL_SpinLock *

lock

)

Lock a spin lock by setting it to a non-zero value.

Parameters

lock	Points to the lock.

SDL_AtomicSet()

DECLSPEC int SDLCALL SDL_AtomicSet	(	SDL_atomic_t *	a,
		int	v
	)

Set an atomic variable to a value.

Returns

The previous value of the atomic variable.

SDL_AtomicSetPtr()

DECLSPEC void* SDLCALL SDL_AtomicSetPtr	(	void **	a,
		void *	v
	)

Set a pointer to a value atomically.

Returns

The previous value of the pointer.

SDL_AtomicTryLock()

DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock

(

SDL_SpinLock *

lock

)

Try to lock a spin lock by setting it to a non-zero value.

Parameters

lock	Points to the lock.

Returns

SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.

SDL_AtomicUnlock()

DECLSPEC void SDLCALL SDL_AtomicUnlock

(

SDL_SpinLock *

lock

)

Unlock a spin lock by setting it to 0. Always returns immediately.

Parameters

lock	Points to the lock.

SDL_MemoryBarrierAcquireFunction()

DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction

(

void

)

SDL_MemoryBarrierReleaseFunction()

DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction

(

void

)

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