SDL 2.0
SDL_atomic.h File Reference
#include "SDL_stdinc.h"
#include "SDL_platform.h"
#include "begin_code.h"
#include "close_code.h"
+ Include dependency graph for SDL_atomic.h:
+ This graph shows which files directly or indirectly include this file:

Go to the source code of this file.

Data Structures

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...
 

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.

#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...
 
typedef int SDL_SpinLock
 
SDL_bool SDL_AtomicTryLock (SDL_SpinLock *lock)
 Try to lock a spin lock by setting it to a non-zero value. More...
 
void SDL_AtomicLock (SDL_SpinLock *lock)
 Lock a spin lock by setting it to a non-zero value. More...
 
void SDL_AtomicUnlock (SDL_SpinLock *lock)
 Unlock a spin lock by setting it to 0. Always returns immediately. More...
 
void SDL_MemoryBarrierReleaseFunction (void)
 
void SDL_MemoryBarrierAcquireFunction (void)
 
SDL_bool 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...
 
int SDL_AtomicSet (SDL_atomic_t *a, int v)
 Set an atomic variable to a value. More...
 
int SDL_AtomicGet (SDL_atomic_t *a)
 Get the value of an atomic variable. More...
 
int SDL_AtomicAdd (SDL_atomic_t *a, int v)
 Add to an atomic variable. More...
 
SDL_bool SDL_AtomicCASPtr (void **a, void *oldval, void *newval)
 Set a pointer to a new value if it is currently an old value. More...
 
voidSDL_AtomicSetPtr (void **a, void *v)
 Set a pointer to a value atomically. More...
 
voidSDL_AtomicGetPtr (void **a)
 Get the value of a pointer atomically. 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()

int 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

Definition at line 238 of file SDL_atomic.c.

239{
240#ifdef HAVE_MSC_ATOMICS
241 return _InterlockedExchangeAdd((long*)&a->value, v);
242#elif defined(HAVE_WATCOM_ATOMICS)
243 return _SDL_xadd_watcom(&a->value, v);
244#elif defined(HAVE_GCC_ATOMICS)
245 return __sync_fetch_and_add(&a->value, v);
246#elif defined(__SOLARIS__)
247 int pv = a->value;
248 membar_consumer();
249#if defined(_LP64)
250 atomic_add_64((volatile uint64_t*)&a->value, v);
251#elif !defined(_LP64)
252 atomic_add_32((volatile uint32_t*)&a->value, v);
253#endif
254 return pv;
255#else
256 int value;
257 do {
258 value = a->value;
259 } while (!SDL_AtomicCAS(a, value, (value + v)));
260 return value;
261#endif
262}
SDL_bool 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.
Definition: SDL_atomic.c:130
unsigned int uint32_t
unsigned long long uint64_t
const GLdouble * v
Definition: SDL_opengl.h:2064
GLboolean GLboolean GLboolean GLboolean a
GLsizei const GLfloat * value

References SDL_AtomicCAS().

◆ SDL_AtomicCAS()

SDL_bool 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!

Definition at line 130 of file SDL_atomic.c.

131{
132#ifdef HAVE_MSC_ATOMICS
133 return (_InterlockedCompareExchange((long*)&a->value, (long)newval, (long)oldval) == (long)oldval);
134#elif defined(HAVE_WATCOM_ATOMICS)
135 return (SDL_bool) _SDL_cmpxchg_watcom(&a->value, newval, oldval);
136#elif defined(HAVE_GCC_ATOMICS)
137 return (SDL_bool) __sync_bool_compare_and_swap(&a->value, oldval, newval);
138#elif defined(__MACOSX__) /* this is deprecated in 10.12 sdk; favor gcc atomics. */
139 return (SDL_bool) OSAtomicCompareAndSwap32Barrier(oldval, newval, &a->value);
140#elif defined(__SOLARIS__) && defined(_LP64)
141 return (SDL_bool) ((int) atomic_cas_64((volatile uint64_t*)&a->value, (uint64_t)oldval, (uint64_t)newval) == oldval);
142#elif defined(__SOLARIS__) && !defined(_LP64)
143 return (SDL_bool) ((int) atomic_cas_32((volatile uint32_t*)&a->value, (uint32_t)oldval, (uint32_t)newval) == oldval);
144#elif EMULATE_CAS
146
147 enterLock(a);
148 if (a->value == oldval) {
149 a->value = newval;
151 }
152 leaveLock(a);
153
154 return retval;
155#else
156 #error Please define your platform.
157#endif
158}
static SDL_INLINE void leaveLock(void *a)
Definition: SDL_atomic.c:120
static SDL_INLINE void enterLock(void *a)
Definition: SDL_atomic.c:112
SDL_bool
Definition: SDL_stdinc.h:162
@ SDL_TRUE
Definition: SDL_stdinc.h:164
@ SDL_FALSE
Definition: SDL_stdinc.h:163
SDL_bool retval

References enterLock(), leaveLock(), retval, SDL_FALSE, and SDL_TRUE.

Referenced by SDL_AtomicAdd(), SDL_AtomicGet(), and SDL_AtomicSet().

◆ SDL_AtomicCASPtr()

SDL_bool 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!

Definition at line 161 of file SDL_atomic.c.

162{
163#if defined(HAVE_MSC_ATOMICS) && (_M_IX86)
164 return (_InterlockedCompareExchange((long*)a, (long)newval, (long)oldval) == (long)oldval);
165#elif defined(HAVE_MSC_ATOMICS) && (!_M_IX86)
166 return (_InterlockedCompareExchangePointer(a, newval, oldval) == oldval);
167#elif defined(HAVE_WATCOM_ATOMICS)
168 return (SDL_bool) _SDL_cmpxchg_watcom((int *)a, (long)newval, (long)oldval);
169#elif defined(HAVE_GCC_ATOMICS)
170 return __sync_bool_compare_and_swap(a, oldval, newval);
171#elif defined(__MACOSX__) && defined(__LP64__) /* this is deprecated in 10.12 sdk; favor gcc atomics. */
172 return (SDL_bool) OSAtomicCompareAndSwap64Barrier((int64_t)oldval, (int64_t)newval, (int64_t*) a);
173#elif defined(__MACOSX__) && !defined(__LP64__) /* this is deprecated in 10.12 sdk; favor gcc atomics. */
174 return (SDL_bool) OSAtomicCompareAndSwap32Barrier((int32_t)oldval, (int32_t)newval, (int32_t*) a);
175#elif defined(__SOLARIS__)
176 return (SDL_bool) (atomic_cas_ptr(a, oldval, newval) == oldval);
177#elif EMULATE_CAS
179
180 enterLock(a);
181 if (*a == oldval) {
182 *a = newval;
184 }
185 leaveLock(a);
186
187 return retval;
188#else
189 #error Please define your platform.
190#endif
191}
signed int int32_t
signed long long int64_t

References enterLock(), leaveLock(), retval, SDL_FALSE, and SDL_TRUE.

Referenced by SDL_AtomicGetPtr(), and SDL_AtomicSetPtr().

◆ SDL_AtomicGet()

int SDL_AtomicGet ( SDL_atomic_t a)

Get the value of an atomic variable.

Definition at line 265 of file SDL_atomic.c.

266{
267#ifdef HAVE_ATOMIC_LOAD_N
268 return __atomic_load_n(&a->value, __ATOMIC_SEQ_CST);
269#else
270 int value;
271 do {
272 value = a->value;
273 } while (!SDL_AtomicCAS(a, value, value));
274 return value;
275#endif
276}

References SDL_AtomicCAS().

◆ SDL_AtomicGetPtr()

void * SDL_AtomicGetPtr ( void **  a)

Get the value of a pointer atomically.

Definition at line 279 of file SDL_atomic.c.

280{
281#ifdef HAVE_ATOMIC_LOAD_N
282 return __atomic_load_n(a, __ATOMIC_SEQ_CST);
283#else
284 void *value;
285 do {
286 value = *a;
287 } while (!SDL_AtomicCASPtr(a, value, value));
288 return value;
289#endif
290}
SDL_bool SDL_AtomicCASPtr(void **a, void *oldval, void *newval)
Set a pointer to a new value if it is currently an old value.
Definition: SDL_atomic.c:161

References SDL_AtomicCASPtr().

◆ SDL_AtomicLock()

void SDL_AtomicLock ( SDL_SpinLock lock)

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

Parameters
lockPoints to the lock.

Definition at line 137 of file SDL_spinlock.c.

138{
139 int iterations = 0;
140 /* FIXME: Should we have an eventual timeout? */
141 while (!SDL_AtomicTryLock(lock)) {
142 if (iterations < 32) {
143 iterations++;
145 } else {
146 /* !!! FIXME: this doesn't definitely give up the current timeslice, it does different things on various platforms. */
147 SDL_Delay(0);
148 }
149 }
150}
#define SDL_Delay
SDL_mutex * lock
Definition: SDL_events.c:76
SDL_bool SDL_AtomicTryLock(SDL_SpinLock *lock)
Try to lock a spin lock by setting it to a non-zero value.
Definition: SDL_spinlock.c:51
#define PAUSE_INSTRUCTION()
Definition: SDL_spinlock.c:133
static int iterations
Definition: testsprite2.c:45

References iterations, lock, PAUSE_INSTRUCTION, SDL_AtomicTryLock(), and SDL_Delay.

◆ SDL_AtomicSet()

int SDL_AtomicSet ( SDL_atomic_t a,
int  v 
)

Set an atomic variable to a value.

Returns
The previous value of the atomic variable.

Definition at line 194 of file SDL_atomic.c.

195{
196#ifdef HAVE_MSC_ATOMICS
197 return _InterlockedExchange((long*)&a->value, v);
198#elif defined(HAVE_WATCOM_ATOMICS)
199 return _SDL_xchg_watcom(&a->value, v);
200#elif defined(HAVE_GCC_ATOMICS)
201 return __sync_lock_test_and_set(&a->value, v);
202#elif defined(__SOLARIS__) && defined(_LP64)
203 return (int) atomic_swap_64((volatile uint64_t*)&a->value, (uint64_t)v);
204#elif defined(__SOLARIS__) && !defined(_LP64)
205 return (int) atomic_swap_32((volatile uint32_t*)&a->value, (uint32_t)v);
206#else
207 int value;
208 do {
209 value = a->value;
210 } while (!SDL_AtomicCAS(a, value, v));
211 return value;
212#endif
213}

References SDL_AtomicCAS().

◆ SDL_AtomicSetPtr()

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

Set a pointer to a value atomically.

Returns
The previous value of the pointer.

Definition at line 216 of file SDL_atomic.c.

217{
218#if defined(HAVE_MSC_ATOMICS) && (_M_IX86)
219 return (void *) _InterlockedExchange((long *)a, (long) v);
220#elif defined(HAVE_MSC_ATOMICS) && (!_M_IX86)
221 return _InterlockedExchangePointer(a, v);
222#elif defined(HAVE_WATCOM_ATOMICS)
223 return (void *) _SDL_xchg_watcom((int *)a, (long)v);
224#elif defined(HAVE_GCC_ATOMICS)
225 return __sync_lock_test_and_set(a, v);
226#elif defined(__SOLARIS__)
227 return atomic_swap_ptr(a, v);
228#else
229 void *value;
230 do {
231 value = *a;
232 } while (!SDL_AtomicCASPtr(a, value, v));
233 return value;
234#endif
235}

References SDL_AtomicCASPtr().

◆ SDL_AtomicTryLock()

SDL_bool SDL_AtomicTryLock ( SDL_SpinLock lock)

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

Parameters
lockPoints to the lock.
Returns
SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.

Definition at line 51 of file SDL_spinlock.c.

52{
53#if SDL_ATOMIC_DISABLED
54 /* Terrible terrible damage */
55 static SDL_mutex *_spinlock_mutex;
56
57 if (!_spinlock_mutex) {
58 /* Race condition on first lock... */
59 _spinlock_mutex = SDL_CreateMutex();
60 }
61 SDL_LockMutex(_spinlock_mutex);
62 if (*lock == 0) {
63 *lock = 1;
64 SDL_UnlockMutex(_spinlock_mutex);
65 return SDL_TRUE;
66 } else {
67 SDL_UnlockMutex(_spinlock_mutex);
68 return SDL_FALSE;
69 }
70
71#elif defined(_MSC_VER)
72 SDL_COMPILE_TIME_ASSERT(locksize, sizeof(*lock) == sizeof(long));
73 return (InterlockedExchange((long*)lock, 1) == 0);
74
75#elif defined(__WATCOMC__) && defined(__386__)
76 return _SDL_xchg_watcom(lock, 1) == 0;
77
78#elif HAVE_GCC_ATOMICS || HAVE_GCC_SYNC_LOCK_TEST_AND_SET
79 return (__sync_lock_test_and_set(lock, 1) == 0);
80
81#elif defined(__GNUC__) && defined(__arm__) && \
82 (defined(__ARM_ARCH_4__) || defined(__ARM_ARCH_4T__) || \
83 defined(__ARM_ARCH_5__) || defined(__ARM_ARCH_5TE__) || \
84 defined(__ARM_ARCH_5TEJ__))
85 int result;
86 __asm__ __volatile__ (
87 "swp %0, %1, [%2]\n"
88 : "=&r,&r" (result) : "r,0" (1), "r,r" (lock) : "memory");
89 return (result == 0);
90
91#elif defined(__GNUC__) && defined(__arm__)
92 int result;
93 __asm__ __volatile__ (
94 "ldrex %0, [%2]\nteq %0, #0\nstrexeq %0, %1, [%2]"
95 : "=&r" (result) : "r" (1), "r" (lock) : "cc", "memory");
96 return (result == 0);
97
98#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
99 int result;
100 __asm__ __volatile__(
101 "lock ; xchgl %0, (%1)\n"
102 : "=r" (result) : "r" (lock), "0" (1) : "cc", "memory");
103 return (result == 0);
104
105#elif defined(__MACOSX__) || defined(__IPHONEOS__)
106 /* Maybe used for PowerPC, but the Intel asm or gcc atomics are favored. */
107 return OSAtomicCompareAndSwap32Barrier(0, 1, lock);
108
109#elif defined(__SOLARIS__) && defined(_LP64)
110 /* Used for Solaris with non-gcc compilers. */
111 return (SDL_bool) ((int) atomic_cas_64((volatile uint64_t*)lock, 0, 1) == 0);
112
113#elif defined(__SOLARIS__) && !defined(_LP64)
114 /* Used for Solaris with non-gcc compilers. */
115 return (SDL_bool) ((int) atomic_cas_32((volatile uint32_t*)lock, 0, 1) == 0);
116
117#else
118#error Please implement for your platform.
119 return SDL_FALSE;
120#endif
121}
#define SDL_LockMutex
#define SDL_CreateMutex
#define SDL_UnlockMutex
GLuint64EXT * result
#define SDL_COMPILE_TIME_ASSERT(name, x)
Definition: SDL_stdinc.h:312

References lock, SDL_COMPILE_TIME_ASSERT, SDL_CreateMutex, SDL_FALSE, SDL_LockMutex, SDL_TRUE, and SDL_UnlockMutex.

Referenced by SDL_AtomicLock().

◆ SDL_AtomicUnlock()

void SDL_AtomicUnlock ( SDL_SpinLock lock)

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

Parameters
lockPoints to the lock.

Definition at line 153 of file SDL_spinlock.c.

154{
155#if defined(_MSC_VER)
156 _ReadWriteBarrier();
157 *lock = 0;
158
159#elif defined(__WATCOMC__) && defined(__386__)
161 *lock = 0;
162
163#elif HAVE_GCC_ATOMICS || HAVE_GCC_SYNC_LOCK_TEST_AND_SET
164 __sync_lock_release(lock);
165
166#elif defined(__SOLARIS__)
167 /* Used for Solaris when not using gcc. */
168 *lock = 0;
169 membar_producer();
170
171#else
172 *lock = 0;
173#endif
174}
#define SDL_CompilerBarrier()
Definition: SDL_atomic.h:132

References lock, and SDL_CompilerBarrier.

◆ SDL_MemoryBarrierAcquireFunction()

void SDL_MemoryBarrierAcquireFunction ( void  )

Definition at line 303 of file SDL_atomic.c.

304{
306}
#define SDL_MemoryBarrierAcquire()
Definition: SDL_atomic.h:208

References SDL_MemoryBarrierAcquire.

◆ SDL_MemoryBarrierReleaseFunction()

void 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

Definition at line 658 of file SDL_dynapi_procs.h.

References SDL_MemoryBarrierRelease.