Atomic manipulation of multithreaded programming

In a multithreaded environment, access to shared variables can be implemented using the lock free technology based on compare and swap, the benefit of which is high efficiency.

An extract of atomic operation

1.1 Android

Source: system/core/libcutils/atomic.c (for X86):

1 #elifDefined (__i386__) | | Defined (__x86_64__)2 3 voidAndroid_atomic_write (int32_t value,volatileint32_t*addr) {4 int32_t OldValue;5      Do {6OldValue = *addr;7} while(Android_atomic_cmpxchg (oldValue, value, addr));8 }9 Tenint32_t Android_atomic_inc (volatileint32_t*addr) { One int32_t OldValue; A      Do { -OldValue = *addr; -} while(Android_atomic_cmpxchg (OldValue, oldvalue+1, addr)); the     returnOldValue; - } -  -int32_t Android_atomic_dec (volatileint32_t*addr) { + int32_t OldValue; -      Do { +OldValue = *addr; A} while(Android_atomic_cmpxchg (OldValue, oldvalue-1, addr)); at     returnOldValue; - } -  -int32_t Android_atomic_add (int32_t value,volatileint32_t*addr) { - int32_t OldValue; -      Do { inOldValue = *addr; -} while(Android_atomic_cmpxchg (OldValue, oldvalue+value, addr)); to     returnOldValue; + } -  the intAndroid_atomic_cmpxchg (int32_t oldvalue, int32_t newvalue,volatileint32_t*addr) { *     intXchg; $AsmvolatilePanax Notoginseng     ( -         "lock; Cmpxchg%%ecx, (%%edx);" the         "Setne%%al;" +         "Andl $,%%eax" A:"=a"(XCHG) the:"a"(OldValue),"C"(NewValue),"D"(addr) +     ); -     returnXchg; $}

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ANDROID_ATOMIC_CMPXCHG is implemented using the GNU C embedded assembler, using the X86 provided by the Atomic support for CAS directive. The oldvalue is placed in the EAX register, NewValue is placed in ecx, and addr (pointer) is placed in edx. The CMPXCHG instruction first compares the memory that addr points to oldvalue (EAX), and if they are equal, puts NewValue (ECX) in the memory pointed to by addr and sets the Z flag 1. The result of the operation of the Setne and ANDL instructions is simple: If the z flag is set, then the EAX is 0, otherwise 1. The final eax of the program execution is placed in the XCHG variable.

It can be seen that the self-increment and decrement operations are implemented based on ANDROID_ATOMIC_CMPXCHG, which successfully returns 1, and the failure returns 0, for example:

5 ; int B = Android_atomic_cmpxchg (A, A +1, &a);p rintf ("%d,%d\n"  , A, b);         // output when successful: 6,1 // output when failed: 5,0

1.2 IOS

Header files: #include <libkern/OSAtomic.h>

Operation	Function Name	Description
Add	OSAtomicAdd32 Osatomicadd32barrier OSAtomicAdd64 Osatomicadd64barrier	Adds together and stores the result in one of the specified variables.
Increment	OSAtomicIncrement32 Osatomicincrement32barrier OSAtomicIncrement64 Osatomicincrement64barrier	Increments the specified integer value by 1.
Decrement	OSAtomicDecrement32 Osatomicdecrement32barrier OSAtomicDecrement64 Osatomicdecrement64barrier	Decrements the specified integer value by 1.
Logical OR	OSAtomicOr32 Osatomicor32barrier	Performs a logical OR between the specified 32-bit value and a 32-bit mask.
Logical and	OSAtomicAnd32 Osatomicand32barrier	Performs a logical and between the specified 32-bit value and a 32-bit mask.
Logical XOR	OSAtomicXor32 Osatomicxor32barrier	Performs a logical XOR between the specified 32-bit value and a 32-bit mask.
Compare and swap	OSAtomicCompareAndSwap32 Osatomiccompareandswap32barrier OSATOMICCOMPAREANDSWAP64 Osatomiccompareandswap64barrier Osatomiccompareandswapptr Osatomiccompareandswapptrbarrier Osatomiccompareandswapint Osatomiccompareandswapintbarrier Osatomiccompareandswaplong Osatomiccompareandswaplongbarrier	Compares a variable against the Specified old value. If The values is equal, this function assigns the specified new value to the variable; Otherwise, it does nothing. The comparison and assignment is done as one atomic operation and the function returns a Boolean value indicating whether The swap actually occurred.
Test and set	Osatomictestandset Osatomictestandsetbarrier	Tests a bit I n the specified variable, sets that bit to 1, and returns the value of the old bit as a Boolean value. Bits is tested according to the formula (0x80 >> (N & 7)) of a Byte ((char*) address + (n >> 3)) where n is t The He bit number and address are a pointer to the variable. This formula effectively breaks up the variable to 8-bit sized chunks and orders the bits in each chunk in reverse. For example, to test the Lowest-order bit (bit 0) of a 32-bit integer, you would actually specify 7 for the bit number; Similarly, to test the highest order bit (bit), you would specify for the bit number.
Test and clear	Osatomictestandclear Osatomictestandclearbarrier	Tests a bit in the specified variable, sets this bit to 0, and returns the value of the old bit as a Boolean value. Bits is tested according to the formula (0x80 >> (N & 7)) of a Byte ((char*) address + (n >> 3)) where n is t The He bit number and address are a pointer to the variable. This formula effectively breaks up the variable to 8-bit sized chunks and orders the bits in each chunk in reverse. For example, to test the Lowest-order bit (bit 0) of a 32-bit integer, you would actually specify 7 for the bit number; Similarly, to test the highest order bit (bit), you would specify for the bit number.

These operations, may be concerned about the function of the return value, the return value in the programming of the description of the document is good, more convenient.  The official website document explains this: thearithmetic operations return the new value, after the operation have been performed. The Boolean operations come in the styles, one of the which returns the new value, and one of the which (the "Orig" versions) Retu  RNs 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 is in empty.

1.3 Windows

The operations on the Windows platform are generally similar, and the functions of the InterlockedIncrement, InterlockedDecrement series are known, and it is important to note that the two functions return the new value after the operation, not the original value before the operation.

Header	Windows.h
Library	Coredll.lib
Windows Embedded CE	Windows CE. NET 4.0 and later
Windows Mobile	Windows Mobile Version 5.0 and later

Atomic manipulation of multithreaded programming