Android atomic operations (x86)

 

• In android, cmpxchg performs atomic operations, such as auto-addition and auto-subtraction.
• Cmpxchg exists in 486 and more advanced CPUs. Other types of CPUs, such as Arm, do not have this command.

Cmpxchg commands are encapsulated in Functionsandroid_atomic_cmpxchgThe function is located in atomic. c and is preprocessed.

#elif defined(__i386__) || defined(__x86_64__)

Protected.

1. source code analysis:

 

The reference link of the annotation is:

• Gnu asm template syntax
• Lock
• Cmpxchg instruction
• Setne

Therefore, the function is used:If oldvalue = * addr, The newvalue is assigned to * addr, and 1 is returned. Otherwise, 0 is returned..

(1 indicates that the new value has been assigned to the variable/register, and 0 indicates that no value is assigned)

Example 2: 2.1 oldval = newval

int32_t a = 5;int b = android_atomic_cmpxchg( a, a+1, &a );printf("%d, %d\n", a, b);

The result is:

6, 1

2.2 oldval! = Newval

int32_t a = 5;int b= android_atomic_cmpxchg( a, a, &a );printf("%d, %d\n", a, b);

The result is:

5, 0

3 Reference

Other functions suchandroid_atomic_inc,android_atomic_decAnd so on.android_atomic_cmpxchgTo implement these functions.atomic.cMedium