__compare_and_swap, __builtin_ppc_compare_and_swap, __compare_and_swaplp, __builtin_ppc_compare_and_swaplp

Purpose

Conditionally updates a single word or doubleword variable atomically.

Prototype

int __builtin_ppc_compare_and_swap (volatile int* addr, int* old_val_addr, int new_val);

int __builtin_ppc_compare_and_swaplp (volatile long* addr, long* old_val_addr, long new_val);

int __compare_and_swap (volatile int* addr, int* old_val_addr, int new_val);

int __compare_and_swaplp (volatile long* addr, long* old_val_addr, long new_val);

Note:

The built-in function in the form of __name is a synonym of the built-in function in the form of __builtin_ppc_name.
The built-in function in the form of __name is provided for compatibility with IBM® XL C/C++ for AIX® 16.1.0 or earlier releases. This built-in function form might be deprecated in the future.

Parameters

addr: The address of the variable to be copied. Must be aligned on a 4-byte boundary for a single word and on an 8-byte boundary for a doubleword.
old_val_addr: The memory location into which the value in addr is to be copied.
new_val: The value to be conditionally assigned to the variable in addr,

Return value

Returns true (1) if the value in addr was equal to old_value and has been set to the new value. Returns false (0) if the value in addr was not equal to old_value and has been left unchanged. In either case, the contents of the memory location specified by addr are copied into the memory location specified by old_val_addr.

Usage

The __compare_and_swap function is useful when a single word value must be updated only if it has not been changed since it was last read. If you use __compare_and_swap as a locking primitive, insert a call to the __isync built-in function at the start of any critical sections.

__compare_and_swaplp is valid only in 64-bit mode.

Note:

This function generates the hardware instruction but does not act as an instruction movement barrier within the compiler. If that is needed, you must also use the __fence function.