compare_and_swap and compare_and_swaplp Subroutines

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Purpose

Conditionally updates or returns a variable atomically.

Library

Standard C library (libc.a)

Syntax

#include <sys/atomic_op.h>
boolean_t compare_and_swap ( addr, old_val_addr, new_val)
atomic_p addr;
int *old_val_addr;
int new_val;

boolean_t compare_and_swaplp ( addr, old_val_addr, new_val)
atomic_l addr;
long *old_val_addr;
long new_val;

Description

The compare_and_swap and compare_and_swaplp subroutines perform an atomic operation that compares the contents of a variable with a stored old value. If the values are equal, a new value is stored in the variable and TRUE is returned. If the values are not equal, the old value is set to the current value of the variable and FALSE is returned.

For 32-bit applications, the compare_and_swap and compare_and_swaplp subroutines are identical and operate on a word aligned single word (32-bit variable aligned on a 4-byte boundary).

For 64-bit applications, the compare_and_swap subroutine operates on a word aligned single word (32-bit variable aligned on a 4-byte boundary) and the compare_and_swaplp subroutine operates on a double word aligned double word (64-bit variable aligned on an 8-byte boundary).

The compare_and_swap and compare_and_swaplp subroutines are useful when a word value must be updated only if it has not been changed since it was last read.

Note: If the compare_and_swap or the compare_and_swaplp subroutine is used as a locking primitive, insert an isync at the start of any critical sections.

Parameters

Item Description
addr Specifies the address of the variable.
old_val_addr Specifies the address of the old value to be checked against (and conditionally updated with) the value of the variable.
new_val Specifies the new value to be conditionally assigned to the variable.

Return Values

Item Description
TRUE Indicates that the variable was equal to the old value, and has been set to the new value.
FALSE Indicates that the variable was not equal to the old value, and that its current value has been returned to the location where the old value was previously stored.