Conditionally updates or returns a single word variable atomically.
Standard C library (libc.a)
boolean_t compare_and_swap ( word_addr, old_val_addr, new_val)
The compare_and_swap subroutine performs an atomic operation which compares the contents of a single word variable with a stored old value. If the values are equal, a new value is stored in the single word variable and TRUE is returned; otherwise, the old value is set to the current value of the single word variable and FALSE is returned.
The compare_and_swap subroutine is useful when a word value must be updated only if it has not been changed since it was last read.
Note: The word containing the single word variable must be aligned on a full word boundary.
Note: If compare_and_swap is used as a locking primitive, insert an isync at the start of any critical sections.
|word_addr||Specifies the address of the single word variable.|
|old_val_addr||Specifies the address of the old value to be checked against (and conditionally updated with) the value of the single word variable.|
|new_val||Specifies the new value to be conditionally assigned to the single word variable.|
|TRUE||Indicates that the single word variable was equal to the old value, and has been set to the new value.|
|FALSE||Indicates that the single word variable was not equal to the old value, and that its current value has been returned in the location where the old value was previously stored.|
The compare_and_swap subroutine is part of the Base Operating System (BOS) Runtime.
The fetch_and_add (fetch_and_add Subroutine) subroutine, fetch_and_and (fetch_and_and or fetch_and_or Subroutine) subroutine, fetch_and_or (fetch_and_and or fetch_and_or Subroutine) subroutine.