Conditionally updates or returns a variable atomically.
#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;
| 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. |
The compare_and_swap kernel services performs an atomic (uninterruptible) operation which compares the contents of a variable with a stored old value; if equal, a new value is stored in the variable, and TRUE is returned, otherwise the old value is set to the current value of the variable, and FALSE is returned.
The compare_and_swap kernel service operates on a single word (32 bit) variable while the compare_and_swaplp kernel service operates on a double word (64 bit) variable.
The compare_and_swap kernel services are particularly useful in operations on singly linked lists, where a list pointer must not be updated if it has been changed by another thread since it was read.
The compare_and_swap kernel services can be called from either the process or interrupt environment.
| 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 in the location where the old value was stored. |