UCAS(9) 						   BSD Kernel Developer's Manual						   UCAS(9)

NAME

     ucas -- atomic memory operations on user-space address

SYNOPSIS

     #include <sys/systm.h>

     int
     ucas_ptr(volatile void *uptr, void *old, void *new, void *retp);

     int
     ucas_int(volatile int *uptr, int old, int new, int *retp);

DESCRIPTION

     These functions provide compare-and-swap (CAS) functionality on user-space address.

     Except that they can be safely used for the kernel to access user-space address, they are semantically equivalents of atomic_cas(3).

     uptr  The pointer to the variable.  This should be a user-space pointer.

     old   The value to compare with the variable.

     new   The value to store to the variable.

     retp  The pointer to the memory to store the old value of the variable.

RETURN VALUES

     On success, these functions return 0.  In that case, the caller can consult the value returned via retp to check the result of the CAS opera-
     tion.  Otherwise, these functions return an appropriate errno(9) error code, typically EFAULT.

SEE ALSO

     atomic_cas(3), intro(9)

BUGS

     Conceptually, the retp argument of ucas_ptr() would be of void **.  The current prototype is a compromise for usability.

BSD
								 October 24, 2011							       BSD