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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 con‐
     sult 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

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