NAME

     casueword, casueword32, casuword, casuword32 — fetch, compare and store data from user-space

SYNOPSIS

     #include <sys/types.h>
     #include <sys/systm.h>

     int
     casueword(volatile u_long *base, u_long oldval, u_long *oldvalp, u_long newval);

     int
     casueword32(volatile uint32_t *base, uint32_t oldval, uint32_t *oldvalp, uint32_t newval);

     u_long
     casuword(volatile u_long *base, u_long oldval, u_long newval);

     uint32_t
     casuword32(volatile uint32_t *base, uint32_t oldval, uint32_t newval);

DESCRIPTION

     The casueword functions are designed to perform atomic compare-and-swap operation on the
     value in the usermode memory of the current process.

     The casueword routines reads the value from user memory with address base, and compare the
     value read with oldval.  If the values are equal, newval is written to the *base.  In case
     of casueword32() and casueword(), old value is stored into the (kernel-mode) variable
     pointed by *oldvalp.  The userspace value must be naturally aligned.

     The callers of casuword() and casuword32() functions cannot distinguish between -1 read from
     userspace and function failure.

RETURN VALUES

     The casuword() and casuword32() functions return the data fetched or -1 on failure.  The
     casueword() and casueword32() functions return 0 on success, -1 on failure to access memory,
     and 1 when comparison or store failed.  The store can fail on load-linked/store-conditional
     architectures.

SEE ALSO

     atomic(9), fetch(9), store(9)