CASU(9)                FreeBSD Kernel Developer's Manual               CASU(9)

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)

FreeBSD 13.1-RELEASE-p6         April 19, 2019         FreeBSD 13.1-RELEASE-p6