atomic_cas(3) - NetBSD Manual Pages

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ATOMIC_CAS(3)           NetBSD Library Functions Manual          ATOMIC_CAS(3)


NAME

     atomic_cas, atomic_cas_32, atomic_cas_uint, atomic_cas_ulong,
     atomic_cas_ptr, atomic_cas_64, atomic_cas_32_ni, atomic_cas_uint_ni,
     atomic_cas_ulong_ni, atomic_cas_ptr_ni, atomic_cas_64_ni -- atomic com-
     pare-and-swap operations


SYNOPSIS

     #include <sys/atomic.h>

     uint32_t
     atomic_cas_32(volatile uint32_t *ptr, uint32_t expected, uint32_t new);

     unsigned int
     atomic_cas_uint(volatile unsigned int *ptr, unsigned int expected,
         unsigned int new);

     unsigned long
     atomic_cas_ulong(volatile unsigned long *ptr, unsigned long expected,
         unsigned long new);

     void *
     atomic_cas_ptr(volatile void *ptr, void *expected, void *new);

     uint64_t
     atomic_cas_64(volatile uint64_t *ptr, uint64_t expected, uint64_t new);

     uint32_t
     atomic_cas_32_ni(volatile uint32_t *ptr, uint32_t expected,
         uint32_t new);

     unsigned int
     atomic_cas_uint_ni(volatile unsigned int *ptr, unsigned int expected,
         unsigned int new);

     unsigned long
     atomic_cas_ulong_ni(volatile unsigned long *ptr, unsigned long expected,
         unsigned long new);

     void *
     atomic_cas_ptr_ni(volatile void *ptr, void *expected, void *new);

     uint64_t
     atomic_cas_64_ni(volatile uint64_t *ptr, uint64_t expected,
         uint64_t new);


DESCRIPTION

     The atomic_cas family of functions perform an atomic conditional assign-
     ment.  The value new is assigned to the variable referenced by ptr.  The
     assignment succeeds if and only if its current value matches the value
     expected.  If the value is different, the assignment fails and no change
     is made.  This operation is sometimes known as ``compare-and-swap''.
     These functions always return the value found via ptr.  Callers test for
     success by comparing the return value to the value passed as expected; if
     they are equal then the new value was stored; if they are not, the value
     was not changed.

     The non-interlocked variants, *_ni(), guarantee atomicity within the same
     CPU with respect to interrupts and preemption.  They are not atomic with
     respect to different CPUs.  These can be used to avoid interprocessor
     synchronization overhead in some cases; for example, they are suitable
     for synchronized operations on a variable shared by a thread and an
     interrupt that are bound to the same CPU.

     The 64-bit variants of these functions are available only on platforms
     that can support atomic 64-bit memory access.  Applications can check for
     the availability of 64-bit atomic memory operations by testing if the
     pre-processor macro __HAVE_ATOMIC64_OPS is defined.


SEE ALSO

     atomic_ops(3)


HISTORY

     The atomic_cas functions first appeared in NetBSD 5.0.


NOTES

     On some architectures, a *_ni() variant is merely an alias for the corre-
     sponding standard compare-and-swap operation.  While the non-interlocked
     variant behaves correctly on those architectures, it does not avoid the
     interprocessor synchronization overhead.

NetBSD 8.3                     February 2, 2014                     NetBSD 8.3