Page 1
SYNC(3C) SYNC(3C)
NAME
sync: __synchronize, __fetch_and_add, __fetch_and_sub, __fetch_and_or,
__fetch_and_and, __fetch_and_xor, __fetch_and_nand, __add_and_fetch,
__sub_and_fetch, __or_and_fetch, __and_and_fetch, __xor_and_fetch,
__nand_and_fetch, __lock_test_and_set, __lock_release- C synchronization
primitives for multiprocessing
SYNOPSIS
See summaries below
DESCRIPTION
The intrinsics described here provide a variety of primitive
synchronization operations. Besides performing the particular
synchronization operation, each of these intrinsics has two key
properties:
The function performed is guaranteed to be atomic (typically achieved
by implementing the operation using a sequence of load-linked/store-
conditional instructions in a loop).
Associated with each instrinsic are certain memory barrier properties
that restrict the movement of memory references to visible data
across the intrinsic operation (by either the compiler or the
processor).
A visible memory reference is a reference to a data object
potentially accessible by another thread executing in the same shared
address space. A visible data object may be one of the following:
C/C++ global data
Fortran COMMON data
data declared extern
volatile data
static data (either file-scope or function-scope)
data accessible via function parameters
automatic data (local-scope) that has had its address taken and
assigned to some object which is visible (recursively).
The memory barrier semantics of an intrinsic may be one of the
following three types:
acquire barrier
Disallows the movement of memory references to visible data from
Page 2
SYNC(3C) SYNC(3C)
after the intrinsic (in program order) to before the intrinsic
(this behavior is desirable at lock-acquire operations, hence the
name).
release barrier
Disallows the movement of memory references to visible data from
before the intrinsic (in program order) to after the intrinsic
(this behavior is desirable at lock-release operations, hence the
name).
full barrier
disallows the movement of memory references to visible data past
the intrinsic (in either direction), and is thus both an acquire
and a release barrier. A barrier only restricts the movement of
memory references to visible data across the intrinsic operation:
between synchronization operations (or in their absence), memory
references to visible data may be freely reordered subject to the
usual data-dependence constraints.
Caution: Conditional execution of a synchronization intrinsic (such as
within a if or a while statement) does not prevent the movement of memory
references to visible data past the overall if or while construct.
Atomic fetch-and-op Operations
type __fetch_and_add (type* ptr, type value, ...)
type __fetch_and_sub (type* ptr, type value, ...)
type __fetch_and_or (type* ptr, type value, ...)
type __fetch_and_and (type* ptr, type value, ...)
type __fetch_and_xor (type* ptr, type value, ...)
type __fetch_and_nand(type* ptr, type value, ...)
Where type may be one of int, long, long long, unsigned int, unsigned
long, or unsigned long long.
The ellipses (...) refers to an optional list of variables protected
by the memory barrier.
Behavior:
Atomically performs the specified operation with the given value
on *ptr, and returns the old value of *ptr. (i.e.)
{ tmp = *ptr; *ptr <op>= value; return tmp; }
Page 3
SYNC(3C) SYNC(3C)
Full barrier.
Atomic op-and-fetch Operations
type __add_and_fetch (type* ptr, type value, ...)
type __sub_and_fetch (type* ptr, type value, ...)
type __or_and_fetch (type* ptr, type value, ...)
type __and_and_fetch (type* ptr, type value, ...)
type __xor_and_fetch (type* ptr, type value, ...)
type __nand_and_fetch(type* ptr, type value, ...)
Where type may be one of int, long, long long, unsigned int, unsigned
long, or unsigned long long. The ellipses (...) refers to an optional
list of variables protected by the memory barrier.
Behavior:
Atomically performs the specified operation with the given value
on *ptr, and returns the new value of *ptr. (i.e.)
{ *ptr <op>= value; return *ptr; }
Full barrier.
Atomic compare-and-swap Operation
int __compare_and_swap (type* ptr, type oldvalue, type newvalue, ...)
Where type may be one of int, long, long long, unsigned int, unsigned
long, or unsigned long long. The ellipses (...) refers to an optional
list of variables protected by the memory barrier.
Behavior:
Atomically do the following: compare *ptr to oldvalue. If equal,
store the new value and return 1, otherwise return 0. (i.e.)
if (*ptr != oldvalue) return 0;
else {
*ptr = newvalue;
return 1; }
Page 4
SYNC(3C) SYNC(3C)
Full barrier.
Atomic synchronize Operation
__synchronize (...)
The ellipses (...) refers to an optional list of variables protected by
the memory barrier.
Behavior:
Full barrier
Atomic lock-test-and-set Operation
type __lock_test_and_set (type* ptr, type value, ...)
Where type may be one of int, long, long long, unsigned int, unsigned
long, or unsigned long long. The ellipses (...) refers to an optional
list of variables protected by the memory barrier.
Behavior
Atomically store the supplied value in *ptr and return the old
value of *ptr. (i.e.)
{ tmp = *ptr; *ptr = value; return tmp; }
Acquire barrier
Atomic lock_release Operation
void __lock_release (type* ptr, ...)
Where type may be one of int, long, long long, unsigned int, unsigned
long, or unsigned long long. The ellipses (...) refers to an optional
list of variables protected by the memory barrier.
Behavior:
Set *ptr to 0. (i.e.) { *ptr = 0 }
Release barrier
Page 5