您的位置：首页 > 运维架构 > Linux

linux实现原子操作的内建函数

2012-03-29 11:57 387 查看
http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html

Next:
Object Size Checking, Previous:
Offsetof, Up:
C Extensions

5.44 Built-in functions for atomic memory access

The following builtins are intended to be compatible with those described in the
Intel Itanium Processor-specific Application Binary Interface, section 7.4. As such, they depart from the normal GCC practice of using the “__builtin_” prefix, and further that they are overloaded such that they work on multiple types.

The definition given in the Intel documentation allows only for the use of the types
int
, 
long
, 
long long
as well as their unsigned counterparts. GCC will allow any integral scalar or pointer type that is 1, 2, 4 or 8 bytes in length.

Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning will be generated and a call an external function will be generated. The external function will carry the same name
as the builtin, with an additional suffix `_n' where
n is the size of the data type.

In most cases, these builtins are considered a full barrier. That is, no memory operand will be moved across the operation, either forward or backward. Further, instructions will be issued as necessary to prevent the processor from speculating
loads across the operation and from queuing stores after the operation.

All of the routines are are described in the Intel documentation to take “an optional list of variables protected by the memory barrier”. It's not clear what is meant by that; it could mean that
only the following variables are protected, or it could mean that these variables should in addition be protected. At present GCC ignores this list and protects all variables which are globally accessible. If in the future we make some use of this
list, an empty list will continue to mean all globally accessible variables.

type
__sync_fetch_and_add (
type
*ptr,
type
value, ...)
type
__sync_fetch_and_sub (
type
*ptr,
type
value, ...)
type
__sync_fetch_and_or (
type
*ptr,
type
value, ...)
type
__sync_fetch_and_and (
type
*ptr,
type
value, ...)
type
__sync_fetch_and_xor (
type
*ptr,
type
value, ...)
type
__sync_fetch_and_nand (
type
*ptr,
type
value, ...)
These
builtins perform the operation suggested by the name, and returns the value that had previously been in memory. That is,
{ tmp = *ptr; *ptr op= value; return tmp; }
{ tmp = *ptr; *ptr = ~tmp & value; return tmp; }   // nand


type
__sync_add_and_fetch (
type
*ptr,
type
value, ...)
type
__sync_sub_and_fetch (
type
*ptr,
type
value, ...)
type
__sync_or_and_fetch (
type
*ptr,
type
value, ...)
type
__sync_and_and_fetch (
type
*ptr,
type
value, ...)
type
__sync_xor_and_fetch (
type
*ptr,
type
value, ...)
type
__sync_nand_and_fetch (
type
*ptr,
type
value, ...)
These
builtins perform the operation suggested by the name, and return the new value. That is,
{ *ptr op= value; return *ptr; }
{ *ptr = ~*ptr & value; return *ptr; }   // nand


bool __sync_bool_compare_and_swap (
type
*ptr,

type
oldval
type
newval, ...)
type
__sync_val_compare_and_swap (
type
*ptr,
type
oldval
type
newval, ...)

These builtins perform an atomic compare and swap. That is, if the current value of
*
ptr is oldval, then write newval into
*
ptr.
The “bool” version returns true if the comparison is successful and newval was written. The “val” version returns the contents of
*
ptr before the operation.

__sync_synchronize (...)
This builtin issues a full memory barrier.

type
__sync_lock_test_and_set (
type
*ptr,
type
value, ...)
This builtin, as described by Intel, is not a traditional test-and-set operation, but rather an atomic exchange operation. It writes
value into 
*
ptr, and returns the previous contents of
*
ptr.
Many targets have only minimal support for such locks, and do not support a full exchange operation. In this case, a target may support reduced functionality here by which the
only valid value to store is the immediate constant 1. The exact value actually stored in
*
ptr is implementation defined.

This builtin is not a full barrier, but rather an acquire barrier. This means that references after the builtin cannot move to (or be speculated to) before the builtin, but previous memory stores may not be globally visible yet, and previous memory
loads may not yet be satisfied.

void __sync_lock_release (
type
*ptr,...)

This builtin releases the lock acquired by
__sync_lock_test_and_set
. Normally this means writing the constant 0 to
*
ptr.
This builtin is not a full barrier, but rather a release barrier. This means that all previous memory stores are globally visible, and all previous memory loads have been satisfied, but following memory reads are not prevented from being speculated
to before the barrier.

 
内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理 点击这里给我发消息
标签：  linux variables documentation exchange list function
相关文章推荐
新的分享
章节导航