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2.6.4 atomic Construct

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The atomic directive ensures that a specific memory location is updated atomically, rather than exposing it to the possibility of multiple, simultaneous writing threads. The syntax of the atomic directive is as follows:

#pragma omp atomic new-lineexpression-stmt

The expression statement must have one of the following forms:

x binop= expr

x++

++x

x--

--x

In the preceding expressions:

  • x is an lvalue expression with scalar type.

  • expr is an expression with scalar type, and it does not reference the object designated by x.

  • binop is not an overloaded operator and is one of +, *, -, /, &, ^, |, <<, or >>.

Although it is implementation-defined whether an implementation replaces all atomic directives with critical directives that have the same unique name, the atomic directive permits better optimization. Often hardware instructions are available that can perform the atomic update with the least overhead.

Only the load and store of the object designated by x are atomic; the evaluation of expr is not atomic. To avoid race conditions, all updates of the location in parallel should be protected with the atomic directive, except those that are known to be free of race conditions.

Restrictions to the atomic directive are as follows:

  • All atomic references to the storage location x throughout the program are required to have a compatible type.

Examples:

extern float a[], *p = a, b;
/* Protect against races among multiple updates. */
#pragma omp atomic
a[index[i]] += b;
/* Protect against races with updates through a. */
#pragma omp atomic
p[i] -= 1.0f;

extern union {int n; float x;} u;
/* ERROR - References through incompatible types. */
#pragma omp atomic
u.n++;
#pragma omp atomic
u.x -= 1.0f;