The default clause allows the user to affect the data-sharing attributes of variables. The syntax of the default clause is as follows:
default(shared | none)
Specifying default(shared) is equivalent to explicitly listing each currently visible variable in a shared clause, unless it is threadprivate or const-qualified. In the absence of an explicit default clause, the default behavior is the same as if default(shared) were specified.
Specifying default(none) requires that at least one of the following must be true for every reference to a variable in the lexical extent of the parallel construct:
The variable is explicitly listed in a data-sharing attribute clause of a construct that contains the reference.
The variable is declared within the parallel construct.
The variable is threadprivate.
The variable has a const-qualified type.
The variable is the loop control variable for a for loop that immediately follows a for or parallel for directive, and the variable reference appears inside the loop.
Specifying a variable on a firstprivate, lastprivate, or reduction clause of an enclosed directive causes an implicit reference to the variable in the enclosing context. Such implicit references are also subject to the requirements listed above.
Only a single default clause may be specified on a parallel directive.
A variable's default data-sharing attribute can be overridden by using the private, firstprivate, lastprivate, reduction, and shared clauses, as demonstrated by the following example:
#pragma omp parallel for default(shared) firstprivate(i)\ private(x) private(r) lastprivate(i)