#pragma omp section, #pragma omp sections

Purpose

The omp sections directive distributes work among threads bound to a defined parallel region.

Syntax

Parameters

clause is any of the following clauses:

private(list)

Declares the scope of the data variables in list to be private to each thread. Data variables in list are separated by commas.

firstprivate(list)

Declares the scope of the data variables in list to be private to each thread. Each new private object is initialized as if there was an implied declaration within the statement block. Data variables in list are separated by commas.

lastprivate(list)

Declares the scope of the data variables in list to be private to each thread. The final value of each variable in list, if assigned, will be the value assigned to that variable in the last section. Variables not assigned a value will have an indeterminate value. Data variables in list are separated by commas.

reduction(reduction-identifier:list)

Specifies that for each data variable in list, a private copy is created. At the end of the statement block, the final values of all private copies of the reduction variable are combined in a manner appropriate to the reduction-identifier, and the result is placed back in the original value of the shared reduction variable.

Scalar variables and array sections are supported in list. Items in list are separated by commas. reduction-identifier is either an identifier, id-expression, or one of the following operators that are implicitly declared: +, -, *, &, |, ^, &&, and ||. If you use a reduction-identifier that is not implicitly declared, you must use the omp declare reduction directive to declare the reduction-identifier beforehand.

The following table lists each reduction-identifier that is implicitly declared at every scope for arithmetic types and its semantic initializer value. The actual initializer value is that value as expressed in the data type of the reduction list item.

Table 1. Implicitly declared C/C++ reduction-identifiers

Identifier	Initializer	Combiner
+	omp_priv=0	omp_out+=omp_in
*	omp_priv=1	omp_out*=omp_in
-	omp_priv=0	omp_out-=omp_in
&	omp_priv=~0	omp_out&=omp_in
|	omp_priv=0	omp_out|=omp_in
^	omp_priv=0	omp_out^=omp_in
&&	omp_priv=1	omp_out=omp_in&&omp_out
||	omp_priv=0	omp_out=omp_in||omp_out
max	omp_priv=Least representable number in the reduction list item type	omp_out=omp_in>omp_out? omp_in:omp_out
min	omp_priv=Largest representable number in the reduction list item type	omp_out=omp_in<omp_out? omp_in:omp_out

omp_in and omp_out correspond to two identifiers that refer to storage of the type of the list item. omp_out holds the final value of the combiner operation. Any reduction-identifier that is defined with the omp declare reduction directive is also valid. In that case, the initializer and combiner of the reduction-identifier are specified by the initializer-clause and the combiner in the omp declare reduction directive.

Restrictions:

• A list item that appears in a reduction clause of a worksharing construct must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
• A list item that appears in a reduction clause of the innermost enclosing worksharing or parallel construct may not be accessed in an explicit task.
• Any number of reduction clauses can be specified on the directive, but a list item can appear only once in the reduction clauses for that directive.
• For a reduction-identifier declared with the omp declare reduction construct, the directive must appear before its use in a reduction clause.
• If a list item is an array section, it must specify contiguous storage and it cannot be a zero-length array section.
• If a list item is an array section, accesses to the elements of the array outside the specified array section are not allowed.
• The type of a list item that appears in a reduction clause must be valid for the reduction-identifier. For a max or min reduction in C, the type of the list item must be an allowed arithmetic data type: char, int, float, double, or _Bool, possibly modified with long, short, signed, or unsigned. For a max or min reduction in C++, the type of the list item must be an allowed arithmetic data type: char, wchar_t, int, float, double, or bool, possibly modified with long, short, signed, or unsigned.
• A list item that appears in a reduction clause must not be const-qualified.
• If a list item in a reduction clause on a worksharing construct has a reference type then it must bind to the same object for all threads of the team.
• The reduction-identifier for any list item must be unambiguous and accessible.

nowait

Use this clause to avoid the implied barrier at the end of the sections directive. This is useful if you have multiple independent work-sharing sections within a given parallel region. Only one nowait clause can appear on a given sections directive.

Usage

The omp section directive is optional for the first program code segment inside the omp sections directive. Following segments must be preceded by an omp section directive. All omp section directives must appear within the lexical construct of the program source code segment associated with the omp sections directive.

When program execution reaches a omp sections directive, program segments defined by the following omp section directive are distributed for parallel execution among available threads. A barrier is implicitly defined at the end of the larger program region associated with the omp sections directive unless the nowait clause is specified.