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I know I can tell OpenMP not to share variables by default within a parallel region by using
#pragma omp parallel default none
But is there a way to set this globally? It seems as though the global default is that everything that isn't declared private is shared, and, at least in my application, there are many more things that should be private than should be shared.
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The default(none) clause forces a programmer to explicitly specify the data-sharing attributes of all variables in a parallel region. Using this clause then forces the programmer to think about data-sharing attributes. This is beneficial because the code is clearer and has less bugs.
The default(firstprivate) clause causes all variables in the construct that have implicitly determined data-sharing attributes to be firstprivate. The default(private) clause causes all variables referenced in the construct that have implicitly determined data-sharing attributes to be private.
firstprivate. Specifies that each thread should have its own instance of a variable, and that the variable should be initialized with the value of the variable, because it exists before the parallel construct.
# pragma omp critical , (name) where name can optionally be used to identify the critical region. Identifiers naming a critical region have external linkage and occupy a namespace distinct from that used by ordinary identifiers.
All variables in OpenMP are shared by default. If you want a set of private variables you will need to specify these variables in a parallel pragma directive in a private clause. If you use
#pragma omp parallel default none
You need to specify the private variables and shared variables. For instance:
#pragma omp parallel default(none) private(i,j) shared(a,b)
References:
[1] http://en.wikipedia.org/wiki/OpenMP#OpenMP_clauses
[2] https://computing.llnl.gov/tutorials/openMP/#ClausesDirectives
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