OpenMP Directives: Data Scope Attribute Clauses

• Also called Data-sharing Attribute Clauses

• An important consideration for OpenMP programming is the understanding and use of data scoping.

• Because OpenMP is based upon the shared memory programming model, most variables are shared by default.

• Global variables include:
  • Fortran: COMMON blocks, SAVE variables, MODULE variables
  • C: File scope variables, static
• Private variables include:
  • Loop index variables
  • Stack variables in subroutines called from parallel regions
  • Fortran: Automatic variables within a statement block

The OpenMP Data Scope Attribute Clauses are used to explicitly define how variables should be scoped. They include: * PRIVATE * FIRSTPRIVATE * LASTPRIVATE * SHARED * DEFAULT * REDUCTION * COPYIN

• Data Scope Attribute Clauses are used in conjunction with several directives (PARALLEL, DO/for, and SECTIONS) to control the scoping of enclosed variables.

• These constructs provide the ability to control the data environment during execution of parallel constructs.
  • They define how and which data variables in the serial section of the program are transferred to the parallel sections of the program (and back)
  • They define which variables will be visible to all threads in the parallel sections and which variables will be privately allocated to all threads.

• Data Scope Attribute Clauses are effective only within their lexical/static extent.

• Important: Please consult the latest OpenMP specs for important details and discussion on this topic.

• A Clauses / Directives Summary Table is provided for convenience.