B3 - variables and synchronisation

• each thread has its own stack variables, used when subroutines are called
• all other variables may be chosen to be:
  • shared: a single storage location accessed by all the threads
  • private: multiple storage locations, one per threads
  • reduction: for variables that are the target of an arithmetic reduction operation, eg: final summation of temporary private variables at the end of the parallel construct
• default behaviour can be specified for variables not included in one of the lists
• default only supports shared/none:

#pragma omp parallel default(shared) private(i)
#pragma omp parallel default(none) shared (n, b, c) private(i)

• recommended to use default(none in all but very simple programs

• helpful for catching unintended sharing

• can also declare thread-private variables within regions only run by the master thread:

int i, j, k;
#pragma omp threadprivate(i, j)

these persist into parallel regions

• all but the master thread's versions are inaccessible in serial regions
• note that the compiler probably will not allocate thread-private variables in memory local to each thread, so they may be much