The routines that can be used to modify the execution environment are listed below.

omp_set_num_threads(num_threads)

The omp_set_num_threads routine is used to set the nthreads OpenMP internal variable. It should be called outside a parallel region. If it is called within a parallel region, its effect is implementation-defined. The argument must be a positive integer. Setting the internal variable to a particular value does not guarantee that the next parallel region will have that number of threads, because a parallel construct may have its own num_threads clause.

omp_get_num_threads

The omp_get_num_threads routine is not the corresponding getter for omp_set_num_threads. It can be called anywhere in the program and returns the number of threads in the innermost parallel region to which it belongs. If it is called outside a parallel region, it returns 1.

int omp_get_max_threads()

The omp_get_max_threads routine is the corresponding getter for omp_set_num_threads. It returns the present value of the nthreads OpenMP internal variable. It should be called outside a parallel region. If it is called within a parallel region, its result is implementation-defined.

int omp_get_thread_number()

The omp_get_thread_number routine returns the OpenMP thread number of the thread from which it is called. OpenMP numbers its threads from 0 to one less than the number that would be returned by omp_get_num_threads if it were called at this point in the program. These numbers should not be confused with the numbers assigned to threads by the operating system.

int omp_get_num_procs()

The omp_get_num_procs routine returns the number of processors (cores) available to the program.

logical omp_in_parallel()

The omp_in_parallel routine returns the logical "or" of the if-clauses of all the enclosing parallel regions. Any parallel region that doesn't have an if-clause contributes .TRUE. to this evaluation. The routine returns .FALSE. if called outside any parallel region. When called in a C program it returns the integer equivalents (1, 0) of true and false.

omp_set_dynamic(dynamic)

The omp_set_dynamic routine sets the dynamic OpenMP internal variable if its argument evaluates to true and if the OpenMP implementation supports dynamic scheduling of threads. It should be called outside a parallel region. If it is called within a parallel region, its effect is implementation-defined.

logical omp_get_dynamic()

The omp_get_dynamic routine gets the dynamic OpenMP internal variable. It should be called outside a parallel region. If it is called within a parallel region, its effect is implementation-defined. If the implementation doesn't support dynamic scheduling of the number of threads, it will always return false.

omp_set_nested(nested)

The omp_set_nested routine sets the nested OpenMP internal variable if its argument evaluates to true and if the OpenMP implementation supports nesting of parallel regions. It should be called outside a parallel region. If it is called within a parallel region, its effect is implementation-defined.

logical omp_get nested()

The omp_get_nested routine gets the nested OpenMP internal variable. It should be called outside a parallel region. If it is called within a parallel region, its effect is implementation-defined. If the implementation doesn't support nesting of parallel regions, it will always return false.