Run-Time Library Routines

Overview:

• The OpenMP API includes an ever-growing number of run-time library routines.

• These routines are used for a variety of purposes as shown in the table below:

Routine	Purpose
OMP_SET_NUM_THREADS	Sets the number of threads that will be used in the next parallel region
OMP_GET_NUM_THREADS	Returns the number of threads that are currently in the team executing the parallel region from which it is called
OMP_GET_MAX_THREADS	Returns the maximum value that can be returned by a call to the OMP_GET_NUM_THREADS function
OMP_GET_THREAD_NUM	Returns the thread number of the thread, within the team, making this call.
OMP_GET_THREAD_LIMIT	Returns the maximum number of OpenMP threads available to a program
OMP_GET_NUM_PROCS	Returns the number of processors that are available to the program
OMP_IN_PARALLEL	Used to determine if the section of code which is executing is parallel or not
OMP_SET_DYNAMIC	Enables or disables dynamic adjustment (by the run time system) of the number of threads available for execution of parallel regions
OMP_GET_DYNAMIC	Used to determine if dynamic thread adjustment is enabled or not
OMP_SET_NESTED	Used to enable or disable nested parallelism
OMP_GET_NESTED	Used to determine if nested parallelism is enabled or not
OMP_SET_SCHEDULE	Sets the loop scheduling policy when "runtime" is used as the schedule kind in the OpenMP directive
OMP_GET_SCHEDULE	Returns the loop scheduling policy when "runtime" is used as the schedule kind in the OpenMP directive
OMP_SET_MAX_ACTIVE_LEVELS	Sets the maximum number of nested parallel regions
OMP_GET_MAX_ACTIVE_LEVELS	Returns the maximum number of nested parallel regions
OMP_GET_LEVEL	Returns the current level of nested parallel regions
OMP_GET_ANCESTOR_THREAD_NUM	Returns, for a given nested level of the current thread, the thread number of ancestor thread
OMP_GET_TEAM_SIZE	Returns, for a given nested level of the current thread, the size of the thread team
OMP_GET_ACTIVE_LEVEL	Returns the number of nested, active parallel regions enclosing the task that contains the call
OMP_IN_FINAL	Returns true if the routine is executed in the final task region; otherwise it returns false
OMP_INIT_LOCK	Initializes a lock associated with the lock variable
OMP_DESTROY_LOCK	Disassociates the given lock variable from any locks
OMP_SET_LOCK	Acquires ownership of a lock
OMP_UNSET_LOCK	Releases a lock
OMP_TEST_LOCK	Attempts to set a lock, but does not block if the lock is unavailable
OMP_INIT_NEST_LOCK	Initializes a nested lock associated with the lock variable
OMP_DESTROY_NEST_LOCK	Disassociates the given nested lock variable from any locks
OMP_SET_NEST_LOCK	Acquires ownership of a nested lock
OMP_UNSET_NEST_LOCK	Releases a nested lock
OMP_TEST_NEST_LOCK	Attempts to set a nested lock, but does not block if the lock is unavailable
OMP_GET_WTIME	Provides a portable wall clock timing routine
OMP_GET_WTICK	Returns a double-precision floating point value equal to the number of seconds between successive clock ticks

For C/C++, all of the run-time library routines are actual subroutines. For Fortran, some are actually functions, and some are subroutines. For example:

Fortran

INTEGER FUNCTION OMP_GET_NUM_THREADS()

C/C++

#include <omp.h>
int omp_get_num_threads(void)

• Note that for C/C++, you usually need to include the <omp.h> header file.

• Fortran routines are not case sensitive, but C/C++ routines are.

• For the Lock routines/functions:
  • The lock variable must be accessed only through the locking routines
  • For Fortran, the lock variable should be of type integer and of a kind large enough to hold an address.
  • For C/C++, the lock variable must have type omp_lock_t or type omp_nest_lock_t, depending on the function being used.
• Implementation notes:
  • Implementations may or may not support all OpenMP API features. For example, if nested parallelism is supported, it may be only nominal, in that a nested parallel region may only have one thread.
  • Consult your implementation’s documentation for details - or experiment and find out for yourself if you can’t find it in the documentation.
• The run-time library routines are discussed in more detail in Appendix A.