This manual documents libgomp, the GNU Offloading and Multi Processing Runtime library.  This is the GNU implementation of the OpenMP and OpenACC APIs for parallel and accelerator programming in C/C++ and Fortran.

Published by the Free Software Foundation 51 Franklin Street, Fifth Floor Boston, MA 02110-1301 USA

Copyright © 2006-2015 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Funding Free Software'', the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below).  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software.  Copies published by the Free Software Foundation raise funds for GNU development.

Introduction
************

This manual documents the usage of libgomp, the GNU Offloading and Multi Processing Runtime Library.  This includes the GNU implementation of the OpenMP (http://www.openmp.org) Application Programming Interface (API) for multi-platform shared-memory parallel programming in C/C++ and Fortran, and the GNU implementation of the OpenACC (http://www.openacc.org/) Application Programming Interface (API) for offloading of code to accelerator devices in C/C++ and Fortran.

Originally, libgomp implemented the GNU OpenMP Runtime Library.  Based on this, support for OpenACC and offloading (both OpenACC and OpenMP 4's target construct) has been added later on, and the library's name changed to GNU Offloading and Multi Processing Runtime Library.

Enabling OpenMP
***************

To activate the OpenMP extensions for C/C++ and Fortran, the compile-time flag -fopenmp must be specified.  This enables the OpenMP directive #pragma omp in C/C++ and !$omp directives in free form, c$omp, *$omp and !$omp directives in fixed form, !$ conditional compilation sentinels in free form and c$, *$ and !$ sentinels in fixed form, for Fortran.  The flag also arranges for automatic linking of the OpenMP runtime library (Runtime Library Routines).

A complete description of all OpenMP directives accepted may be found in the OpenMP Application Program Interface (http://www.openmp.org) manual, version 4.0.

Runtime Library Routines
************************

The runtime routines described here are defined by Section 3 of the OpenMP specification in version 4.0.  The routines are structured in following three parts:

omp_get_active_level -- Number of parallel regions
==================================================

Description:
     This function returns the nesting level for the active parallel blocks, which enclose the calling call.

C/C++

     Prototype:   int omp_get_active_level(void);

Fortran:

     Interface:   integer function omp_get_active_level()

See also:
     omp_get_level, omp_get_max_active_levels, omp_set_max_active_levels

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.20.

omp_get_ancestor_thread_num -- Ancestor thread ID
=================================================

Description:
     This function returns the thread identification number for the given nesting level of the current thread.  For values of LEVEL outside zero to omp_get_level -1 is returned; if LEVEL is omp_get_level the result is identical to omp_get_thread_num.

C/C++

     Prototype:   int omp_get_ancestor_thread_num(int level);

Fortran:

     Interface:   integer function omp_get_ancestor_thread_num(level)
       integer level

See also:
     omp_get_level, omp_get_thread_num, omp_get_team_size

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.18.

omp_get_cancellation -- Whether cancellation support is enabled
===============================================================

Description:
     This function returns true if cancellation is activated, false otherwise.  Here, true and false represent their language-specific counterparts.  Unless OMP_CANCELLATION is set true, cancellations are deactivated.

C/C++:

     Prototype:   int omp_get_cancellation(void);

Fortran:

     Interface:   logical function omp_get_cancellation()

See also:
     OMP_CANCELLATION

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.9.

omp_get_default_device -- Get the default device for target regions
===================================================================

Description:
     Get the default device for target regions without device clause.

C/C++:

     Prototype:   int omp_get_default_device(void);

Fortran:

     Interface:   integer function omp_get_default_device()

See also:
     OMP_DEFAULT_DEVICE, omp_set_default_device

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.24.

omp_get_dynamic -- Dynamic teams setting
========================================

Description:
     This function returns true if enabled, false otherwise. Here, true and false represent their language-specific counterparts.

     The dynamic team setting may be initialized at startup by the OMP_DYNAMIC environment variable or at runtime using omp_set_dynamic.  If undefined, dynamic adjustment is disabled by default.

C/C++:

     Prototype:   int omp_get_dynamic(void);

Fortran:

     Interface:   logical function omp_get_dynamic()

See also:
     omp_set_dynamic, OMP_DYNAMIC

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.8.

omp_get_level -- Obtain the current nesting level
=================================================

Description:
     This function returns the nesting level for the parallel blocks, which enclose the calling call.

C/C++

     Prototype:   int omp_get_level(void);

Fortran:

     Interface:   integer function omp_level()

See also:
     omp_get_active_level

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.17.

omp_get_max_active_levels -- Maximum number of active regions
=============================================================

Description:
     This function obtains the maximum allowed number of nested, active parallel regions.

C/C++

     Prototype:   int omp_get_max_active_levels(void);

Fortran:

     Interface:   integer function omp_get_max_active_levels()

See also:
     omp_set_max_active_levels, omp_get_active_level

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.16.

omp_get_max_threads -- Maximum number of threads of parallel region
===================================================================

Description:
     Return the maximum number of threads used for the current parallel region that does not use the clause num_threads.

C/C++:

     Prototype:   int omp_get_max_threads(void);

Fortran:

     Interface:   integer function omp_get_max_threads()

See also:
     omp_set_num_threads, omp_set_dynamic, omp_get_thread_limit

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.3.

omp_get_nested -- Nested parallel regions
=========================================

Description:
     This function returns true if nested parallel regions are enabled, false otherwise.  Here, true and false represent their language-specific counterparts.

     Nested parallel regions may be initialized at startup by the OMP_NESTED environment variable or at runtime using omp_set_nested.  If undefined, nested parallel regions are disabled by default.

C/C++:

     Prototype:   int omp_get_nested(void);

Fortran:

     Interface:   logical function omp_get_nested()

See also:
     omp_set_nested, OMP_NESTED

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.11.

omp_get_num_devices -- Number of target devices
===============================================

Description:
     Returns the number of target devices.

C/C++:

     Prototype:   int omp_get_num_devices(void);

Fortran:

     Interface:   integer function omp_get_num_devices()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.25.

omp_get_num_procs -- Number of processors online
================================================

Description:
     Returns the number of processors online on that device.

C/C++:

     Prototype:   int omp_get_num_procs(void);

Fortran:

     Interface:   integer function omp_get_num_procs()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.5.

omp_get_num_teams -- Number of teams
====================================

Description:
     Returns the number of teams in the current team region.

C/C++:

     Prototype:   int omp_get_num_teams(void);

Fortran:

     Interface:   integer function omp_get_num_teams()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.26.

omp_get_num_threads -- Size of the active team
==============================================

Description:
     Returns the number of threads in the current team.  In a sequential section of the program omp_get_num_threads returns 1.

     The default team size may be initialized at startup by the OMP_NUM_THREADS environment variable.  At runtime, the size of the current team may be set either by the NUM_THREADS clause or by omp_set_num_threads.  If none of the above were used to define a specific value and OMP_DYNAMIC is disabled, one thread per CPU online is used.

C/C++:

     Prototype:   int omp_get_num_threads(void);

Fortran:

     Interface:   integer function omp_get_num_threads()

See also:
     omp_get_max_threads, omp_set_num_threads, OMP_NUM_THREADS

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.2.

omp_get_proc_bind -- Whether theads may be moved between CPUs
=============================================================

Description:
     This functions returns the currently active thread affinity policy, which is set via OMP_PROC_BIND.  Possible values are omp_proc_bind_false, omp_proc_bind_true, omp_proc_bind_master, omp_proc_bind_close and omp_proc_bind_spread.

C/C++:

     Prototype:   omp_proc_bind_t omp_get_proc_bind(void);

Fortran:

     Interface:   integer(kind=omp_proc_bind_kind) function omp_get_proc_bind()

See also:
     OMP_PROC_BIND, OMP_PLACES, GOMP_CPU_AFFINITY,

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.22.

omp_get_schedule -- Obtain the runtime scheduling method
========================================================

Description:
     Obtain the runtime scheduling method.  The KIND argument will be set to the value omp_sched_static, omp_sched_dynamic, omp_sched_guided or omp_sched_auto.  The second argument, MODIFIER, is set to the chunk size.

C/C++

     Prototype:   void omp_get_schedule(omp_sched_t *kind, int *modifier);

Fortran:

     Interface:   subroutine omp_get_schedule(kind, modifier)
       integer(kind=omp_sched_kind) kind
       integer modifier

See also:
     omp_set_schedule, OMP_SCHEDULE

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.13.

omp_get_team_num -- Get team number
===================================

Description:
     Returns the team number of the calling thread.

C/C++:

     Prototype:   int omp_get_team_num(void);

Fortran:

     Interface:   integer function omp_get_team_num()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.27.

omp_get_team_size -- Number of threads in a team
================================================

Description:
     This function returns the number of threads in a thread team to which either the current thread or its ancestor belongs.  For values of LEVEL outside zero to omp_get_level, -1 is returned; if LEVEL is zero, 1 is returned, and for omp_get_level, the result is identical to omp_get_num_threads.

C/C++:

     Prototype:   int omp_get_team_size(int level);

Fortran:

     Interface:   integer function omp_get_team_size(level)
       integer level

See also:
     omp_get_num_threads, omp_get_level, omp_get_ancestor_thread_num

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.19.

omp_get_thread_limit -- Maximum number of threads
=================================================

Description:
     Return the maximum number of threads of the program.

C/C++:

     Prototype:   int omp_get_thread_limit(void);

Fortran:

     Interface:   integer function omp_get_thread_limit()

See also:
     omp_get_max_threads, OMP_THREAD_LIMIT

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.14.

omp_get_thread_num -- Current thread ID
=======================================

Description:
     Returns a unique thread identification number within the current team. In a sequential parts of the program, omp_get_thread_num always returns 0.  In parallel regions the return value varies from 0 to omp_get_num_threads-1 inclusive.  The return value of the master thread of a team is always 0.

C/C++:

     Prototype:   int omp_get_thread_num(void);

Fortran:

     Interface:   integer function omp_get_thread_num()

See also:
     omp_get_num_threads, omp_get_ancestor_thread_num

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.4.

omp_in_parallel -- Whether a parallel region is active
======================================================

Description:
     This function returns true if currently running in parallel, false otherwise.  Here, true and false represent their language-specific counterparts.

C/C++:

     Prototype:   int omp_in_parallel(void);

Fortran:

     Interface:   logical function omp_in_parallel()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.6.

omp_in_final -- Whether in final or included task region
========================================================

Description:
     This function returns true if currently running in a final or included task region, false otherwise.  Here, true and false represent their language-specific counterparts.

C/C++:

     Prototype:   int omp_in_final(void);

Fortran:

     Interface:   logical function omp_in_final()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.21.

omp_is_initial_device -- Whether executing on the host device
=============================================================

Description:
     This function returns true if currently running on the host device, false otherwise.  Here, true and false represent their language-specific counterparts.

C/C++:

     Prototype:   int omp_is_initial_device(void);

Fortran:

     Interface:   logical function omp_is_initial_device()

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.28.

omp_set_default_device -- Set the default device for target regions
===================================================================

Description:
     Set the default device for target regions without device clause.  The argument shall be a nonnegative device number.

C/C++:

     Prototype:   void omp_set_default_device(int device_num);

Fortran:

     Interface:   subroutine omp_set_default_device(device_num)
       integer device_num

See also:
     OMP_DEFAULT_DEVICE, omp_get_default_device

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.23.

omp_set_dynamic -- Enable/disable dynamic teams
===============================================

Description:
     Enable or disable the dynamic adjustment of the number of threads within a team.  The function takes the language-specific equivalent of true and false, where true enables dynamic adjustment of team sizes and false disables it.

C/C++:

     Prototype:   void omp_set_dynamic(int dynamic_threads);

Fortran:

     Interface:   subroutine omp_set_dynamic(dynamic_threads)
       logical, intent(in) :: dynamic_threads

See also:
     OMP_DYNAMIC, omp_get_dynamic

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.7.

omp_set_max_active_levels -- Limits the number of active parallel regions
=========================================================================

Description:
     This function limits the maximum allowed number of nested, active parallel regions.

C/C++

     Prototype:   void omp_set_max_active_levels(int max_levels);

Fortran:

     Interface:   subroutine omp_set_max_active_levels(max_levels)
       integer max_levels

See also:
     omp_get_max_active_levels, omp_get_active_level

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.15.

omp_set_nested -- Enable/disable nested parallel regions
========================================================

Description:
     Enable or disable nested parallel regions, i.e., whether team members are allowed to create new teams.  The function takes the language-specific equivalent of true and false, where true enables dynamic adjustment of team sizes and false disables it.

C/C++:

     Prototype:   void omp_set_nested(int nested);

Fortran:

     Interface:   subroutine omp_set_nested(nested)
       logical, intent(in) :: nested

See also:
     OMP_NESTED, omp_get_nested

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.10.

omp_set_num_threads -- Set upper team size limit
================================================

Description:
     Specifies the number of threads used by default in subsequent parallel sections, if those do not specify a num_threads clause.  The argument of omp_set_num_threads shall be a positive integer.

C/C++:

     Prototype:   void omp_set_num_threads(int num_threads);

Fortran:

     Interface:   subroutine omp_set_num_threads(num_threads)
       integer, intent(in) :: num_threads

See also:
     OMP_NUM_THREADS, omp_get_num_threads, omp_get_max_threads

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.1.

omp_set_schedule -- Set the runtime scheduling method
=====================================================

Description:
     Sets the runtime scheduling method.  The KIND argument can have the value omp_sched_static, omp_sched_dynamic, omp_sched_guided or omp_sched_auto.  Except for omp_sched_auto, the chunk size is set to the value of MODIFIER if positive, or to the default value if zero or negative. For omp_sched_auto the MODIFIER argument is ignored.

C/C++

     Prototype:   void omp_set_schedule(omp_sched_t kind, int modifier);

Fortran:

     Interface:   subroutine omp_set_schedule(kind, modifier)
       integer(kind=omp_sched_kind) kind
       integer modifier

See also:
     omp_get_schedule OMP_SCHEDULE

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.2.12.

omp_init_lock -- Initialize simple lock
=======================================

Description:
     Initialize a simple lock.  After initialization, the lock is in an unlocked state.

C/C++:

     Prototype:   void omp_init_lock(omp_lock_t *lock);

Fortran:

     Interface:   subroutine omp_init_lock(svar)
       integer(omp_lock_kind), intent(out) :: svar

See also:
     omp_destroy_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.1.

omp_set_lock -- Wait for and set simple lock
============================================

Description:
     Before setting a simple lock, the lock variable must be initialized by omp_init_lock.  The calling thread is blocked until the lock is available.  If the lock is already held by the current thread, a deadlock occurs.

C/C++:

     Prototype:   void omp_set_lock(omp_lock_t *lock);

Fortran:

     Interface:   subroutine omp_set_lock(svar)
       integer(omp_lock_kind), intent(inout) :: svar

See also:
     omp_init_lock, omp_test_lock, omp_unset_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.3.

omp_test_lock -- Test and set simple lock if available
======================================================

Description:
     Before setting a simple lock, the lock variable must be initialized by omp_init_lock.  Contrary to omp_set_lock, omp_test_lock does not block if the lock is not available.  This function returns true upon success, false otherwise.  Here, true and false represent their language-specific counterparts.

C/C++:

     Prototype:   int omp_test_lock(omp_lock_t *lock);

Fortran:

     Interface:   logical function omp_test_lock(svar)
       integer(omp_lock_kind), intent(inout) :: svar

See also:
     omp_init_lock, omp_set_lock, omp_set_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.5.

omp_unset_lock -- Unset simple lock
===================================

Description:
     A simple lock about to be unset must have been locked by omp_set_lock or omp_test_lock before.  In addition, the lock must be held by the thread calling omp_unset_lock.  Then, the lock becomes unlocked.  If one or more threads attempted to set the lock before, one of them is chosen to, again, set the lock to itself.

C/C++:

     Prototype:   void omp_unset_lock(omp_lock_t *lock);

Fortran:

     Interface:   subroutine omp_unset_lock(svar)
       integer(omp_lock_kind), intent(inout) :: svar

See also:
     omp_set_lock, omp_test_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.4.

omp_destroy_lock -- Destroy simple lock
=======================================

Description:
     Destroy a simple lock.  In order to be destroyed, a simple lock must be in the unlocked state.

C/C++:

     Prototype:   void omp_destroy_lock(omp_lock_t *lock);

Fortran:

     Interface:   subroutine omp_destroy_lock(svar)
       integer(omp_lock_kind), intent(inout) :: svar

See also:
     omp_init_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.2.

omp_init_nest_lock -- Initialize nested lock
============================================

Description:
     Initialize a nested lock.  After initialization, the lock is in an unlocked state and the nesting count is set to zero.

C/C++:

     Prototype:   void omp_init_nest_lock(omp_nest_lock_t *lock);

Fortran:

     Interface:   subroutine omp_init_nest_lock(nvar)
       integer(omp_nest_lock_kind), intent(out) :: nvar

See also:
     omp_destroy_nest_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.1.

omp_set_nest_lock -- Wait for and set nested lock
=================================================

Description:
     Before setting a nested lock, the lock variable must be initialized by omp_init_nest_lock.  The calling thread is blocked until the lock is available.  If the lock is already held by the current thread, the nesting count for the lock is incremented.

C/C++:

     Prototype:   void omp_set_nest_lock(omp_nest_lock_t *lock);

Fortran:

     Interface:   subroutine omp_set_nest_lock(nvar)
       integer(omp_nest_lock_kind), intent(inout) :: nvar

See also:
     omp_init_nest_lock, omp_unset_nest_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.3.

omp_test_nest_lock -- Test and set nested lock if available
===========================================================

Description:
     Before setting a nested lock, the lock variable must be initialized by omp_init_nest_lock.  Contrary to omp_set_nest_lock, omp_test_nest_lock does not block if the lock is not available. If the lock is already held by the current thread, the new nesting count is returned.  Otherwise, the return value equals zero.

C/C++:

     Prototype:   int omp_test_nest_lock(omp_nest_lock_t *lock);

Fortran:

     Interface:   logical function omp_test_nest_lock(nvar)
       integer(omp_nest_lock_kind), intent(inout) :: nvar

See also:
     omp_init_lock, omp_set_lock, omp_set_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.5.

omp_unset_nest_lock -- Unset nested lock
========================================

Description:
     A nested lock about to be unset must have been locked by omp_set_nested_lock or omp_test_nested_lock before.  In addition, the lock must be held by the thread calling omp_unset_nested_lock.  If the nesting count drops to zero, the lock becomes unlocked.  If one ore more threads attempted to set the lock before, one of them is chosen to, again, set the lock to itself.

C/C++:

     Prototype:   void omp_unset_nest_lock(omp_nest_lock_t *lock);

Fortran:

     Interface:   subroutine omp_unset_nest_lock(nvar)
       integer(omp_nest_lock_kind), intent(inout) :: nvar

See also:
     omp_set_nest_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.4.

omp_destroy_nest_lock -- Destroy nested lock
============================================

Description:
     Destroy a nested lock.  In order to be destroyed, a nested lock must be in the unlocked state and its nesting count must equal zero.

C/C++:

     Prototype:   void omp_destroy_nest_lock(omp_nest_lock_t *);

Fortran:

     Interface:   subroutine omp_destroy_nest_lock(nvar)
       integer(omp_nest_lock_kind), intent(inout) :: nvar

See also:
     omp_init_lock

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.3.2.

omp_get_wtick -- Get timer precision
====================================

Description:
     Gets the timer precision, i.e., the number of seconds between two successive clock ticks.

C/C++:

     Prototype:   double omp_get_wtick(void);

Fortran:

     Interface:   double precision function omp_get_wtick()

See also:
     omp_get_wtime

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.4.2.

omp_get_wtime -- Elapsed wall clock time
========================================

Description:
     Elapsed wall clock time in seconds.  The time is measured per thread, no guarantee can be made that two distinct threads measure the same time. Time is measured from some "time in the past", which is an arbitrary time guaranteed not to change during the execution of the program.

C/C++:

     Prototype:   double omp_get_wtime(void);

Fortran:

     Interface:   double precision function omp_get_wtime()

See also:
     omp_get_wtick

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 3.4.1.

Environment Variables
*********************

The environment variables which beginning with OMP_ are defined by section 4 of the OpenMP specification in version 4.0, while those beginning with GOMP_ are GNU extensions.

OMP_CANCELLATION -- Set whether cancellation is activated
=========================================================

Description:
     If set to TRUE, the cancellation is activated.  If set to FALSE or if unset, cancellation is disabled and the cancel construct is ignored.

See also:
     omp_get_cancellation

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.11

OMP_DISPLAY_ENV -- Show OpenMP version and environment variables
================================================================

Description:
     If set to TRUE, the OpenMP version number and the values associated with the OpenMP environment variables are printed to stderr. If set to VERBOSE, it additionally shows the value of the environment variables which are GNU extensions.  If undefined or set to FALSE, this information will not be shown.

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.12

OMP_DEFAULT_DEVICE -- Set the device used in target regions
===========================================================

Description:
     Set to choose the device which is used in a target region, unless the value is overridden by omp_set_default_device or by a device clause.  The value shall be the nonnegative device number. If no device with the given device number exists, the code is executed on the host.  If unset, device number 0 will be used.

See also:
     omp_get_default_device, omp_set_default_device,

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.11

OMP_DYNAMIC -- Dynamic adjustment of threads
============================================

Description:
     Enable or disable the dynamic adjustment of the number of threads within a team.  The value of this environment variable shall be TRUE or FALSE.  If undefined, dynamic adjustment is disabled by default.

See also:
     omp_set_dynamic

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.3

OMP_MAX_ACTIVE_LEVELS -- Set the maximum number of nested parallel regions
==========================================================================

Description:
     Specifies the initial value for the maximum number of nested parallel regions.  The value of this variable shall be a positive integer. If undefined, the number of active levels is unlimited.

See also:
     omp_set_max_active_levels

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.9

OMP_NESTED -- Nested parallel regions
=====================================

Description:
     Enable or disable nested parallel regions, i.e., whether team members are allowed to create new teams.  The value of this environment variable shall be TRUE or FALSE.  If undefined, nested parallel regions are disabled by default.

See also:
     omp_set_nested

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.6

OMP_NUM_THREADS -- Specifies the number of threads to use
=========================================================

Description:
     Specifies the default number of threads to use in parallel regions.  The value of this variable shall be a comma-separated list of positive integers; the value specified the number of threads to use for the corresponding nested level.  If undefined one thread per CPU is used.

See also:
     omp_set_num_threads

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.2

OMP_PROC_BIND -- Whether theads may be moved between CPUs
=========================================================

Description:
     Specifies whether threads may be moved between processors.  If set to TRUE, OpenMP theads should not be moved; if set to FALSE they may be moved.  Alternatively, a comma separated list with the values MASTER, CLOSE and SPREAD can be used to specify the thread affinity policy for the corresponding nesting level.  With MASTER the worker threads are in the same place partition as the master thread.  With CLOSE those are kept close to the master thread in contiguous place partitions.  And with SPREAD a sparse distribution across the place partitions is used.

     When undefined, OMP_PROC_BIND defaults to TRUE when OMP_PLACES or GOMP_CPU_AFFINITY is set and FALSE otherwise.

See also:
     OMP_PLACES, GOMP_CPU_AFFINITY, omp_get_proc_bind

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.4

OMP_PLACES -- Specifies on which CPUs the theads should be placed
=================================================================

Description:
     The thread placement can be either specified using an abstract name or by an explicit list of the places.  The abstract names threads, cores and sockets can be optionally followed by a positive number in parentheses, which denotes the how many places shall be created.  With threads each place corresponds to a single hardware thread; cores to a single core with the corresponding number of hardware threads; and with sockets the place corresponds to a single socket.  The resulting placement can be shown by setting the OMP_DISPLAY_ENV environment variable.

     Alternatively, the placement can be specified explicitly as comma-separated list of places.  A place is specified by set of nonnegative numbers in curly braces, denoting the denoting the hardware threads.  The hardware threads belonging to a place can either be specified as comma-separated list of nonnegative thread numbers or using an interval.  Multiple places can also be either specified by a comma-separated list of places or by an interval.  To specify an interval, a colon followed by the count is placed after after the hardware thread number or the place.  Optionally, the length can be followed by a colon and the stride number -- otherwise a unit stride is assumed.  For instance, the following specifies the same places list: "{0,1,2}, {3,4,6}, {7,8,9}, {10,11,12}"; "{0:3}, {3:3}, {7:3}, {10:3}"; and "{0:2}:4:3".

     If OMP_PLACES and GOMP_CPU_AFFINITY are unset and OMP_PROC_BIND is either unset or false, threads may be moved between CPUs following no placement policy.

See also:
     OMP_PROC_BIND, GOMP_CPU_AFFINITY, omp_get_proc_bind, OMP_DISPLAY_ENV

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.5

OMP_STACKSIZE -- Set default thread stack size
==============================================

Description:
     Set the default thread stack size in kilobytes, unless the number is suffixed by B, K, M or G, in which case the size is, respectively, in bytes, kilobytes, megabytes or gigabytes.  This is different from pthread_attr_setstacksize which gets the number of bytes as an argument.  If the stack size cannot be set due to system constraints, an error is reported and the initial stack size is left unchanged.  If undefined, the stack size is system dependent.

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.7

OMP_SCHEDULE -- How threads are scheduled
=========================================

Description:
     Allows to specify schedule type and chunk size. The value of the variable shall have the form: type[,chunk] where type is one of static, dynamic, guided or auto The optional chunk size shall be a positive integer.  If undefined, dynamic scheduling and a chunk size of 1 is used.

See also:
     omp_set_schedule

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Sections 2.7.1 and 4.1

OMP_THREAD_LIMIT -- Set the maximum number of threads
=====================================================

Description:
     Specifies the number of threads to use for the whole program.  The value of this variable shall be a positive integer.  If undefined, the number of threads is not limited.

See also:
     OMP_NUM_THREADS, omp_get_thread_limit

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.10

OMP_WAIT_POLICY -- How waiting threads are handled
==================================================

Description:
     Specifies whether waiting threads should be active or passive.  If the value is PASSIVE, waiting threads should not consume CPU power while waiting; while the value is ACTIVE specifies that they should.  If undefined, threads wait actively for a short time before waiting passively.

See also:
     GOMP_SPINCOUNT

Reference:
     OpenMP specification v4.0 (http://www.openmp.org/), Section 4.8

GOMP_CPU_AFFINITY -- Bind threads to specific CPUs
==================================================

Description:
     Binds threads to specific CPUs.  The variable should contain a space-separated or comma-separated list of CPUs.  This list may contain different kinds of entries: either single CPU numbers in any order, a range of CPUs (M-N) or a range with some stride (M-N:S).  CPU numbers are zero based.  For example, GOMP_CPU_AFFINITY="0 3 1-2 4-15:2" will bind the initial thread to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12, and 14 respectively and then start assigning back from the beginning of the list.  GOMP_CPU_AFFINITY=0 binds all threads to CPU 0.

     There is no libgomp library routine to determine whether a CPU affinity specification is in effect.  As a workaround, language-specific library functions, e.g., getenv in C or GET_ENVIRONMENT_VARIABLE in Fortran, may be used to query the setting of the GOMP_CPU_AFFINITY environment variable.  A defined CPU affinity on startup cannot be changed or disabled during the runtime of the application.

     If both GOMP_CPU_AFFINITY and OMP_PROC_BIND are set, OMP_PROC_BIND has a higher precedence.  If neither has been set and OMP_PROC_BIND is unset, or when OMP_PROC_BIND is set to FALSE, the host system will handle the assignment of threads to CPUs.

See also:
     OMP_PLACES, OMP_PROC_BIND

GOMP_DEBUG -- Enable debugging output
=====================================

Description:
     Enable debugging output.  The variable should be set to 0 (disabled, also the default if not set), or 1 (enabled).

     If enabled, some debugging output will be printed during execution. This is currently not specified in more detail, and subject to change.

GOMP_STACKSIZE -- Set default thread stack size
===============================================

Description:
     Set the default thread stack size in kilobytes.  This is different from pthread_attr_setstacksize which gets the number of bytes as an argument.  If the stack size cannot be set due to system constraints, an error is reported and the initial stack size is left unchanged.  If undefined, the stack size is system dependent.

See also:
     OMP_STACKSIZE

Reference:
     GCC Patches Mailinglist (http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00493.html), GCC Patches Mailinglist (http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00496.html)

GOMP_SPINCOUNT -- Set the busy-wait spin count
==============================================

Description:
     Determines how long a threads waits actively with consuming CPU power before waiting passively without consuming CPU power.  The value may be either INFINITE, INFINITY to always wait actively or an integer which gives the number of spins of the busy-wait loop.  The integer may optionally be followed by the following suffixes acting as multiplication factors: k (kilo, thousand), M (mega, million), G (giga, billion), or T (tera, trillion). If undefined, 0 is used when OMP_WAIT_POLICY is PASSIVE, 300,000 is used when OMP_WAIT_POLICY is undefined and 30 billion is used when OMP_WAIT_POLICY is ACTIVE. If there are more OpenMP threads than available CPUs, 1000 and 100 spins are used for OMP_WAIT_POLICY being ACTIVE or undefined, respectively; unless the GOMP_SPINCOUNT is lower or OMP_WAIT_POLICY is PASSIVE.

See also:
     OMP_WAIT_POLICY

The libgomp ABI
***************

The following sections present notes on the external ABI as presented by libgomp.  Only maintainers should need them.

Implementing MASTER construct
=============================

     if (omp_get_thread_num () == 0)
       block

Alternately, we generate two copies of the parallel subfunction and only include this in the version run by the master thread. Surely this is not worthwhile though...

Implementing CRITICAL construct
===============================

Without a specified name,

       void GOMP_critical_start (void);
       void GOMP_critical_end (void);

so that we don't get COPY relocations from libgomp to the main application.

With a specified name, use omp_set_lock and omp_unset_lock with name being transformed into a variable declared like

       omp_lock_t gomp_critical_user_<name> __attribute__((common))

Ideally the ABI would specify that all zero is a valid unlocked state, and so we wouldn't need to initialize this at startup.

Implementing ATOMIC construct
=============================

The target should implement the __sync builtins.

Failing that we could add

       void GOMP_atomic_enter (void)
       void GOMP_atomic_exit (void)

which reuses the regular lock code, but with yet another lock object private to the library.

Implementing FLUSH construct
============================

Expands to the __sync_synchronize builtin.

Implementing BARRIER construct
==============================

       void GOMP_barrier (void)

Implementing THREADPRIVATE construct
====================================

In _most_ cases we can map this directly to __thread.  Except that OMP allows constructors for C++ objects.  We can either refuse to support this (how often is it used?) or we can implement something akin to .ctors.

Even more ideally, this ctor feature is handled by extensions to the main pthreads library.  Failing that, we can have a set of entry points to register ctor functions to be called.

Implementing PRIVATE clause
===========================

In association with a PARALLEL, or within the lexical extent of a PARALLEL block, the variable becomes a local variable in the parallel subfunction.

In association with FOR or SECTIONS blocks, create a new automatic variable within the current function.  This preserves the semantic of new variable creation.

Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses
====================================================================

This seems simple enough for PARALLEL blocks.  Create a private struct for communicating between the parent and subfunction. In the parent, copy in values for scalar and "small" structs; copy in addresses for others TREE_ADDRESSABLE types.  In the subfunction, copy the value into the local variable.

It is not clear what to do with bare FOR or SECTION blocks. The only thing I can figure is that we do something like:

     #pragma omp for firstprivate(x) lastprivate(y)
     for (int i = 0; i < n; ++i)
       body;

which becomes

     {
       int x = x, y;

       // for stuff

       if (i == n)
         y = y;
     }

where the "x=x" and "y=y" assignments actually have different uids for the two variables, i.e. not something you could write directly in C.  Presumably this only makes sense if the "outer" x and y are global variables.

COPYPRIVATE would work the same way, except the structure broadcast would have to happen via SINGLE machinery instead.

Implementing REDUCTION clause
=============================

The private struct mentioned in the previous section should have a pointer to an array of the type of the variable, indexed by the thread's TEAM_ID.  The thread stores its final value into the array, and after the barrier, the master thread iterates over the array to collect the values.

Implementing PARALLEL construct
===============================

       #pragma omp parallel
       {
         body;
       }

becomes

       void subfunction (void *data)
       {
         use data;
         body;
       }

       setup data;
       GOMP_parallel_start (subfunction, &data, num_threads);
       subfunction (&data);
       GOMP_parallel_end ();

       void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)

The FN argument is the subfunction to be run in parallel.

The DATA argument is a pointer to a structure used to communicate data in and out of the subfunction, as discussed above with respect to FIRSTPRIVATE et al.

The NUM_THREADS argument is 1 if an IF clause is present and false, or the value of the NUM_THREADS clause, if present, or 0.

The function needs to create the appropriate number of threads and/or launch them from the dock.  It needs to create the team structure and assign team ids.

       void GOMP_parallel_end (void)

Tears down the team and returns us to the previous omp_in_parallel() state.

Implementing FOR construct
==========================

       #pragma omp parallel for
       for (i = lb; i <= ub; i++)
         body;

becomes

       void subfunction (void *data)
       {
         long _s0, _e0;
         while (GOMP_loop_static_next (&_s0, &_e0))
         {
           long _e1 = _e0, i;
           for (i = _s0; i < _e1; i++)
             body;
         }
         GOMP_loop_end_nowait ();
       }

       GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);
       subfunction (NULL);
       GOMP_parallel_end ();

       #pragma omp for schedule(runtime)
       for (i = 0; i < n; i++)
         body;

becomes

       {
         long i, _s0, _e0;
         if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))
           do {
             long _e1 = _e0;
             for (i = _s0, i < _e0; i++)
               body;
           } while (GOMP_loop_runtime_next (&_s0, _&e0));
         GOMP_loop_end ();
       }

Note that while it looks like there is trickiness to propagating a non-constant STEP, there isn't really.  We're explicitly allowed to evaluate it as many times as we want, and any variables involved should automatically be handled as PRIVATE or SHARED like any other variables.  So the expression should remain evaluable in the subfunction.  We can also pull it into a local variable if we like, but since its supposed to remain unchanged, we can also not if we like.

If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be able to get away with no work-sharing context at all, since we can simply perform the arithmetic directly in each thread to divide up the iterations.  Which would mean that we wouldn't need to call any of these routines.

There are separate routines for handling loops with an ORDERED clause.  Bookkeeping for that is non-trivial...

Implementing ORDERED construct
==============================

       void GOMP_ordered_start (void)
       void GOMP_ordered_end (void)

Implementing SECTIONS construct
===============================

A block as

       #pragma omp sections
       {
         #pragma omp section
         stmt1;
         #pragma omp section
         stmt2;
         #pragma omp section
         stmt3;
       }

becomes

       for (i = GOMP_sections_start (3); i != 0; i = GOMP_sections_next ())
         switch (i)
           {
           case 1:
             stmt1;
             break;
           case 2:
             stmt2;
             break;
           case 3:
             stmt3;
             break;
           }
       GOMP_barrier ();

Implementing SINGLE construct
=============================

A block like

       #pragma omp single
       {
         body;
       }

becomes

       if (GOMP_single_start ())
         body;
       GOMP_barrier ();

while

       #pragma omp single copyprivate(x)
         body;

becomes

       datap = GOMP_single_copy_start ();
       if (datap == NULL)
         {
           body;
           data.x = x;
           GOMP_single_copy_end (&data);
         }
       else
         x = datap->x;
       GOMP_barrier ();

Reporting Bugs
**************

Bugs in the GNU Offloading and Multi Processing Runtime Library should be reported via Bugzilla (http://gcc.gnu.org/bugzilla/).  Please add "openacc", or "openmp", or both to the keywords field in the bug report, as appropriate.

     [file gpl_v3.texi not included]

     [file fdl.texi not included]

     [file funding.texi not included]