gcc/libgomp/libgomp.texi

\input texinfo @c -*-texinfo-*-

@c %**start of header
@setfilename libgomp.info
@settitle GNU libgomp
@c %**end of header


@copying
Copyright @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
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(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

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     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.
@end copying

@ifinfo
@dircategory GNU Libraries
@direntry
* libgomp: (libgomp).          GNU Offloading and Multi Processing Runtime Library.
@end direntry

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

@insertcopying
@end ifinfo


@setchapternewpage odd

@titlepage
@title GNU Offloading and Multi Processing Runtime Library
@subtitle The GNU OpenMP and OpenACC Implementation
@page
@vskip 0pt plus 1filll
@comment For the @value{version-GCC} Version*
@sp 1
Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor@*
Boston, MA 02110-1301, USA@*
@sp 1
@insertcopying
@end titlepage

@summarycontents
@contents
@page


@node Top
@top Introduction
@cindex Introduction

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



@comment
@comment  When you add a new menu item, please keep the right hand
@comment  aligned to the same column.  Do not use tabs.  This provides
@comment  better formatting.
@comment
@menu
* Enabling OpenMP::            How to enable OpenMP for your applications.
* Runtime Library Routines::   The OpenMP runtime application programming 
                               interface.
* Environment Variables::      Influencing runtime behavior with environment 
                               variables.
* The libgomp ABI::            Notes on the external ABI presented by libgomp.
* Reporting Bugs::             How to report bugs in the GNU Offloading and
                               Multi Processing Runtime Library.
* Copying::                    GNU general public license says
                               how you can copy and share libgomp.
* GNU Free Documentation License::
                               How you can copy and share this manual.
* Funding::                    How to help assure continued work for free 
                               software.
* Library Index::              Index of this documentation.
@end menu


@c ---------------------------------------------------------------------
@c Enabling OpenMP
@c ---------------------------------------------------------------------

@node Enabling OpenMP
@chapter Enabling OpenMP

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

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


@c ---------------------------------------------------------------------
@c Runtime Library Routines
@c ---------------------------------------------------------------------

@node Runtime Library Routines
@chapter 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:

@menu
Control threads, processors and the parallel environment.  They have C
linkage, and do not throw exceptions.

* omp_get_active_level::        Number of active parallel regions
* omp_get_ancestor_thread_num:: Ancestor thread ID
* omp_get_cancellation::        Whether cancellation support is enabled
* omp_get_default_device::      Get the default device for target regions
* omp_get_dynamic::             Dynamic teams setting
* omp_get_level::               Number of parallel regions
* omp_get_max_active_levels::   Maximum number of active regions
* omp_get_max_threads::         Maximum number of threads of parallel region
* omp_get_nested::              Nested parallel regions
* omp_get_num_devices::         Number of target devices
* omp_get_num_procs::           Number of processors online
* omp_get_num_teams::           Number of teams
* omp_get_num_threads::         Size of the active team
* omp_get_proc_bind::           Whether theads may be moved between CPUs
* omp_get_schedule::            Obtain the runtime scheduling method
* omp_get_team_num::            Get team number
* omp_get_team_size::           Number of threads in a team
* omp_get_thread_limit::        Maximum number of threads
* omp_get_thread_num::          Current thread ID
* omp_in_parallel::             Whether a parallel region is active
* omp_in_final::                Whether in final or included task region
* omp_is_initial_device::       Whether executing on the host device
* omp_set_default_device::      Set the default device for target regions
* omp_set_dynamic::             Enable/disable dynamic teams
* omp_set_max_active_levels::   Limits the number of active parallel regions
* omp_set_nested::              Enable/disable nested parallel regions
* omp_set_num_threads::         Set upper team size limit
* omp_set_schedule::            Set the runtime scheduling method

Initialize, set, test, unset and destroy simple and nested locks.

* omp_init_lock::            Initialize simple lock
* omp_set_lock::             Wait for and set simple lock
* omp_test_lock::            Test and set simple lock if available
* omp_unset_lock::           Unset simple lock
* omp_destroy_lock::         Destroy simple lock
* omp_init_nest_lock::       Initialize nested lock
* omp_set_nest_lock::        Wait for and set simple lock
* omp_test_nest_lock::       Test and set nested lock if available
* omp_unset_nest_lock::      Unset nested lock
* omp_destroy_nest_lock::    Destroy nested lock

Portable, thread-based, wall clock timer.

* omp_get_wtick::            Get timer precision.
* omp_get_wtime::            Elapsed wall clock time.
@end menu



@node omp_get_active_level
@section @code{omp_get_active_level} -- Number of parallel regions
@table @asis
@item @emph{Description}:
This function returns the nesting level for the active parallel blocks,
which enclose the calling call.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_active_level(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_active_level()}
@end multitable

@item @emph{See also}:
@ref{omp_get_level}, @ref{omp_get_max_active_levels}, @ref{omp_set_max_active_levels}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.20.
@end table



@node omp_get_ancestor_thread_num
@section @code{omp_get_ancestor_thread_num} -- Ancestor thread ID
@table @asis
@item @emph{Description}:
This function returns the thread identification number for the given
nesting level of the current thread.  For values of @var{level} outside
zero to @code{omp_get_level} -1 is returned; if @var{level} is
@code{omp_get_level} the result is identical to @code{omp_get_thread_num}.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_ancestor_thread_num(int level);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_ancestor_thread_num(level)}
@item                   @tab @code{integer level}
@end multitable

@item @emph{See also}:
@ref{omp_get_level}, @ref{omp_get_thread_num}, @ref{omp_get_team_size}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.18.
@end table



@node omp_get_cancellation
@section @code{omp_get_cancellation} -- Whether cancellation support is enabled
@table @asis
@item @emph{Description}:
This function returns @code{true} if cancellation is activated, @code{false}
otherwise.  Here, @code{true} and @code{false} represent their language-specific
counterparts.  Unless @env{OMP_CANCELLATION} is set true, cancellations are
deactivated.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_cancellation(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_get_cancellation()}
@end multitable

@item @emph{See also}:
@ref{OMP_CANCELLATION}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.9.
@end table



@node omp_get_default_device
@section @code{omp_get_default_device} -- Get the default device for target regions
@table @asis
@item @emph{Description}:
Get the default device for target regions without device clause.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_default_device(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_default_device()}
@end multitable

@item @emph{See also}:
@ref{OMP_DEFAULT_DEVICE}, @ref{omp_set_default_device}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.24.
@end table



@node omp_get_dynamic
@section @code{omp_get_dynamic} -- Dynamic teams setting
@table @asis
@item @emph{Description}:
This function returns @code{true} if enabled, @code{false} otherwise. 
Here, @code{true} and @code{false} represent their language-specific 
counterparts.

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

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_dynamic(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_get_dynamic()}
@end multitable

@item @emph{See also}:
@ref{omp_set_dynamic}, @ref{OMP_DYNAMIC}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.8.
@end table



@node omp_get_level
@section @code{omp_get_level} -- Obtain the current nesting level
@table @asis
@item @emph{Description}:
This function returns the nesting level for the parallel blocks,
which enclose the calling call.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_level(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_level()}
@end multitable

@item @emph{See also}:
@ref{omp_get_active_level}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.17.
@end table



@node omp_get_max_active_levels
@section @code{omp_get_max_active_levels} -- Maximum number of active regions
@table @asis
@item @emph{Description}:
This function obtains the maximum allowed number of nested, active parallel regions.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_max_active_levels(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_max_active_levels()}
@end multitable

@item @emph{See also}:
@ref{omp_set_max_active_levels}, @ref{omp_get_active_level}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.16.
@end table



@node omp_get_max_threads
@section @code{omp_get_max_threads} -- Maximum number of threads of parallel region
@table @asis
@item @emph{Description}:
Return the maximum number of threads used for the current parallel region
that does not use the clause @code{num_threads}.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_max_threads(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_max_threads()}
@end multitable

@item @emph{See also}:
@ref{omp_set_num_threads}, @ref{omp_set_dynamic}, @ref{omp_get_thread_limit}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.3.
@end table



@node omp_get_nested
@section @code{omp_get_nested} -- Nested parallel regions
@table @asis
@item @emph{Description}:
This function returns @code{true} if nested parallel regions are
enabled, @code{false} otherwise.  Here, @code{true} and @code{false}
represent their language-specific counterparts.

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

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_nested(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_get_nested()}
@end multitable

@item @emph{See also}:
@ref{omp_set_nested}, @ref{OMP_NESTED}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.11.
@end table



@node omp_get_num_devices
@section @code{omp_get_num_devices} -- Number of target devices
@table @asis
@item @emph{Description}:
Returns the number of target devices.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_num_devices(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_num_devices()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.25.
@end table



@node omp_get_num_procs
@section @code{omp_get_num_procs} -- Number of processors online
@table @asis
@item @emph{Description}:
Returns the number of processors online on that device.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_num_procs(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_num_procs()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.5.
@end table



@node omp_get_num_teams
@section @code{omp_get_num_teams} -- Number of teams
@table @asis
@item @emph{Description}:
Returns the number of teams in the current team region.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_num_teams(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_num_teams()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.26.
@end table



@node omp_get_num_threads
@section @code{omp_get_num_threads} -- Size of the active team
@table @asis
@item @emph{Description}:
Returns the number of threads in the current team.  In a sequential section of
the program @code{omp_get_num_threads} returns 1.

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

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_num_threads(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_num_threads()}
@end multitable

@item @emph{See also}:
@ref{omp_get_max_threads}, @ref{omp_set_num_threads}, @ref{OMP_NUM_THREADS}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.2.
@end table



@node omp_get_proc_bind
@section @code{omp_get_proc_bind} -- Whether theads may be moved between CPUs
@table @asis
@item @emph{Description}:
This functions returns the currently active thread affinity policy, which is
set via @env{OMP_PROC_BIND}.  Possible values are @code{omp_proc_bind_false},
@code{omp_proc_bind_true}, @code{omp_proc_bind_master},
@code{omp_proc_bind_close} and @code{omp_proc_bind_spread}.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{omp_proc_bind_t omp_get_proc_bind(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer(kind=omp_proc_bind_kind) function omp_get_proc_bind()}
@end multitable

@item @emph{See also}:
@ref{OMP_PROC_BIND}, @ref{OMP_PLACES}, @ref{GOMP_CPU_AFFINITY},

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.22.
@end table



@node omp_get_schedule
@section @code{omp_get_schedule} -- Obtain the runtime scheduling method
@table @asis
@item @emph{Description}:
Obtain the runtime scheduling method.  The @var{kind} argument will be
set to the value @code{omp_sched_static}, @code{omp_sched_dynamic},
@code{omp_sched_guided} or @code{omp_sched_auto}.  The second argument,
@var{modifier}, is set to the chunk size.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_get_schedule(omp_sched_t *kind, int *modifier);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_get_schedule(kind, modifier)}
@item                   @tab @code{integer(kind=omp_sched_kind) kind}
@item                   @tab @code{integer modifier}
@end multitable

@item @emph{See also}:
@ref{omp_set_schedule}, @ref{OMP_SCHEDULE}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.13.
@end table



@node omp_get_team_num
@section @code{omp_get_team_num} -- Get team number
@table @asis
@item @emph{Description}:
Returns the team number of the calling thread.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_team_num(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_team_num()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.27.
@end table



@node omp_get_team_size
@section @code{omp_get_team_size} -- Number of threads in a team
@table @asis
@item @emph{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 @var{level}
outside zero to @code{omp_get_level}, -1 is returned; if @var{level} is zero,
1 is returned, and for @code{omp_get_level}, the result is identical
to @code{omp_get_num_threads}.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_team_size(int level);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_team_size(level)}
@item                   @tab @code{integer level}
@end multitable

@item @emph{See also}:
@ref{omp_get_num_threads}, @ref{omp_get_level}, @ref{omp_get_ancestor_thread_num}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.19.
@end table



@node omp_get_thread_limit
@section @code{omp_get_thread_limit} -- Maximum number of threads
@table @asis
@item @emph{Description}:
Return the maximum number of threads of the program.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_thread_limit(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_thread_limit()}
@end multitable

@item @emph{See also}:
@ref{omp_get_max_threads}, @ref{OMP_THREAD_LIMIT}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.14.
@end table



@node omp_get_thread_num
@section @code{omp_get_thread_num} -- Current thread ID
@table @asis
@item @emph{Description}:
Returns a unique thread identification number within the current team.
In a sequential parts of the program, @code{omp_get_thread_num}
always returns 0.  In parallel regions the return value varies
from 0 to @code{omp_get_num_threads}-1 inclusive.  The return
value of the master thread of a team is always 0.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_get_thread_num(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{integer function omp_get_thread_num()}
@end multitable

@item @emph{See also}:
@ref{omp_get_num_threads}, @ref{omp_get_ancestor_thread_num}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.4.
@end table



@node omp_in_parallel
@section @code{omp_in_parallel} -- Whether a parallel region is active
@table @asis
@item @emph{Description}:
This function returns @code{true} if currently running in parallel,
@code{false} otherwise.  Here, @code{true} and @code{false} represent
their language-specific counterparts.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_in_parallel(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_in_parallel()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.6.
@end table


@node omp_in_final
@section @code{omp_in_final} -- Whether in final or included task region
@table @asis
@item @emph{Description}:
This function returns @code{true} if currently running in a final
or included task region, @code{false} otherwise.  Here, @code{true}
and @code{false} represent their language-specific counterparts.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_in_final(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_in_final()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.21.
@end table



@node omp_is_initial_device
@section @code{omp_is_initial_device} -- Whether executing on the host device
@table @asis
@item @emph{Description}:
This function returns @code{true} if currently running on the host device,
@code{false} otherwise.  Here, @code{true} and @code{false} represent
their language-specific counterparts.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_is_initial_device(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_is_initial_device()}
@end multitable

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.28.
@end table



@node omp_set_default_device
@section @code{omp_set_default_device} -- Set the default device for target regions
@table @asis
@item @emph{Description}:
Set the default device for target regions without device clause.  The argument
shall be a nonnegative device number.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_default_device(int device_num);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_default_device(device_num)}
@item                   @tab @code{integer device_num}
@end multitable

@item @emph{See also}:
@ref{OMP_DEFAULT_DEVICE}, @ref{omp_get_default_device}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.23.
@end table



@node omp_set_dynamic
@section @code{omp_set_dynamic} -- Enable/disable dynamic teams
@table @asis
@item @emph{Description}:
Enable or disable the dynamic adjustment of the number of threads 
within a team.  The function takes the language-specific equivalent
of @code{true} and @code{false}, where @code{true} enables dynamic 
adjustment of team sizes and @code{false} disables it.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_dynamic(int dynamic_threads);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_dynamic(dynamic_threads)}
@item                   @tab @code{logical, intent(in) :: dynamic_threads}
@end multitable

@item @emph{See also}:
@ref{OMP_DYNAMIC}, @ref{omp_get_dynamic}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.7.
@end table



@node omp_set_max_active_levels
@section @code{omp_set_max_active_levels} -- Limits the number of active parallel regions
@table @asis
@item @emph{Description}:
This function limits the maximum allowed number of nested, active
parallel regions.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_max_active_levels(int max_levels);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_max_active_levels(max_levels)}
@item                   @tab @code{integer max_levels}
@end multitable

@item @emph{See also}:
@ref{omp_get_max_active_levels}, @ref{omp_get_active_level}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.15.
@end table



@node omp_set_nested
@section @code{omp_set_nested} -- Enable/disable nested parallel regions
@table @asis
@item @emph{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 @code{true} and @code{false}, where @code{true} enables 
dynamic adjustment of team sizes and @code{false} disables it.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_nested(int nested);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_nested(nested)}
@item                   @tab @code{logical, intent(in) :: nested}
@end multitable

@item @emph{See also}:
@ref{OMP_NESTED}, @ref{omp_get_nested}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.10.
@end table



@node omp_set_num_threads
@section @code{omp_set_num_threads} -- Set upper team size limit
@table @asis
@item @emph{Description}:
Specifies the number of threads used by default in subsequent parallel 
sections, if those do not specify a @code{num_threads} clause.  The
argument of @code{omp_set_num_threads} shall be a positive integer.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_num_threads(int num_threads);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_num_threads(num_threads)}
@item                   @tab @code{integer, intent(in) :: num_threads}
@end multitable

@item @emph{See also}:
@ref{OMP_NUM_THREADS}, @ref{omp_get_num_threads}, @ref{omp_get_max_threads}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.1.
@end table



@node omp_set_schedule
@section @code{omp_set_schedule} -- Set the runtime scheduling method
@table @asis
@item @emph{Description}:
Sets the runtime scheduling method.  The @var{kind} argument can have the
value @code{omp_sched_static}, @code{omp_sched_dynamic},
@code{omp_sched_guided} or @code{omp_sched_auto}.  Except for
@code{omp_sched_auto}, the chunk size is set to the value of
@var{modifier} if positive, or to the default value if zero or negative.
For @code{omp_sched_auto} the @var{modifier} argument is ignored.

@item @emph{C/C++}
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_schedule(omp_sched_t kind, int modifier);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_schedule(kind, modifier)}
@item                   @tab @code{integer(kind=omp_sched_kind) kind}
@item                   @tab @code{integer modifier}
@end multitable

@item @emph{See also}:
@ref{omp_get_schedule}
@ref{OMP_SCHEDULE}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.2.12.
@end table



@node omp_init_lock
@section @code{omp_init_lock} -- Initialize simple lock
@table @asis
@item @emph{Description}:
Initialize a simple lock.  After initialization, the lock is in
an unlocked state.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_init_lock(omp_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_init_lock(svar)}
@item                   @tab @code{integer(omp_lock_kind), intent(out) :: svar}
@end multitable

@item @emph{See also}:
@ref{omp_destroy_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.1.
@end table



@node omp_set_lock
@section @code{omp_set_lock} -- Wait for and set simple lock
@table @asis
@item @emph{Description}:
Before setting a simple lock, the lock variable must be initialized by 
@code{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_lock(omp_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_lock(svar)}
@item                   @tab @code{integer(omp_lock_kind), intent(inout) :: svar}
@end multitable

@item @emph{See also}:
@ref{omp_init_lock}, @ref{omp_test_lock}, @ref{omp_unset_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.3.
@end table



@node omp_test_lock
@section @code{omp_test_lock} -- Test and set simple lock if available
@table @asis
@item @emph{Description}:
Before setting a simple lock, the lock variable must be initialized by 
@code{omp_init_lock}.  Contrary to @code{omp_set_lock}, @code{omp_test_lock} 
does not block if the lock is not available.  This function returns
@code{true} upon success, @code{false} otherwise.  Here, @code{true} and
@code{false} represent their language-specific counterparts.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_test_lock(omp_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_test_lock(svar)}
@item                   @tab @code{integer(omp_lock_kind), intent(inout) :: svar}
@end multitable

@item @emph{See also}:
@ref{omp_init_lock}, @ref{omp_set_lock}, @ref{omp_set_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.5.
@end table



@node omp_unset_lock
@section @code{omp_unset_lock} -- Unset simple lock
@table @asis
@item @emph{Description}:
A simple lock about to be unset must have been locked by @code{omp_set_lock}
or @code{omp_test_lock} before.  In addition, the lock must be held by the
thread calling @code{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_unset_lock(omp_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_unset_lock(svar)}
@item                   @tab @code{integer(omp_lock_kind), intent(inout) :: svar}
@end multitable

@item @emph{See also}:
@ref{omp_set_lock}, @ref{omp_test_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.4.
@end table



@node omp_destroy_lock
@section @code{omp_destroy_lock} -- Destroy simple lock
@table @asis
@item @emph{Description}:
Destroy a simple lock.  In order to be destroyed, a simple lock must be
in the unlocked state. 

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_destroy_lock(omp_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_destroy_lock(svar)}
@item                   @tab @code{integer(omp_lock_kind), intent(inout) :: svar}
@end multitable

@item @emph{See also}:
@ref{omp_init_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.2.
@end table



@node omp_init_nest_lock
@section @code{omp_init_nest_lock} -- Initialize nested lock
@table @asis
@item @emph{Description}:
Initialize a nested lock.  After initialization, the lock is in
an unlocked state and the nesting count is set to zero.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_init_nest_lock(omp_nest_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_init_nest_lock(nvar)}
@item                   @tab @code{integer(omp_nest_lock_kind), intent(out) :: nvar}
@end multitable

@item @emph{See also}:
@ref{omp_destroy_nest_lock}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.1.
@end table


@node omp_set_nest_lock
@section @code{omp_set_nest_lock} -- Wait for and set nested lock
@table @asis
@item @emph{Description}:
Before setting a nested lock, the lock variable must be initialized by 
@code{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_set_nest_lock(omp_nest_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_set_nest_lock(nvar)}
@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: nvar}
@end multitable

@item @emph{See also}:
@ref{omp_init_nest_lock}, @ref{omp_unset_nest_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.3.
@end table



@node omp_test_nest_lock
@section @code{omp_test_nest_lock} -- Test and set nested lock if available
@table @asis
@item @emph{Description}:
Before setting a nested lock, the lock variable must be initialized by 
@code{omp_init_nest_lock}.  Contrary to @code{omp_set_nest_lock},
@code{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{int omp_test_nest_lock(omp_nest_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{logical function omp_test_nest_lock(nvar)}
@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: nvar}
@end multitable


@item @emph{See also}:
@ref{omp_init_lock}, @ref{omp_set_lock}, @ref{omp_set_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.5.
@end table



@node omp_unset_nest_lock
@section @code{omp_unset_nest_lock} -- Unset nested lock
@table @asis
@item @emph{Description}:
A nested lock about to be unset must have been locked by @code{omp_set_nested_lock}
or @code{omp_test_nested_lock} before.  In addition, the lock must be held by the
thread calling @code{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_unset_nest_lock(omp_nest_lock_t *lock);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_unset_nest_lock(nvar)}
@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: nvar}
@end multitable

@item @emph{See also}:
@ref{omp_set_nest_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.4.
@end table



@node omp_destroy_nest_lock
@section @code{omp_destroy_nest_lock} -- Destroy nested lock
@table @asis
@item @emph{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{void omp_destroy_nest_lock(omp_nest_lock_t *);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{subroutine omp_destroy_nest_lock(nvar)}
@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: nvar}
@end multitable

@item @emph{See also}:
@ref{omp_init_lock}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.3.2.
@end table



@node omp_get_wtick
@section @code{omp_get_wtick} -- Get timer precision
@table @asis
@item @emph{Description}:
Gets the timer precision, i.e., the number of seconds between two 
successive clock ticks.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{double omp_get_wtick(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{double precision function omp_get_wtick()}
@end multitable

@item @emph{See also}:
@ref{omp_get_wtime}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.4.2.
@end table



@node omp_get_wtime
@section @code{omp_get_wtime} -- Elapsed wall clock time
@table @asis
@item @emph{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.

@item @emph{C/C++}:
@multitable @columnfractions .20 .80
@item @emph{Prototype}: @tab @code{double omp_get_wtime(void);}
@end multitable

@item @emph{Fortran}:
@multitable @columnfractions .20 .80
@item @emph{Interface}: @tab @code{double precision function omp_get_wtime()}
@end multitable

@item @emph{See also}:
@ref{omp_get_wtick}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 3.4.1.
@end table



@c ---------------------------------------------------------------------
@c Environment Variables
@c ---------------------------------------------------------------------

@node Environment Variables
@chapter Environment Variables

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

@menu
* OMP_CANCELLATION::      Set whether cancellation is activated
* OMP_DISPLAY_ENV::       Show OpenMP version and environment variables
* OMP_DEFAULT_DEVICE::    Set the device used in target regions
* OMP_DYNAMIC::           Dynamic adjustment of threads
* OMP_MAX_ACTIVE_LEVELS:: Set the maximum number of nested parallel regions
* OMP_NESTED::            Nested parallel regions
* OMP_NUM_THREADS::       Specifies the number of threads to use
* OMP_PROC_BIND::         Whether theads may be moved between CPUs
* OMP_PLACES::            Specifies on which CPUs the theads should be placed
* OMP_STACKSIZE::         Set default thread stack size
* OMP_SCHEDULE::          How threads are scheduled
* OMP_THREAD_LIMIT::      Set the maximum number of threads
* OMP_WAIT_POLICY::       How waiting threads are handled
* GOMP_CPU_AFFINITY::     Bind threads to specific CPUs
* GOMP_DEBUG::            Enable debugging output
* GOMP_STACKSIZE::        Set default thread stack size
* GOMP_SPINCOUNT::        Set the busy-wait spin count
@end menu


@node OMP_CANCELLATION
@section @env{OMP_CANCELLATION} -- Set whether cancellation is activated
@cindex Environment Variable
@table @asis
@item @emph{Description}:
If set to @code{TRUE}, the cancellation is activated.  If set to @code{FALSE} or
if unset, cancellation is disabled and the @code{cancel} construct is ignored.

@item @emph{See also}:
@ref{omp_get_cancellation}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.11
@end table



@node OMP_DISPLAY_ENV
@section @env{OMP_DISPLAY_ENV} -- Show OpenMP version and environment variables
@cindex Environment Variable
@table @asis
@item @emph{Description}:
If set to @code{TRUE}, the OpenMP version number and the values
associated with the OpenMP environment variables are printed to @code{stderr}.
If set to @code{VERBOSE}, it additionally shows the value of the environment
variables which are GNU extensions.  If undefined or set to @code{FALSE},
this information will not be shown.


@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.12
@end table



@node OMP_DEFAULT_DEVICE
@section @env{OMP_DEFAULT_DEVICE} -- Set the device used in target regions
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Set to choose the device which is used in a @code{target} region, unless the
value is overridden by @code{omp_set_default_device} or by a @code{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.


@item @emph{See also}:
@ref{omp_get_default_device}, @ref{omp_set_default_device},

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.11
@end table



@node OMP_DYNAMIC
@section @env{OMP_DYNAMIC} -- Dynamic adjustment of threads
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Enable or disable the dynamic adjustment of the number of threads 
within a team.  The value of this environment variable shall be 
@code{TRUE} or @code{FALSE}.  If undefined, dynamic adjustment is
disabled by default.

@item @emph{See also}:
@ref{omp_set_dynamic}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.3
@end table



@node OMP_MAX_ACTIVE_LEVELS
@section @env{OMP_MAX_ACTIVE_LEVELS} -- Set the maximum number of nested parallel regions
@cindex Environment Variable
@table @asis
@item @emph{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.

@item @emph{See also}:
@ref{omp_set_max_active_levels}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.9
@end table



@node OMP_NESTED
@section @env{OMP_NESTED} -- Nested parallel regions
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{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 @code{TRUE} or @code{FALSE}.  If undefined, nested parallel 
regions are disabled by default.

@item @emph{See also}:
@ref{omp_set_nested}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.6
@end table



@node OMP_NUM_THREADS
@section @env{OMP_NUM_THREADS} -- Specifies the number of threads to use
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{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.

@item @emph{See also}:
@ref{omp_set_num_threads}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.2
@end table



@node OMP_PROC_BIND
@section @env{OMP_PROC_BIND} -- Whether theads may be moved between CPUs
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Specifies whether threads may be moved between processors.  If set to
@code{TRUE}, OpenMP theads should not be moved; if set to @code{FALSE}
they may be moved.  Alternatively, a comma separated list with the
values @code{MASTER}, @code{CLOSE} and @code{SPREAD} can be used to specify
the thread affinity policy for the corresponding nesting level.  With
@code{MASTER} the worker threads are in the same place partition as the
master thread.  With @code{CLOSE} those are kept close to the master thread
in contiguous place partitions.  And with @code{SPREAD} a sparse distribution
across the place partitions is used.

When undefined, @env{OMP_PROC_BIND} defaults to @code{TRUE} when
@env{OMP_PLACES} or @env{GOMP_CPU_AFFINITY} is set and @code{FALSE} otherwise.

@item @emph{See also}:
@ref{OMP_PLACES}, @ref{GOMP_CPU_AFFINITY}, @ref{omp_get_proc_bind}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.4
@end table



@node OMP_PLACES
@section @env{OMP_PLACES} -- Specifies on which CPUs the theads should be placed
@cindex Environment Variable
@table @asis
@item @emph{Description}:
The thread placement can be either specified using an abstract name or by an
explicit list of the places.  The abstract names @code{threads}, @code{cores}
and @code{sockets} can be optionally followed by a positive number in
parentheses, which denotes the how many places shall be created.  With
@code{threads} each place corresponds to a single hardware thread; @code{cores}
to a single core with the corresponding number of hardware threads; and with
@code{sockets} the place corresponds to a single socket.  The resulting
placement can be shown by setting the @env{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:
@code{"@{0,1,2@}, @{3,4,6@}, @{7,8,9@}, @{10,11,12@}"};
@code{"@{0:3@}, @{3:3@}, @{7:3@}, @{10:3@}"}; and @code{"@{0:2@}:4:3"}.

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

@item @emph{See also}:
@ref{OMP_PROC_BIND}, @ref{GOMP_CPU_AFFINITY}, @ref{omp_get_proc_bind},
@ref{OMP_DISPLAY_ENV}

@item @emph{Reference}:
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.5
@end table



@node OMP_STACKSIZE
@section @env{OMP_STACKSIZE} -- Set default thread stack size
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Set the default thread stack size in kilobytes, unless the number
is suffixed by @code{B}, @code{K}, @code{M} or @code{G}, in which
case the size is, respectively, in bytes, kilobytes, megabytes
or gigabytes.  This is different from @code{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.

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.7
@end table



@node OMP_SCHEDULE
@section @env{OMP_SCHEDULE} -- How threads are scheduled
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{Description}:
Allows to specify @code{schedule type} and @code{chunk size}. 
The value of the variable shall have the form: @code{type[,chunk]} where
@code{type} is one of @code{static}, @code{dynamic}, @code{guided} or @code{auto}
The optional @code{chunk} size shall be a positive integer.  If undefined,
dynamic scheduling and a chunk size of 1 is used.

@item @emph{See also}:
@ref{omp_set_schedule}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Sections 2.7.1 and 4.1
@end table



@node OMP_THREAD_LIMIT
@section @env{OMP_THREAD_LIMIT} -- Set the maximum number of threads
@cindex Environment Variable
@table @asis
@item @emph{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.

@item @emph{See also}:
@ref{OMP_NUM_THREADS}, @ref{omp_get_thread_limit}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.10
@end table



@node OMP_WAIT_POLICY
@section @env{OMP_WAIT_POLICY} -- How waiting threads are handled
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Specifies whether waiting threads should be active or passive.  If
the value is @code{PASSIVE}, waiting threads should not consume CPU
power while waiting; while the value is @code{ACTIVE} specifies that
they should.  If undefined, threads wait actively for a short time
before waiting passively.

@item @emph{See also}:
@ref{GOMP_SPINCOUNT}

@item @emph{Reference}: 
@uref{http://www.openmp.org/, OpenMP specification v4.0}, Section 4.8
@end table



@node GOMP_CPU_AFFINITY
@section @env{GOMP_CPU_AFFINITY} -- Bind threads to specific CPUs
@cindex Environment Variable
@table @asis
@item @emph{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,
@code{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.  @code{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., @code{getenv} in C or @code{GET_ENVIRONMENT_VARIABLE} in 
Fortran, may be used to query the setting of the @code{GOMP_CPU_AFFINITY} 
environment variable.  A defined CPU affinity on startup cannot be changed 
or disabled during the runtime of the application.

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

@item @emph{See also}:
@ref{OMP_PLACES}, @ref{OMP_PROC_BIND}
@end table



@node GOMP_DEBUG
@section @env{GOMP_DEBUG} -- Enable debugging output
@cindex Environment Variable
@table @asis
@item @emph{Description}:
Enable debugging output.  The variable should be set to @code{0}
(disabled, also the default if not set), or @code{1} (enabled).

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



@node GOMP_STACKSIZE
@section @env{GOMP_STACKSIZE} -- Set default thread stack size
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{Description}:
Set the default thread stack size in kilobytes.  This is different from
@code{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.

@item @emph{See also}:
@ref{OMP_STACKSIZE}

@item @emph{Reference}: 
@uref{http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00493.html, 
GCC Patches Mailinglist}, 
@uref{http://gcc.gnu.org/ml/gcc-patches/2006-06/msg00496.html,
GCC Patches Mailinglist}
@end table



@node GOMP_SPINCOUNT
@section @env{GOMP_SPINCOUNT} -- Set the busy-wait spin count
@cindex Environment Variable
@cindex Implementation specific setting
@table @asis
@item @emph{Description}:
Determines how long a threads waits actively with consuming CPU power
before waiting passively without consuming CPU power.  The value may be
either @code{INFINITE}, @code{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: @code{k} (kilo, thousand), @code{M} (mega,
million), @code{G} (giga, billion), or @code{T} (tera, trillion).
If undefined, 0 is used when @env{OMP_WAIT_POLICY} is @code{PASSIVE},
300,000 is used when @env{OMP_WAIT_POLICY} is undefined and
30 billion is used when @env{OMP_WAIT_POLICY} is @code{ACTIVE}.
If there are more OpenMP threads than available CPUs, 1000 and 100
spins are used for @env{OMP_WAIT_POLICY} being @code{ACTIVE} or
undefined, respectively; unless the @env{GOMP_SPINCOUNT} is lower
or @env{OMP_WAIT_POLICY} is @code{PASSIVE}.

@item @emph{See also}:
@ref{OMP_WAIT_POLICY}
@end table



@c ---------------------------------------------------------------------
@c The libgomp ABI
@c ---------------------------------------------------------------------

@node The libgomp ABI
@chapter The libgomp ABI

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

@menu
* Implementing MASTER construct::
* Implementing CRITICAL construct::
* Implementing ATOMIC construct::
* Implementing FLUSH construct::
* Implementing BARRIER construct::
* Implementing THREADPRIVATE construct::
* Implementing PRIVATE clause::
* Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses::
* Implementing REDUCTION clause::
* Implementing PARALLEL construct::
* Implementing FOR construct::
* Implementing ORDERED construct::
* Implementing SECTIONS construct::
* Implementing SINGLE construct::
@end menu


@node Implementing MASTER construct
@section Implementing MASTER construct

@smallexample
if (omp_get_thread_num () == 0)
  block
@end smallexample

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...



@node Implementing CRITICAL construct
@section Implementing CRITICAL construct

Without a specified name,

@smallexample
  void GOMP_critical_start (void);
  void GOMP_critical_end (void);
@end smallexample

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

@smallexample
  omp_lock_t gomp_critical_user_<name> __attribute__((common))
@end smallexample

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



@node Implementing ATOMIC construct
@section Implementing ATOMIC construct

The target should implement the @code{__sync} builtins.

Failing that we could add

@smallexample
  void GOMP_atomic_enter (void)
  void GOMP_atomic_exit (void)
@end smallexample

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



@node Implementing FLUSH construct
@section Implementing FLUSH construct

Expands to the @code{__sync_synchronize} builtin.



@node Implementing BARRIER construct
@section Implementing BARRIER construct

@smallexample
  void GOMP_barrier (void)
@end smallexample


@node Implementing THREADPRIVATE construct
@section Implementing THREADPRIVATE construct

In _most_ cases we can map this directly to @code{__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.



@node Implementing PRIVATE clause
@section 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.



@node Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses
@section 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:

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

which becomes

@smallexample
@{
  int x = x, y;

  // for stuff

  if (i == n)
    y = y;
@}
@end smallexample

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.



@node Implementing REDUCTION clause
@section 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 @var{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.


@node Implementing PARALLEL construct
@section Implementing PARALLEL construct

@smallexample
  #pragma omp parallel
  @{
    body;
  @}
@end smallexample

becomes

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

  setup data;
  GOMP_parallel_start (subfunction, &data, num_threads);
  subfunction (&data);
  GOMP_parallel_end ();
@end smallexample

@smallexample
  void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)
@end smallexample

The @var{FN} argument is the subfunction to be run in parallel.

The @var{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 @var{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.

@smallexample
  void GOMP_parallel_end (void)
@end smallexample

Tears down the team and returns us to the previous @code{omp_in_parallel()} state.



@node Implementing FOR construct
@section Implementing FOR construct

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

becomes

@smallexample
  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 ();
@end smallexample

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

becomes

@smallexample
  @{
    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 ();
  @}
@end smallexample

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...



@node Implementing ORDERED construct
@section Implementing ORDERED construct

@smallexample
  void GOMP_ordered_start (void)
  void GOMP_ordered_end (void)
@end smallexample



@node Implementing SECTIONS construct
@section Implementing SECTIONS construct

A block as 

@smallexample
  #pragma omp sections
  @{
    #pragma omp section
    stmt1;
    #pragma omp section
    stmt2;
    #pragma omp section
    stmt3;
  @}
@end smallexample

becomes

@smallexample
  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 ();
@end smallexample


@node Implementing SINGLE construct
@section Implementing SINGLE construct

A block like 

@smallexample
  #pragma omp single
  @{
    body;
  @}
@end smallexample

becomes

@smallexample
  if (GOMP_single_start ())
    body;
  GOMP_barrier ();
@end smallexample

while 

@smallexample
  #pragma omp single copyprivate(x)
    body;
@end smallexample

becomes

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



@c ---------------------------------------------------------------------
@c Reporting Bugs
@c ---------------------------------------------------------------------

@node Reporting Bugs
@chapter Reporting Bugs

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



@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------

@include gpl_v3.texi



@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------

@include fdl.texi



@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------

@include funding.texi

@c ---------------------------------------------------------------------
@c Index
@c ---------------------------------------------------------------------

@node Library Index
@unnumbered Library Index

@printindex cp

@bye