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

@c %**start of header

@setfilename libgomp.info

@settitle GNU libgomp

@c %**end of header

@copying

Copyright @copyright{} 2006 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 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being ``GNU General Public License'' and ``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.

@end copying

@ifinfo

@dircategory GNU Libraries

@direntry

* libgomp: (libgomp).                    GNU OpenMP runtime library

@end direntry

This manual documents the GNU implementation of the OpenMP API for

multi-platform shared-memory parallel 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 The GNU OpenMP 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 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.

@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 GNU OpenMP.

* 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.

* 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 2.5.

@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

specifications in version 2.5.

Control threads, processors and the parallel environment.

@menu

* omp_get_dynamic::          Dynamic teams setting

* omp_get_max_threads::      Maximum number of threads

* omp_get_nested::           Nested parallel regions

* omp_get_num_procs::        Number of processors online

* omp_get_num_threads::      Size of the active team

* omp_get_thread_num::       Current thread ID

* omp_in_parallel::          Whether a parallel region is active

* omp_set_dynamic::          Enable/disable dynamic teams

* omp_set_nested::           Enable/disable nested parallel regions

* omp_set_num_threads::      Set upper team size limit

@end menu

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

@menu

* 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

@end menu

Portable, thread-based, wall clock timer.

@menu

* omp_get_wtick::            Get timer precision.

* omp_get_wtime::            Elapsed wall clock time.

@end menu

@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

@code{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();}

@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 specifications v2.5}, section 3.2.8.

@end table

@node omp_get_max_threads

@section @code{omp_get_max_threads} -- Maximum number of threads

@table @asis

@item @emph{Description}:

Return the maximum number of threads used for parallel regions that do

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();}

@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}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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

@code{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();}

@end multitable

@item @emph{Fortran}:

@multitable @columnfractions .20 .80

@item @emph{Interface}: @tab @code{integer 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 specifications v2.5}, section 3.2.10.

@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.

@item @emph{C/C++}:

@multitable @columnfractions .20 .80

@item @emph{Prototype}: @tab @code{int omp_get_num_procs();}

@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 specifications v2.5}, section 3.2.5.

@end table

@node omp_get_num_threads

@section @code{omp_get_num_threads} -- Size of the active team

@table @asis

@item @emph{Description}:

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

@code{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 @code{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();}

@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 specifications v2.5}, section 3.2.2.

@end table

@node omp_get_thread_num

@section @code{omp_get_thread_num} -- Current thread ID

@table @asis

@item @emph{Description}:

Unique thread identification number. 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_max_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();}

@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_max_threads}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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();}

@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 specifications v2.5}, section 3.2.6.

@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);}

@end multitable

@item @emph{Fortran}:

@multitable @columnfractions .20 .80

@item @emph{Interface}: @tab @code{subroutine omp_set_dynamic(set)}

@item                   @tab @code{integer, intent(in) :: set}

@end multitable

@item @emph{See also}:

@ref{OMP_DYNAMIC}, @ref{omp_get_dynamic}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, section 3.2.7.

@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_dynamic(int);}

@end multitable

@item @emph{Fortran}:

@multitable @columnfractions .20 .80

@item @emph{Interface}: @tab @code{subroutine omp_set_dynamic(set)}

@item                   @tab @code{integer, intent(in) :: set}

@end multitable

@item @emph{See also}:

@ref{OMP_NESTED}, @ref{omp_get_nested}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, section 3.2.9.

@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);}

@end multitable

@item @emph{Fortran}:

@multitable @columnfractions .20 .80

@item @emph{Interface}: @tab @code{subroutine omp_set_num_threads(set)}

@item                   @tab @code{integer, intent(in) :: set}

@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 specifications v2.5}, section 3.2.1.

@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(lock)}

@item                   @tab @code{integer(omp_lock_kind), intent(out) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_destroy_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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(lock)}

@item                   @tab @code{integer(omp_lock_kind), intent(out) :: lock}

@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 specifications v2.5}, 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{subroutine omp_test_lock(lock)}

@item                   @tab @code{logical(omp_logical_kind) :: omp_test_lock}

@item                   @tab @code{integer(omp_lock_kind), intent(out) :: lock}

@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 specifications v2.5}, 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

ore more threads attempted to set the lock before, one of them is chosen to,

again, set the lock for 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(lock)}

@item                   @tab @code{integer(omp_lock_kind), intent(out) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_set_lock}, @ref{omp_test_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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 *);}

@end multitable

@item @emph{Fortran}:

@multitable @columnfractions .20 .80

@item @emph{Interface}: @tab @code{subroutine omp_destroy_lock(lock)}

@item                   @tab @code{integer(omp_lock_kind), intent(inout) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_init_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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(lock)}

@item                   @tab @code{integer(omp_nest_lock_kind), intent(out) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_destroy_nest_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, section 3.3.1.

@end table

@node omp_set_nest_lock

@section @code{omp_set_nest_lock} -- Wait for and set simple 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 in 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(lock)}

@item                   @tab @code{integer(omp_nest_lock_kind), intent(out) :: lock}

@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 specifications v2.5}, 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{integer function omp_test_nest_lock(lock)}

@item                   @tab @code{integer(omp_integer_kind) :: omp_test_nest_lock}

@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: lock}

@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 specifications v2.5}, 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 for 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(lock)}

@item                   @tab @code{integer(omp_nest_lock_kind), intent(out) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_set_nest_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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(lock)}

@item                   @tab @code{integer(omp_nest_lock_kind), intent(inout) :: lock}

@end multitable

@item @emph{See also}:

@ref{omp_init_lock}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, 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();}

@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 specifications v2.5}, 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 bee made that two distinct threads measure the same time.

Time is measured from some "time in the past". On POSIX compliant systems

the seconds since the Epoch (00:00:00 UTC, January 1, 1970) are returned.

@item @emph{C/C++}:

@multitable @columnfractions .20 .80

@item @emph{Prototype}: @tab @code{double omp_get_wtime();}

@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 specifications v2.5}, section 3.4.1.

@end table

@c ---------------------------------------------------------------------

@c Environment Variables

@c ---------------------------------------------------------------------

@node Environment Variables

@chapter Environment Variables

The variables @env{OMP_DYNAMIC}, @env{OMP_NESTED}, @env{OMP_NUM_THREADS} and

@env{OMP_SCHEDULE} are defined by section 4 of the OpenMP specifications in

version 2.5, while @env{GOMP_CPU_AFFINITY} and @env{GOMP_STACKSIZE} are GNU

extensions.

@menu

* OMP_DYNAMIC::        Dynamic adjustment of threads

* OMP_NESTED::         Nested parallel regions

* OMP_NUM_THREADS::    Specifies the number of threads to use

* OMP_SCHEDULE::       How threads are scheduled

* GOMP_CPU_AFFINITY::  Bind threads to specific CPUs

* GOMP_STACKSIZE::     Set default thread stack size

@end menu

@node OMP_DYNAMIC

@section @env{OMP_DYNAMIC} -- Dynamic adjustment of threads

@cindex Environment Variable

@cindex Implementation specific setting

@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 specifications v2.5}, section 4.3

@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 specifications v2.5}, section 4.4

@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 positive integer. If undefined one thread

per CPU online is used.

@item @emph{See also}:

@ref{omp_set_num_threads}

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, section 4.2

@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} or @code{guided}.

The optional @code{chunk size} shall be a positive integer. If undefined,

dynamic scheduling and a chunk size of 1 is used.

@item @emph{Reference}:

@uref{http://www.openmp.org/, OpenMP specifications v2.5}, sections 2.5.1 and 4.1

@end table

@node GOMP_CPU_AFFINITY

@section @env{GOMP_CPU_AFFINITY} -- Bind threads to specific CPUs

@cindex Environment Variable

@table @asis

@item @emph{Description}:

A patch for this extension has been submitted, but was not yet applied at the

time of writing.

@item @emph{Reference}:

@uref{http://gcc.gnu.org/ml/gcc-patches/2006-05/msg00982.html,

GCC Patches Mailinglist}

@uref{http://gcc.gnu.org/ml/gcc-patches/2006-05/msg01133.html,

GCC Patches Mailinglist}

@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 in opposition

to @code{pthread_attr_setstacksize} which gets the number of bytes as an

argument. If the stacksize can not be set due to system constraints, an

error is reported and the initial stacksize is left unchanged. If undefined,

the stack size is system dependent.

@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

@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.