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 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.

@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 that's 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 actually 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

Seems simple enough for PARALLEL blocks.  Create a private
struct for communicating between 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.

Not clear at all 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 trickyness 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
@c ---------------------------------------------------------------------

@node Reporting Bugs
@chapter Reporting Bugs

Bugs in the GNU OpenMP implementation should be reported via
@uref{http://gcc.gnu.org/bugzilla/, bugzilla}. In all cases, please add
"openmp" to the keywords field in the bug report.


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

@include gpl.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 Index
@unnumbered Index

@printindex cp

@bye