Provides links to directives used in the OpenMP API.
Jan 24, 2019 In Visual Studio 2019 Preview 2 we have continued to improve the C backend with new features, new and improved optimizations, build throughput improvements, and quality of life changes. New Features Added a new inlining command line switch: -Ob3.Ob3 is a more aggressive version of. OPENMP, C programs which illustrate the use of the OpenMP application program interface for carrying out parallel computations in a shared memory environment. POISSONOPENMP, a C program which computes an approximate solution to the Poisson equation in a rectangle, using the Jacobi iteration to solve the linear system, and OpenMP to carry out the Jacobi iteration in parallel.
Visual C++ supports the following OpenMP directives.
The C compiler currently supports the OpenMP 2.0 standard. However, Visual Studio 2019 also now offers SIMD functionality. To use SIMD, compile by using the /openmp:experimental option. This option enables both the usual OpenMP features, and additional OpenMP SIMD features not available when using the /openmp switch.
Openmp C++ Tutorial
OnlineGDB is online IDE with C compiler. Quick and easy way to compiler c program online. It supports g compiler for c. Jul 01, 2008 What is TC?? Is this the old Turbo C? If so, then the compiler doesn't support OpenMP. OpenMP requires compiler support. You can find a list of compilers off the OpenMP ARB web page. Currently on the upper left hand side of the page there is box labeled 'What's Here' and under it there is a header 'OpenMP Compilers'.
For parallel work-sharing:
| Directive | Description |
|---|---|
| parallel | Defines a parallel region, which is code that will be executed by multiple threads in parallel. |
| for | Causes the work done in a for loop inside a parallel region to be divided among threads. |
| sections | Identifies code sections to be divided among all threads. |
| single | Lets you specify that a section of code should be executed on a single thread, not necessarily the master thread. |
For master and synchronization:
| Directive | Description |
|---|---|
| master | Specifies that only the master thread should execute a section of the program. |
| critical | Specifies that code is only executed on one thread at a time. |
| barrier | Synchronizes all threads in a team; all threads pause at the barrier, until all threads execute the barrier. |
| atomic | Specifies that a memory location that will be updated atomically. |
| flush | Specifies that all threads have the same view of memory for all shared objects. |
| ordered | Specifies that code under a parallelized for loop should be executed like a sequential loop. |
For data environment:
| Directive | Description |
|---|---|
| threadprivate | Specifies that a variable is private to a thread. |
atomic
Specifies that a memory location that will be updated atomically.
Parameters
expression
The statement that has the lvalue, whose memory location you want to protect against more than one write.
Remarks
The atomic directive supports no clauses.
For more information, see 2.6.4 atomic construct.
Example
barrier
Synchronizes all threads in a team; all threads pause at the barrier, until all threads execute the barrier.
Remarks
The barrier directive supports no clauses.
For more information, see 2.6.3 barrier directive.
Example
For a sample of how to use barrier, see master.
critical
Specifies that code is only be executed on one thread at a time.
Parameters
name
(Optional) A name to identify the critical code. The name must be enclosed in parentheses.
Remarks
The critical directive supports no clauses.
For more information, see 2.6.2 critical construct.
Example
flush
Specifies that all threads have the same view of memory for all shared objects.
Parameters
var
(Optional) A comma-separated list of variables that represent objects you want to synchronize. If var isn't specified, all memory is flushed.
Remarks
The flush directive supports no clauses.
For more information, see 2.6.5 flush directive.
Example
for
Causes the work done in a for loop inside a parallel region to be divided among threads.
Parameters
clauses
(Optional) Zero or more clauses, see the Remarks section.
for_statement
A for loop. Undefined behavior will result if user code in the for loop changes the index variable.
Remarks
The for directive supports the following clauses:
If parallel is also specified, clauses can be any clause accepted by the parallel or for directives, except nowait.
For more information, see 2.4.1 for construct.
Example
master
Specifies that only the master thread should execute a section of the program.
Remarks
The master directive supports no clauses.
The single directive lets you specify that a section of code should be executed on a single thread, not necessarily the master thread.
For more information, see 2.6.1 master construct.
Example
ordered
Specifies that code under a parallelized for loop should be executed like a sequential loop.
Remarks
The ordered directive must be within the dynamic extent of a for or parallel for construct with an ordered clause.
The ordered directive supports no clauses.
For more information, see 2.6.6 ordered construct.
Example
parallel
Defines a parallel region, which is code that will be executed by multiple threads in parallel.
Parameters
clauses
(Optional) Zero or more clauses, see the Remarks section.
Remarks
The parallel directive supports the following clauses:
parallel can also be used with the for and sections directives.
For more information, see 2.3 parallel construct.
Example
The following sample shows how to set the number of threads and define a parallel region. The number of threads is equal by default to the number of logical processors on the machine. For example, if you have a machine with one physical processor that has hyperthreading enabled, it will have two logical processors and two threads. The order of output can vary on different machines.
sections
Identifies code sections to be divided among all threads.
Parameters
clauses
(Optional) Zero or more clauses, see the Remarks section.
Remarks
The sections directive can contain zero or more section directives.
The sections directive supports the following clauses:
If parallel is also specified, clauses can be any clause accepted by the parallel or sections directives, except nowait.
For more information, see 2.4.2 sections construct.
Example
single
Lets you specify that a section of code should be executed on a single thread, not necessarily the master thread.
Parameters
clauses
(Optional) Zero or more clauses, see the Remarks section.
Remarks
The single directive supports the following clauses:
The master directive lets you specify that a section of code should be executed only on the master thread.
For more information, see 2.4.3 single construct.
Example
threadprivate
Specifies that a variable is private to a thread.
Parameters
var
A comma-separated list of variables that you want to make private to a thread. var must be either a global- or namespace-scoped variable or a local static variable.
Remarks
The threadprivate directive supports no clauses.
The threadprivate directive is based on the thread attribute using the __declspec keyword; limits on __declspec(thread) apply to threadprivate. For example, a threadprivate variable will exist in any thread started in the process, not just those threads that are part of a thread team spawned by a parallel region. Be aware of this implementation detail; you may notice that constructors for a threadprivate user-defined type are called more often then expected.
You can use threadprivate in a DLL that is statically loaded at process startup, however you can't use threadprivate in any DLL that will be loaded via LoadLibrary such as DLLs that are loaded with /DELAYLOAD (delay load import), which also uses LoadLibrary.
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A threadprivate variable of a destructible type isn't guaranteed to have its destructor called. For example:
Users have no control as to when the threads constituting the parallel region will terminate. If those threads exist when the process exits, the threads won't be notified about the process exit, and the destructor won't be called for threaded_var on any thread except the one that exits (here, the primary thread). So code shouldn't count on proper destruction of threadprivate variables.
For more information, see 2.7.1 threadprivate directive.
Example
For a sample of using threadprivate, see private.
Causes the compiler to process #pragma omp directives in support of OpenMP.
Syntax
/openmp
Remarks
#pragma omp is used to specify Directives and Clauses. If /openmp isn't specified in a compilation, the compiler ignores OpenMP clauses and directives. OpenMP Function calls are processed by the compiler even if /openmp isn't specified.
The C++ compiler currently supports the OpenMP 2.0 standard. However, Visual Studio 2019 also now offers SIMD functionality. To use SIMD, compile by using the /openmp:experimental option. This option enables both the usual OpenMP features, and additional OpenMP SIMD features not available when using the /openmp switch.
Applications compiled by using both /openmp and /clr can only be run in a single application domain process. Multiple application domains aren't supported. That is, when the module constructor (.cctor) is run, it detects if the process is compiled using /openmp, and if the app is loaded into a non-default runtime. For more information, see appdomain, /clr (Common Language Runtime Compilation), and Initialization of Mixed Assemblies.
If you attempt to load an app compiled using both /openmp and /clr into a non-default application domain, a TypeInitializationException exception is thrown outside the debugger, and a OpenMPWithMultipleAppdomainsException exception is thrown in the debugger.
These exceptions can also be raised in the following situations:
If your application is compiled using /clr but not /openmp, and is loaded into a non-default application domain, where the process includes an app compiled using /openmp.
If you pass your /clr app to a utility, such as regasm.exe, which loads its target assemblies into a non-default application domain.
The common language runtime's code access security doesn't work in OpenMP regions. If you apply a CLR code access security attribute outside a parallel region, it won't be in effect in the parallel region.
Microsoft doesn't recommend that you write /openmp apps that allow partially trusted callers. Don't use AllowPartiallyTrustedCallersAttribute, or any CLR code access security attributes.
To set this compiler option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.
Expand the Configuration Properties > C/C++ > Language property page.
Modify the OpenMP Support property.
To set this compiler option programmatically
- See OpenMP.
Example
The following sample shows some of the effects of thread pool startup versus using the thread pool after it has started. Assuming an x64, single core, dual processor, the thread pool takes about 16 ms to start up. After that, there's little extra cost for the thread pool.
When you compile using /openmp, the second call to test2 never runs any longer than if you compile using /openmp-, as there's no thread pool startup. At a million iterations, the /openmp version is faster than the /openmp- version for the second call to test2. At 25 iterations, both /openmp- and /openmp versions register less than the clock granularity.
Openmp Tutorial
If you have only one loop in your application and it runs in less than 15 ms (adjusted for the approximate overhead on your machine), /openmp may not be appropriate. If it's higher, you may want to consider using /openmp.
See also
MSVC Compiler Options
MSVC Compiler Command-Line Syntax
OpenMP in MSVC