# Adding OpenMP* Support to your Application

To add OpenMP* support to your application, do the following:

1. Add the appropriate OpenMP pragmas to your source code.

2. Compile the application with the /Qopenmp (Windows*) or -qopenmp (Linux* and macOS*) option.

3. For applications with large local or temporary arrays, you may need to increase the stack space available at runtime. In addition, you may need to increase the stack allocated to individual threads by using the `OMP_STACKSIZE` environment variable or by setting the corresponding library routines.

You can set other environment variables to control multi-threaded code execution.

## OpenMP Pragma Syntax

To add OpenMP support to your application, first declare the OpenMP header and then add appropriate OpenMP pragmas to your source code.

To declare the OpenMP header, add the following in your code:

    #include <omp.h>

OpenMP pragmas use a specific format and syntax. Intel Extension Routines to OpenMP describes the OpenMP extensions to the specification that have been added to the Intel® C++ Compiler Classic.

The following syntax illustrates using the pragmas in your source.

Example
    <prefix> <pragma> [<clause>, ...] <newline>

where:

- <prefix> - Required for all OpenMP pragmas. The prefix must be `#pragma omp`.

- <pragma> - A valid OpenMP pragma. Must immediately follow the prefix.

- [<clause>] - Optional. Clauses can be in any order and repeated as necessary, unless otherwise restricted.

- <newline> - A required component of pragma syntax. It precedes the structured block that is enclosed by this pragma.

The pragmas are interpreted as comments if you omit the /Qopenmp (Windows) or -qopenmp (Linux and macOS*) option.

The following example demonstrates one way of using an OpenMP pragma to parallelize a loop.

Example
    #include <omp.h>
    void simple_omp(int *a){
      int i;
      #pragma omp parallel for
      for (i=0; i<1024; i++)
        a[i] = i*2;
    }

## Compile the Application

The /Qopenmp (Windows) or -qopenmp (Linux and macOS*) option enables the parallelizer to generate multi-threaded code based on the OpenMP pragmas in the source. The code can be executed in parallel on single processor, multi-processor, or multi-core processor systems.

The /Qopenmp (Windows) or -qopenmp (Linux and macOS*) option works with both -O0 (Linux and macOS*) and /Od (Windows*) and with any optimization level of O1, O2 and O3.

Specifying -O0 (Linux and macOS*) or /Od (Windows) with the /Qopenmp (Windows) or -qopenmp (Linux and macOS*) option helps to debug OpenMP applications.

Compile your application using commands similar to those shown below:

Operating System | Syntax Example
--- | ---
Linux | `icpc -qopenmp source_file`
macOS* | `icpc -qopenmp source_file`
Windows | `icl /Qopenmp source_file`

Assume that you compile the sample above, using commands similar to the following, where the c option instructs the compiler to compile the code without generating an executable:

Operating System | Extended Syntax Example
--- | ---
Linux | `icpc -qopenmp -c parallel.cpp`
macOS* | `icpc -qopenmp -c parallel.cpp`
Windows | `icl /Qopenmp /c parallel.cpp`

Configure the OpenMP Environment

Before you run the multi-threaded code, you can set the number of desired threads using the OpenMP environment variable, `OMP_NUM_THREADS`.

Parent topic: OpenMP* Support

## See Also

c compiler option
O compiler option
OpenMP* Examples
qopenmp, Qopenmp compiler option
Supported Environment Variables