# Add OpenMP* Support

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

1. Add the appropriate OpenMP directives 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 Directive Syntax

To add OpenMP support to your application, first add appropriate OpenMP directives to your source code.

OpenMP directives 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® Fortran Compiler.

To use directives in your source, use this syntax:

    <prefix> <directive> [<clause>[[,]<clause>...]]

where:

- <prefix> - Required for all OpenMP directives. For free form source input, the prefix is `!$OMP` only; for fixed form source input, the prefix is `!$OMP` or C$OMP.

- <directive> - A valid OpenMP directive. Must immediately follow the prefix; for example: `!$OMP PARALLEL`.

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

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

- [,]: Optional. Commas between more than one <clause> are optional.

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

The OpenMP constructs defining a parallel region have one of the following syntax forms:

    !$OMP <directive>
      <structured block of code>
    !$OMP END <directive>
                      # OR
    !$OMP <directive>
      <structured block of code>
                      # OR
    !$OMP <directive>

The following example demonstrates one way of using an OpenMP directive to parallelize a loop:

    subroutine simple_omp(a, N)
      use omp_lib
      integer :: N, a(N)
    !$OMP PARALLEL DO
      do i = 1, N
        a(i) = i*2
      end do
    end subroutine simple_omp

## Compile the Application

The /Qopenmp (Windows) or -qopenmp (Linux and macOS) option enables the parallelizer to generate multi-threaded code based on the OpenMP directives 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 a command similar to one of the following:

Linux

    ifort -qopenmp source_file

macOS

    ifort -qopenmp source_file

Windows

    ifort /Qopenmp source_file

For example, to compile the previous code example without generating an executable, use the c option:

Linux

    ifort -qopenmp -c parallel.f90

macOS

    ifort -qopenmp -c parallel.f90

Windows

    ifort /Qopenmp /c parallel.f90

To build your application with target offload support (introduced since OpenMP 4.0) use compiler options to specify the target for which the regions marked with OpenMP "target" directives must be compiled. For example:

Linux

    ifx -qopenmp -fopenmp-targets=spir64 offload.f90

macOS

    ifx -qopenmp -fopenmp-targets=spir64 offload.f90

Windows

    ifx /Qopenmp /Qopenmp-targets:spir64 offload.f90

Refer to Get Started with OpenMP* Offload to GPU for the Intel® oneAPI DPC/C++ Compiler and Intel® Fortran Compiler for more information.

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