FindOpenMP

Finds Open Multi-Processing (OpenMP) support in a compiler:

find_package(OpenMP [<version>] [COMPONENTS <langs>...] [...])

If the compiler supports OpenMP, the flags required to compile with OpenMP support are returned in variables for the different languages. The variables may be empty if the compiler does not need a special flag to support OpenMP.

Added in version 3.5: Clang support.

Components

This module supports components that can be specified using the standard syntax:

find_package(OpenMP [COMPONENTS <langs>...])

Each of these components controls the various languages to search OpenMP support for. The following components are exposed:

C

Added in version 3.10.

CXX

Added in version 3.10.

Fortran

Added in version 3.10.

CUDA

Added in version 3.31.

The CUDA language component is supported when using a CUDA compiler that supports OpenMP on the host.

If no components are specified, module checks for all of them automatically, depending on the enabled languages in the project.

Imported Targets

This module provides the following Imported Targets:

OpenMP::OpenMP_<lang>

Added in version 3.9.

Target encapsulating the OpenMP usage requirements for language <lang>.

OpenMP::OpenMP_<lang>_Offload

Added in version 4.6.

Target for code whose OpenMP target regions may be offloaded to a device, for <lang> of C, CXX, or Fortran. It links OpenMP::OpenMP_<lang> and, when offload is enabled with the OpenMP_<lang>_OFFLOAD or OpenMP_<lang>_OFFLOAD_FLAGS variables, adds the offload flags when compiling <lang> sources. It also adds them, together with the OpenMP flags, when the link is done by a compiler with the same CMAKE_<LANG>_COMPILER_ID, because compilers need both in order to link device code. When offload is not enabled, it is equivalent to OpenMP::OpenMP_<lang>.

With NVHPC, an executable linked by NVHPC must also link the offload target when it uses a shared library with offloaded code, for example through $<LINK_ONLY:OpenMP::OpenMP_<lang>_Offload> in the library's INTERFACE link libraries.

Linking through a separate linker, as with Visual Studio generators or clang-cl, is not supported.

Input Variables

The following variables may be set to influence this module's behavior:

OpenMP_RUNTIME_MSVC

Added in version 3.30.

Specify the OpenMP Runtime when compiling with the MSVC compiler (cl). If set to a non-empty value, such as experimental or llvm, it will be passed as the value of the -openmp: flag. This has no effect with other compilers, including clang-cl.

OpenMP_<lang>_OFFLOAD

Added in version 4.6.

Enables OpenMP target offload for targets that link OpenMP::OpenMP_<lang>_Offload. If not defined, offload is enabled when OpenMP_<lang>_OFFLOAD_FLAGS is non-empty. If defined, its boolean value decides, so OFF disables offload even when flags are set.

The offload flags are OpenMP_<lang>_OFFLOAD_FLAGS if non-empty, and otherwise a default for the compiler:

• GNU: none. GCC offloads to the devices whose offload compilers are installed, such as gcc-offload-nvptx on Debian and Ubuntu.

• NVHPC: -mp=gpu.

• IntelLLVM with a GNU-like command line (icx, icpx, ifx): -fopenmp-targets=spir64.

• Cray, CrayClang: none. The device comes from the loaded craype-accel-* module.

Other compilers, including Clang, have no default. Enabling offload for them without OpenMP_<lang>_OFFLOAD_FLAGS is an error, even if the package is QUIET or the component is optional.

Targets linking only OpenMP::OpenMP_<lang> are not affected. Projects using the result variables must add the device flags for compiling and linking themselves. Disabling offload only removes what the offload target would add; it does not undo offload enabled by other compiler flags or by the environment. OpenMP_<lang>_FOUND and OpenMP_<lang>_VERSION describe host OpenMP support only; the module does not check that offload works.

OpenMP_<lang>_OFFLOAD_FLAGS

Added in version 4.6.

Compiler flags selecting the device(s) to offload OpenMP target regions to, replacing the compiler's default described above. For example, --offload-arch=sm_80 for Clang, -foffload=nvptx-none for GCC, or -mp=gpu -gpu=cc80 for NVHPC. The value is a command-line string, split into arguments as by the SHELL: prefix of target_compile_options(). An empty or whitespace-only value counts as not set.

Result Variables

This module defines the following variables:

OpenMP_FOUND

Added in version 3.10.

Boolean indicating that OpenMP flags for all requested languages were found. If no components are specified, this variable is set to boolean true if OpenMP settings for all enabled languages were detected.

OpenMP_VERSION

Added in version 4.2.

Minimal version of the OpenMP standard detected among the requested languages, or all enabled languages if no components were specified.

This module will set the following variables per language in the project, where <lang> is one of C, CXX, CUDA, or Fortran:

OpenMP_<lang>_FOUND

Added in version 3.9.

Boolean indicating whether the (requested version of) OpenMP support for <lang> was detected.

OpenMP_<lang>_VERSION

Added in version 3.9.

OpenMP version implemented by the <lang> compiler, if available.

OpenMP_<lang>_VERSION_MAJOR

Added in version 3.9.

Major version of OpenMP implemented by the <lang> compiler, if available.

OpenMP_<lang>_VERSION_MINOR

Added in version 3.9.

Minor version of OpenMP implemented by the <lang> compiler, if available.

OpenMP_<lang>_SPEC_DATE

Added in version 3.7.

Date of the OpenMP specification implemented by the <lang> compiler, if available.

The specification date is formatted as given in the OpenMP standard: yyyymm where yyyy and mm represents the year and month of the OpenMP specification implemented by the <lang> compiler.

OpenMP_<lang>_FLAGS

OpenMP compiler flags for <lang>, separated by spaces.

OpenMP_<lang>_INCLUDE_DIRS

Added in version 3.16.

Directories that must be added to the header search path for <lang> when using OpenMP.

For linking with OpenMP code written in <lang>, the following variables are provided:

OpenMP_<lang>_LIB_NAMES

Added in version 3.9.

A semicolon-separated list of OpenMP programs libraries for <lang>.

OpenMP_<libname>_LIBRARY

Added in version 3.9.

Location of the individual libraries needed for OpenMP support in <lang>. The <libname> names are stored in the OpenMP_<lang>_LIB_NAMES list.

OpenMP_<lang>_LIBRARIES

Added in version 3.9.

A list of libraries needed to link with OpenMP code written in <lang>.

Specifically for Fortran, the module sets the following variables:

OpenMP_Fortran_HAVE_OMPLIB_HEADER

Added in version 3.9.

Boolean indicating whether OpenMP is accessible through the omp_lib.h Fortran header.

OpenMP_Fortran_HAVE_OMPLIB_MODULE

Added in version 3.9.

Boolean indicating whether OpenMP is accessible through the omp_lib Fortran module.

For some compilers, it may be necessary to add a header search path to find the relevant OpenMP headers. This location may be language-specific. Where this is needed, the module may attempt to find the location, but it can be provided directly by setting the OpenMP_<lang>_INCLUDE_DIR cache variable. Note that this variable is an input control to the module. Project code should use the OpenMP_<lang>_INCLUDE_DIRS output variable if it needs to know what include directories are needed, or preferably the OpenMP::OpenMP_<lang> imported target.

For Clang targeting the MSVC ABI, the module searches for the OpenMP runtime library, preferring the one shipped with the compiler. A different library can be provided directly by setting the OpenMP_libomp_LIBRARY cache variable. Programs linked with it need the matching runtime DLL, such as libomp.dll, next to them or on PATH when they run.

Changed in version 4.6: In previous CMake versions, the first libomp found through the LIB environment variable was used, such as the one provided with MSVC for its -openmp:llvm option.

Examples

Finding OpenMP support and linking the imported target to a project target using the C language component:

find_package(OpenMP)
target_link_libraries(project_target PRIVATE OpenMP::OpenMP_C)

Linking a library whose target regions may be offloaded to a GPU:

find_package(OpenMP REQUIRED)
target_link_libraries(kernels PRIVATE OpenMP::OpenMP_CXX_Offload)

and enabling offload when configuring the build, e.g. with Clang:

$ cmake -B build -DOpenMP_CXX_OFFLOAD_FLAGS=--offload-arch=sm_80