# 9.2. Customizing wrapper compiler behavior

The Open MPI wrapper compilers are driven by text files that contain, among other things, the flags that are passed to the underlying compiler. These text files are generated automatically for Open MPI and are customized for the compiler set that was selected when Open MPI was configured; it is not recommended that users edit these files.

However, there are cases where it may be necessary or desirable to edit these files and add to or subtract from the flags that Open MPI selected. These files are installed in `$pkgdatadir`, which defaults to `$prefix/share/openmpi/WRAPPER_NAME-wrapper-data.txt`. Several environment variables are also available for run-time replacement of the wrapper’s default values (from the text files):

Note

You may need to scroll right in the following table.

Wrapper compiler | Compiler | Preprocessor flags | Compiler flags | Linker flags | Linker library flags | Data file
--- | --- | --- | --- | --- | --- | ---
`mpicc` | `OMPI_CC` | `OMPI_CPPFLAGS` | `OMPI_CFLAGS` | `OMPI_LDFLAGS` | `OMPI_LIBS` | `mpicc-wrapper-data.txt`
`mpic++` and `mpiCC` | `OMPI_CXX` | `OMPI_CPPFLAGS` | `OMPI_CXXFLAGS` | `OMPI_LDFLAGS` | `OMPI_LIBS` | `mpic++-wrapper-data.txt` and `mpiCC-wrapper-data.txt`, respectively
`mpifort` | `OMPI_FC` | `OMPI_CPPFLAGS` | `OMPI_FCFLAGS` | `OMPI_LDFLAGS` | `OMPI_LIBS` | `mpifort-wrapper-data.txt`

Caution

Note that changing the underlying compiler may not work at all.

For example, C++ and Fortran compilers are notoriously binary incompatible with each other (sometimes even within multiple releases of the same compiler). If you compile/install Open MPI with C++ compiler vX.Y.Z and then use the `OMPI_CXX` environment variable to change the `mpicxx` wrapper compiler to use the vA.B.C C++ compiler, your application code may not compile and/or link. The traditional method of using multiple different compilers with Open MPI is to install Open MPI multiple times; each installation should be built/installed with a different compiler. This is annoying, but it is beyond the scope of Open MPI to be able to fix.

Note that the values of these fields can be directly influenced by passing flags to Open MPI’s `configure` script. See this section in the Installation guide for more details.

The files cited in the above table use fairly simplistic “key=value” data formats. The following are several fields that are likely to be of interest to end-users:

- `project_short`: Prefix for all environment variables. See below.

- `compiler_env`: Specifies the base name of the environment variable that can be used to override the wrapper’s underlying compiler at run-time. The full name of the environment variable is of the form `<project_short>_<compiler_env>`; see table above.

- `compiler_flags_env`: Specifies the base name of the environment variable that can be used to override the wrapper’s compiler flags at run-time. The full name of the environment variable is of the form `<project_short>_<compiler_flags_env>`; see table above.

- `compiler`: The executable name of the underlying compiler.

- `extra_includes`: Relative to `$installdir`, a list of directories to also list in the preprocessor flags to find header files.

- `preprocessor_flags`: A list of flags passed to the preprocessor.

- `compiler_flags`: A list of flags passed to the compiler.

- `linker_flags`: A list of flags passed to the linker.

- `libs`: A list of libraries passed to the linker.

- `required_file`: If non-empty, check for the presence of this file before continuing. If the file is not there, the wrapper will abort saying that the language is not supported.

- `includedir`: Directory containing Open MPI’s header files. The proper compiler “include” flag is prepended to this directory and added into the preprocessor flags.

- `libdir`: Directory containing Open MPI’s library files. The proper compiler “include” flag is prepended to this directory and added into the linker flags.

- `module_option`: This field only appears in `mpifort`. It is the flag that the Fortran compiler requires to declare where module files are located.