# 4.9.1. `configure` CLI option conventions

`configure` will, by default, search for header files and/or libraries for various optional features (e.g., various HPC network API/support libraries). If the relevant files are found, Open MPI will built support for that feature. If they are not found, Open MPI will skip building support for that feature.

However, if you specify `--with-FOO` (where `FOO` is the corresponding CLI option name for the feature) on the `configure` command line and Open MPI is unable to find relevant support for `FOO`, `configure` will assume that it was unable to provide a feature that was specifically requested, and will therefore abort so that a human can resolve out the issue.

Note

Using `--with-FOO` to force Open MPI’s `configure` script to abort it if can’t find support for a given feature may be preferable to unexpectedly discovering at run-time that Open MPI is missing support for a critical feature.

Additionally, if a search directory is specified for `FOO` in the form `--with-FOO=DIR`, Open MPI will:

1. Search for `FOO`’s header files in `DIR/include`.

2. Search for `FOO`’s library files:

   1. If `--with-FOO-libdir=LIBDIR` was specified, search in `LIBDIR`.

   2. Otherwise, search in `DIR/lib`, and if they are not found there, search again in `DIR/lib64`.

3. If both the relevant header files and libraries are found:

   1. Open MPI will build support for `FOO`.

   2. If the root path where the FOO libraries are found is neither `/usr` nor `/usr/local`, Open MPI will compile itself with RPATH flags pointing to the directory where `FOO`’s libraries are located.

      Important

      Open MPI does not RPATH `/usr/lib[64]` and `/usr/local/lib[64]` because many systems already search these directories for run-time libraries by default; adding RPATH for them could have unintended consequences for the search path ordering.

Caution

The `--with-FOO-libdir=LIBDIR` options are not usually needed; they are typically only needed when `FOO`’s libraries are installed in an “unexpected” location.

Also note the difference between `--with-FOO=DIR` and `--with-FOO-libdir=LIBDIR`: the former is a directory to which suffixes such as `/include` and `/lib` are added, whereas the latter is assumed to be a full library directory name (e.g., `/opt/some_library/lib`).