12.7. Source code tree layout

There are a few notable top-level directories in the source tree:

  • The main sub-projects:

    • oshmem: Top-level OpenSHMEM code base

    • ompi: The Open MPI code base

    • opal: The OPAL code base

  • config: M4 scripts supporting the top-level configure script mpi.h

  • etc: Some miscellaneous text files

  • docs: Source code for Open MPI documentation

  • examples: Trivial MPI / OpenSHMEM example programs

  • 3rd-party: Included copies of required core libraries (either via Git submodules in Git clones or via binary tarballs).


    While it may be considered unusual, we include binary tarballs (instead of Git submodules) for 3rd party projects that are:

    1. Needed by Open MPI for correct operation, and

    2. Not universally included in OS distributions, and

    3. Rarely updated.

Each of the three main source directories (oshmem, ompi, and opal) generate at least a top-level library named liboshmem, libmpi, and libopen-pal, respectively. They can be built as either static or shared libraries. Executables are also produced in subdirectories of some of the trees.

Each of the sub-project source directories have similar (but not identical) directory structures under them:

  • class: C++-like “classes” (using the OPAL class system) specific to this project

  • include: Top-level include files specific to this project

  • mca: MCA frameworks and components specific to this project

  • runtime: Startup and shutdown of this project at runtime

  • tools: Executables specific to this project (currently none in OPAL)

  • util: Random utility code

There are other top-level directories in each of the sub-projects, each having to do with specific logic and code for that project. For example, the MPI API implementations can be found under ompi/mpi/LANGUAGE, where LANGUAGE is c or fortran.

The layout of the mca trees are strictly defined. They are of the form:


To be explicit: it is forbidden to have a directory under the mca trees that does not meet this template (with the exception of base directories, explained below). Hence, only framework and component code can be in the mca trees.

That is, framework and component names must be valid directory names (and C variables; more on that later). For example, the TCP BTL component is located in opal/mca/btl/tcp/.

The name base is reserved; there cannot be a framework or component named base. Directories named base are reserved for the implementation of the MCA and frameworks. Here are a few examples (as of the v5.0.x source tree):

# Main implementation of the MCA

# Implementation of the btl framework

# Implementation of the sysv framework

# Implementation of the pml framework

Under these mandated directories, frameworks and/or components may have arbitrary directory structures, however.