mpicxx.openmpi - Online in the Cloud

PROGRAM:

NAME

mpicxx -- Open MPI C++ wrapper compiler

SYNTAX

mpicxx [-showme|-showme:compile|-showme:link] ...

OPTIONS

--showme
This option comes in several different variants (see below). None of the variants
invokes the underlying compiler; they all provide information on how the underlying
compiler would have been invoked had --showme not been used. The basic --showme
option outputs the command line that would be executed to compile the program.
NOTE: If a non-filename argument is passed on the command line, the -showme option
will not display any additional flags. For example, both "mpicxx --showme" and
"mpicxx --showme my_source.c" will show all the wrapper-supplied flags. But
"mpicxx --showme -v" will only show the underlying compiler name and "-v".

--showme:compile
Output the compiler flags that would have been supplied to the C++ compiler.

--showme:link
Output the linker flags that would have been supplied to the C++ compiler.

--showme:command
Outputs the underlying C++ compiler command (which may be one or more tokens).

--showme:incdirs
Outputs a space-delimited (but otherwise undecorated) list of directories that the
wrapper compiler would have provided to the underlying C++ compiler to indicate
where relevant header files are located.

--showme:libdirs
Outputs a space-delimited (but otherwise undecorated) list of directories that the
wrapper compiler would have provided to the underlying linker to indicate where
relevant libraries are located.

--showme:libs
Outputs a space-delimited (but otherwise undecorated) list of library names that
the wrapper compiler would have used to link an application. For example: "mpi
open-rte open-pal util".

--showme:version
Outputs the version number of Open MPI.

--showme:help
Output a brief usage help message.

See the man page for your underlying C++ compiler for other options that can be passed
through mpicxx.

DESCRIPTION

Conceptually, the role of these commands is quite simple: transparently add relevant
compiler and linker flags to the user's command line that are necessary to compile / link
Open MPI programs, and then invoke the underlying compiler to actually perform the
command.

As such, these commands are frequently referred to as "wrapper" compilers because they do
not actually compile or link applications themselves; they only add in command line flags
and invoke the back-end compiler.

Background
Open MPI is comprised of three software layers: OPAL (Open Portable Access Layer), ORTE
(Open Run-Time Environment), and OMPI (Open MPI). There are wrapper compilers for each
layer; each layer's wrapper only links in the libraries relevant for that layer.
Specifically, each layer provides the following wrapper compilers:

OPAL
opalcc and opalc++

ORTE
ortecc and ortec++

OMPI
mpicc, mpic++, mpicxx, mpiCC (only on systems with case-senstive file systems), and
mpifort (and its legacy/deprecated names mpif77 and mpif90). Note that mpic++,
mpicxx, and mpiCC all invoke the same underlying C++ compiler with the same options.
All are provided as compatibility with other MPI implementations.

Fortran Notes
The Fortran wrapper compiler for MPI (mpifort, and its legacy/deprecated names mpif77 and
mpif90) can compile and link MPI applications that use any/all of the MPI Fortran
bindings: mpif.h, the mpi module, and the mpi_f08 module (assuming Open MPI was installed
with support for each of these Fortran bindings). Specifically: it is no longer necessary
to use different wrapper compilers for applications that use mpif.h vs. applications that
use the mpi module -- just use mpifort for all Fortran MPI applications.

Note, however, that the Fortran compiler may require additional command-line options to
enforce a specific Fortran dialect. For example, in some versions of the IBM XLF
compiler, if xlf90 is the underlying Fortran compiler, -qfixed may be necessary to compile
fixed-format Fortran source files.

Finally, note that mpifort will be inoperative and will return an error on use if Fortran
support was not built into the MP Ilayer.

Overview
mpicxx is a convenience wrappers for the underlying C++ compiler. Translation of an Open
MPI program requires the linkage of the Open MPI-specific libraries which may not reside
in one of the standard search directories of ld(1). It also often requires the inclusion
of header files what may also not be found in a standard location.

mpicxx passes its arguments to the underlying C++ compiler along with the -I, -L and -l
options required by Open MPI programs.

The Open MPI Team strongly encourages using the wrapper compilers instead of attempting to
link to the Open MPI libraries manually. This allows the specific implementation of Open
MPI to change without forcing changes to linker directives in users' Makefiles. Indeed,
the specific set of flags and libraries used by the wrapper compilers depends on how Open
MPI was configured and built; the values can change between different installations of the
same version of Open MPI.

Indeed, since the wrappers are simply thin shells on top of an underlying compiler, there
are very, very few compelling reasons not to use mpicxx. When it is not possible to use
the wrappers directly, the -showme:compile and -showme:link options should be used to
determine what flags the wrappers would have used. For example:

shell$ cc -c file1.c `mpicc -showme:compile`

shell$ cc -c file2.c `mpicc -showme:compile`

shell$ cc file1.o file2.o `mpicc -showme:link` -o my_mpi_program

NOTES

It is possible to make the wrapper compilers multi-lib aware. That is, the libraries and
includes specified may differ based on the compiler flags specified (for example, with the
GNU compilers on Linux, a different library path may be used if -m32 is seen versus -m64
being seen). This is not the default behavior in a standard build, but can be activated
(for example, in a binary package providing both 32 and 64 bit support). More information
can be found at:

https://svn.open-mpi.org/trac/ompi/wiki/compilerwrapper3264

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