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mpic++ -- Open MPI C++ wrapper compiler
- 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 "mpic++ --showme" and "mpic++ --showme my_source.c"
will show all the wrapper-supplied flags. But "mpic++ --showme -v" will only
show the underlying compiler name and "-v".
- Output the compiler
flags that would have been supplied to the C++ compiler.
the linker flags that would have been supplied to the C++ compiler.
- Outputs the underlying C++ compiler command (which may be one or more tokens).
- 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.
- 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.
- 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".
- Outputs the version number of Open
- Output a brief usage help message.
See the man page for
your underlying C++ compiler for other options that can be passed through
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.
MPI is comprised of two software layers: OPAL (Open Portable Access Layer),
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:
- opalcc and opalc++
- 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.
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
mpic++ 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.
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.
since the wrappers are simply thin shells on top of an underlying compiler,
there are very, very few compelling reasons not to use mpic++. 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‘
-c file2.c ‘mpicc -showme:compile‘
shell$ cc file1.o file2.o ‘mpicc -showme:link‘
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:
The string that the wrapper compilers insert into the command line
before invoking the underlying compiler are stored in a text file created
by Open MPI and installed to $pkgdata/mpic++-wrapper-data.txt, where $pkgdata
is typically $prefix/share/openmpi, and $prefix is the top installation
directory of Open MPI.
It is rarely necessary to edit this file, but it
can be examined to gain insight into what flags the wrappers are placing
on the command line.
By default, the wrappers use
the compilers that were selected when Open MPI was configured. These compilers
were either found automatically by Open MPI’s "configure" script, or were
selected by the user in the CC, CXX, F77, and/or FC environment variables
before "configure" was invoked. Additionally, other arguments specific
to the compiler may have been selected by configure.
These values can be
selectively overridden by either editing the text files containing this
configuration information (see the FILES section), or by setting selected
environment variables of the form "OMPI_value".
Valid value names are:
- Flags added when invoking the preprocessor (C or C++)
- Flags added when invoking the linker (C, C++, or Fortran)
added when invoking the linker (C, C++, or Fortran)
- C compiler
- C compiler flags
- C++ compiler
- C++ compiler flags
- Fortran compiler flags
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