Open mpi c++ wrapper compiler
mpic++ [-showme|-showme:compile|-showme:link] ...
--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 "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".
--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.
See the man page for your underlying C++ compiler for other options that can be passed through mpic++.
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.
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), 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 compilers for MPI (mpif77 and mpif90) will be inoperative and will return an error on use if Fortran 77 / Fortran 90 support was not built into the MPI layer.
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.
mpic++ 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 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`
shell$ cc -c file2.c `mpicc -showme:compile`
shell$ cc file1.o file2.o `mpicc -showme:link` -o my_mpi_program
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
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:
CPPFLAGS
Flags added when invoking the preprocessor (C or C++)
LDFLAGS
Flags added when invoking the linker (C, C++, or Fortran)
LIBS
Libraries added when invoking the linker (C, C++, or Fortran)
CC
C compiler
CFLAGS
C compiler flags
CXX
C++ compiler
CXXFLAGS
C++ compiler flags
F77
Fortran 77 compiler
FFLAGS
Fortran 77 compiler flags
FC
Fortran 90 compiler
FCFLAGS
Fortran 90 compiler flags