MPI_Comm_spawn(3OpenMPI)MPI_Comm_spawn(3OpenMPI)NAMEMPI_Comm_spawn - Spawns a number of identical binaries.
SYNTAXC Syntax
#include <mpi.h>
int MPI_Comm_spawn(char *command, char *argv[], int maxprocs,
MPI_Info info, int root, MPI_Comm comm,
MPI_Comm *intercomm, int array_of_errcodes[])
Fortran Syntax
INCLUDE 'mpif.h'
MPI_COMM_SPAWN(COMMAND, ARGV, MAXPROCS, INFO, ROOT, COMM,
INTERCOMM, ARRAY_OF_ERRCODES, IERROR)
CHARACTER*(*) COMMAND, ARGV(*)
INTEGER INFO, MAXPROCS, ROOT, COMM, INTERCOMM,
ARRAY_OF_ERRORCODES(*), IERROR
C++ Syntax
#include <mpi.h>
MPI::Intercomm MPI::Intracomm::Spawn(const char* command,
const char* argv[], int maxprocs, const MPI::Info& info,
int root, int array_of_errcodes[]) const
MPI::Intercomm MPI::Intracomm::Spawn(const char* command,
const char* argv[], int maxprocs, const MPI::Info& info,
int root) const
INPUT PARAMETERS
command Name of program to be spawned (string, significant only at
root).
argv Arguments to command (array of strings, significant only at
root).
maxprocs Maximum number of processes to start (integer, significant
only at root).
info A set of key-value pairs telling the runtime system where and
how to start the processes (handle, significant only at
root).
root Rank of process in which previous arguments are examined
(integer).
comm Intracommunicator containing group of spawning processes
(handle).
OUTPUT PARAMETER
intercomm Intercommunicator between original group and the newly
spawned group (handle).
array_of_errcodes
One code per process (array of integers).
IERROR Fortran only: Error status (integer).
DESCRIPTIONMPI_Comm_spawn tries to start maxprocs identical copies of the MPI pro‐
gram specified by command, establishing communication with them and
returning an intercommunicator. The spawned processes are referred to
as children. The children have their own MPI_COMM_WORLD, which is sepa‐
rate from that of the parents. MPI_Comm_spawn is collective over comm,
and also may not return until MPI_Init has been called in the children.
Similarly, MPI_Init in the children may not return until all parents
have called MPI_Comm_spawn. In this sense, MPI_Comm_spawn in the par‐
ents and MPI_Init in the children form a collective operation over the
union of parent and child processes. The intercommunicator returned by
MPI_Comm_spawn contains the parent processes in the local group and the
child processes in the remote group. The ordering of processes in the
local and remote groups is the same as the as the ordering of the group
of the comm in the parents and of MPI_COMM_WORLD of the children,
respectively. This intercommunicator can be obtained in the children
through the function MPI_Comm_get_parent.
The MPI standard allows an implementation to use the MPI_UNIVERSE_SIZE
attribute of MPI_COMM_WORLD to specify the number of processes that
will be active in a program. Although this implementation of the MPI
standard defines MPI_UNIVERSE_SIZE, it does not allow the user to set
its value. If you try to set the value of MPI_UNIVERSE_SIZE, you will
get an error message.
The command Argument
The command argument is a string containing the name of a program to be
spawned. The string is null-terminated in C. In Fortran, leading and
trailing spaces are stripped. MPI looks for the file first in the work‐
ing directory of the spawning process.
The argv Argument
argv is an array of strings containing arguments that are passed to the
program. The first element of argv is the first argument passed to com‐
mand, not, as is conventional in some contexts, the command itself. The
argument list is terminated by NULL in C and C++ and an empty string in
Fortran. In Fortran, leading and trailing spaces are always stripped,
so that a string consisting of all spaces is considered an empty
string. The constant MPI_ARGV_NULL may be used in C, C++ and Fortran to
indicate an empty argument list. In C and C++, this constant is the
same as NULL.
In C, the MPI_Comm_spawn argument argv differs from the argv argument
of main in two respects. First, it is shifted by one element. Specifi‐
cally, argv[0] of main contains the name of the program (given by com‐
mand). argv[1] of main corresponds to argv[0] in MPI_Comm_spawn,
argv[2] of main to argv[1] of MPI_Comm_spawn, and so on. Second, argv
of MPI_Comm_spawn must be null-terminated, so that its length can be
determined. Passing an argv of MPI_ARGV_NULL to MPI_Comm_spawn results
in main receiving argc of 1 and an argv whose element 0 is the name of
the program.
The maxprocs Argument
Open MPI tries to spawn maxprocs processes. If it is unable to spawn
maxprocs processes, it raises an error of class MPI_ERR_SPAWN. If MPI
is able to spawn the specified number of processes, MPI_Comm_spawn
returns successfully and the number of spawned processes, m, is given
by the size of the remote group of intercomm.
A spawn call with the default behavior is called hard. A spawn call for
which fewer than maxprocs processes may be returned is called soft.
The info Argument
The info argument is an opaque handle of type MPI_Info in C, MPI::Info
in C++ and INTEGER in Fortran. It is a container for a number of user-
speci ed (key,value) pairs. key and value are strings (null-terminated
char* in C, character*(*) in Fortran). Routines to create and manipu‐
late the info argument are described in Section 4.10 of the MPI-2 stan‐
dard.
For the SPAWN calls, info provides additional, implementation-dependent
instructions to MPI and the runtime system on how to start processes.
An application may pass MPI_INFO_NULL in C or Fortran. Portable pro‐
grams not requiring detailed control over process locations should use
MPI_INFO_NULL.
The following values for info are recognized in Open MPI. (The reserved
values mentioned in Section 5.3.4 of the MPI-2 standard are not imple‐
mented.)
Key value Type Description
-------------------------
host char * Host on which the process should be spawned.
wdir char * Directory where the executable is located.
The root Argument
All arguments before the root argument are examined only on the process
whose rank in comm is equal to root. The value of these arguments on
other processes is ignored.
The array_of_errcodes Argument
The array_of_errcodes is an array of length maxprocs in which MPI
reports the status of the processes that MPI was requested to start. If
all maxprocs processes were spawned, array_of_errcodes is filled in
with the value MPI_SUCCESS. If anyof the processes are not spawned,
array_of_errcodes is filled in with the value MPI_ERR_SPAWN. In C or
Fortran, an application may pass MPI_ERRCODES_IGNORE if it is not
interested in the error codes. In C++ this constant does not exist, and
the array_of_errcodes argument may be omitted from the argument list.
NOTES
Completion of MPI_Comm_spawn in the parent does not necessarily mean
that MPI_Init has been called in the children (although the returned
intercommunicator can be used immediately).
ERRORS
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++ func‐
tions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI:Exception object.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does
not guarantee that an MPI program can continue past an error.
SEE ALSOMPI_Comm_spawn_multiple(3)MPI_Comm_get_parent(3)mpirun(1)Open MPI 1.2 September 2006 MPI_Comm_spawn(3OpenMPI)
