MPI_Comm_spawn(3)                  Open MPI                  MPI_Comm_spawn(3)



NAME
       MPI_Comm_spawn - Spawns a number of identical binaries.


SYNTAX
C 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).


DESCRIPTION
       MPI_Comm_spawn tries to start maxprocs identical copies of the MPI
       program 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
       separate 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 parents 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
       working 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
       command, 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.
       Specifically, argv[0] of main  contains the name of the program (given
       by command). 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
       manipulate the info argument are described in Section 4.10 of the MPI-2
       standard.

       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
       programs not requiring detailed control over process locations should
       use MPI_INFO_NULL.

       The following keys for info are recognized in Open MPI. (The reserved
       values mentioned in Section 5.3.4 of the MPI-2 standard are not
       implemented.)

       Key                   Type      Description
       ---                   ----      -----------

       host                  char *    Host on which the process should be spawned.
                                       See the orte_host man page for an
                                       explanation of how this will be used.
       hostfile              char *    Hostfile containing the hosts on which
                                       the processes are to be spawned. See
                                       the orte_hostfile man page for an
                                       explanation of how this will be used.
       wdir                  char *    Directory where the executable is located.
       ompi_prefix           char *    Same as the --prefix command line argument
                                       to mpirun.
       ompi_non_mpi          bool      If set to true, launching a non-MPI
                                       application; the returned communicator
                                       will be MPI_COMM_NULL. Failure to set
                                       this flag when launching a non-MPI
                                       application will cause both the child
                                       and parent jobs to "hang".

       bool info keys are actually strings but are evaluated as follows: if
       the string value is a number, it is converted to an integer and cast to
       a boolean (meaning that zero integers are false and non-zero values are
       true).  If the string value is (case-insensitive) "yes" or "true", the
       boolean is true.  If the string value is (case-insensitive) "no" or
       "false", the boolean is false.  All other string values are
       unrecognized, and therefore false.


       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++
       functions 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 ALSO
       MPI_Comm_spawn_multiple(3)
       MPI_Comm_get_parent(3)
       mpirun(1)




1.4.1                            Jan 14, 2010                MPI_Comm_spawn(3)
