# 17.2.78. MPI_Comm_spawn_multiple

MPI_Comm_spawn_multiple — Spawns multiple binaries, or the same binary with multiple sets of arguments.

## 17.2.78.1. SYNTAX

### 17.2.78.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_spawn_multiple(int count, char *array_of_commands[],
         char **array_of_argv[], const int array_of_maxprocs[], const MPI_Info
         array_of_info[], int root, MPI_Comm comm, MPI_Comm *intercomm,
         int array_of_errcodes[])

### 17.2.78.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_COMM_SPAWN_MULTIPLE(COUNT, ARRAY_OF_COMMANDS, ARRAY_OF_ARGV,
         ARRAY_OF_MAXPROCS, ARRAY_OF_INFO, ROOT, COMM, INTERCOMM,
         ARRAY_OF_ERRCODES, IERROR)
         INTEGER COUNT, ARRAY_OF_INFO(*), ARRAY_OF_MAXPROCS(*), ROOT,
                 COMM, INTERCOMM, ARRAY_OF_ERRCODES(*), IERROR
         CHARACTER*(*) ARRAY_OF_COMMANDS(*), ARRAY_OF_ARGV(COUNT, *)

### 17.2.78.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Comm_spawn_multiple(count, array_of_commands, array_of_argv,
         array_of_maxprocs, array_of_info, root, comm, intercomm,
                 array_of_errcodes, ierror)
         INTEGER, INTENT(IN) :: count, array_of_maxprocs(*), root
         CHARACTER(LEN=*), INTENT(IN) :: array_of_commands(*)
         CHARACTER(LEN=*), INTENT(IN) :: array_of_argv(count, *)
         TYPE(MPI_Info), INTENT(IN) :: array_of_info(*)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         TYPE(MPI_Comm), INTENT(OUT) :: intercomm
         INTEGER :: array_of_errcodes(*)
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.78.2. INPUT PARAMETERS

- `count`: Number of commands (positive integer, significant to MPI only at root — see NOTES).

- `array_of_commands`: Programs to be executed (array of strings, significant only at root).

- `array_of_argv`: Arguments for commands (array of array of strings, significant only at root).

- `array_of_maxprocs`: Maximum number of processes to start for each command (array of integers, significant only at root).

- `array_of_info`: Info objects telling the runtime system where and how to start processes (array of handles, significant only at root).

- `root`: Rank of process in which previous arguments are examined (integer).

- `comm`: Intracommunicator containing group of spawning processes (handle).

## 17.2.78.3. OUTPUT PARAMETERS

- `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).

## 17.2.78.4. DESCRIPTION

MPI_Comm_spawn_multiple is identical to MPI_Comm_spawn except that it can specify multiple executables. The first argument, count, indicates the number of executables. The next three arguments are arrays of the corresponding arguments in MPI_Comm_spawn. The next argument, array_of_info, is an array of info arguments, one for each executable. See the INFO ARGUMENTS section for more information.

For the Fortran version of array_of_argv, the element array_of_argv(i,j) is the jth argument to command number i.

In any language, an application may use the constant MPI_ARGVS_NULL (which is likely to be `(char **)0` in C) to specify that no arguments should be passed to any commands. The effect of setting individual elements of array_of_argv to MPI_ARGV_NULL is not defined. To specify arguments for some commands but not others, the commands without arguments should have a corresponding argv whose first element is null `((char *)0` in C and empty string in Fortran).

All of the spawned processes have the same MPI_COMM_WORLD. Their ranks in MPI_COMM_WORLD correspond directly to the order in which the commands are specified in MPI_Comm_spawn_multiple. Assume that m1 processes are generated by the first command, m2 by the second, etc. The processes corresponding to the first command have ranks 0, 1,…, m1-1. The processes in the second command have ranks m1, m1+1, …, m1+m2-1. The processes in the third have ranks m1+m2, m1+m2+1, …, m1+m2+m3-1, etc.

The array_of_errcodes argument is 1-dimensional array of size

     _ count
    \       n ,
    /_ i=1   i

where i is the ith element of array_of_maxprocs. Command number i corresponds to the i contiguous slots in this array from element

                     _              _
     _ i-1          |   _ i          |
    \       n ,  to |  \      n      | -1
    /_ j=1   i      |  /_ j=1  j     |
                    |_              _|

Error codes are treated as for MPI_Comm_spawn.

## 17.2.78.5. INFO ARGUMENTS

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

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

    host                   char *   Comma-separated list of hosts on which
                                    the processes 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.
    add-host               char *   Add the specified hosts to the list of
                                    hosts known to this job and use it for
                                    the associated processes. This will be
                                    used similarly to the -host option.
    add-hostfile           char *   Hostfile containing hosts to be added
                                    to the list of hosts known to this job
                                    and use it for the associated
                                    process. This will be used similarly
                                    to the -hostfile option.
    wdir                   char *   Directory where the executable is
                                    located. If files are to be
                                    pre-positioned, then this location is
                                    the desired working directory at time
                                    of execution - if not specified, then
                                    it will automatically be set to
                                    ompi_preload_files_dest_dir.
    ompi_prefix            char *   Same as the --prefix command line
                                    argument to mpirun.
    ompi_preload_binary    bool     If set to true, pre-position the
                                    specified executable onto the remote
                                    host. A destination directory must
                                    also be provided.
    ompi_preload_files     char *   A comma-separated list of files that
                                    are to be pre-positioned in addition
                                    to the executable.  Note that this
                                    option does not depend upon
                                    ompi_preload_binary - files can
                                    be moved to the target even if an
                                    executable is not moved.
    ompi_stdin_target      char *   Comma-delimited list of ranks to
                                    receive stdin when forwarded.
    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".
    ompi_param             char *   Pass an OMPI MCA parameter to the
                                    child job.  If that parameter already
                                    exists in the environment, the value
                                    will be overwritten by the provided
                                    value.
    mapper                 char *   Mapper to be used for this job
    map_by                 char *   Mapping directive indicating how
                                    processes are to be mapped (slot,
                                    node, socket, etc.).
    rank_by                char *   Ranking directive indicating how
                                    processes are to be ranked (slot,
                                    node, socket, etc.).
    bind_to                char *   Binding directive indicating how
                                    processes are to be bound (core, slot,
                                    node, socket, etc.).
    path                   char *   List of directories to search for
                                    the executable
    npernode               char *   Number of processes to spawn on
                                    each node of the allocation
    pernode                bool     Equivalent to npernode of 1
    ppr                    char *   Spawn specified number of processes
                                    on each of the identified object type
    env                    char *   Newline-delimited list of envars to
                                    be passed to the spawned procs

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.

Note that if any of the info handles have ompi_non_mpi set to true, then all info handles must have it set to true. If some are set to true, but others are set to false (or are unset), MPI_ERR_INFO will be returned.

Note that in “#PACKAGE_NAME#”, the first array location in array_of_info is applied to all the commands in array_of_commands.

## 17.2.78.6. NOTES

The argument count is interpreted by MPI only at the root, as is array_of_argv. Since the leading dimension of array_of_argv is count, a nonpositive value of count at a nonroot node could theoretically cause a runtime bounds check error, even though array_of_argv should be ignored by the subroutine. If this happens, you should explicitly supply a reasonable value of count on the nonroot nodes.

Similar to MPI_Comm_spawn, it is the application’s responsibility to terminate each individual set of argv in the array_of_argv argument. In C, each argv array is terminated by a NULL pointer. In Fortran, each argv array is terminated by an empty string (note that compilers will not automatically insert this blank string; the application must ensure to have enough space for an empty string entry as the last element of the array).

Other restrictions apply to the array_of_argv parameter; see MPI_Comm_spawn’s description of the argv parameter for more details.

MPI-3.1 implies (but does not directly state) that the argument array_of_commands must be an array of strings of length count. Unlike the array_of_argv parameter, array_of_commands does not need to be terminated with a NULL pointer in C or a blank string in Fortran. Older versions of Open MPI required that array_of_commands be terminated with a blank string in Fortran; that is no longer required in this version of Open MPI.

Calling MPI_Comm_spawn many times would create many sets of children with different MPI_COMM_WORLDs, whereas MPI_Comm_spawn_multiple creates children with a single MPI_COMM_WORLD, so the two methods are not completely equivalent. Also if you need to spawn multiple executables, you may get better performance by using MPI_Comm_spawn_multiple instead of calling MPI_Comm_spawn several times.

## 17.2.78.7. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Comm_spawn

- MPI_Comm_get_parent

- mpirun(1)