plucky (3) MPI_Comm_spawn.openmpi.3.gz

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SYNTAX

   C Syntax
          #include <mpi.h>

          int MPI_Comm_spawn(const char *command, char *argv[], int maxprocs,
               MPI_Info info, int root, MPI_Comm comm,
               MPI_Comm *intercomm, int array_of_errcodes[])

   Fortran Syntax
          USE MPI
          ! or the older form: 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_ERRCODES(*), IERROR

   Fortran 2008 Syntax
          USE mpi_f08
          MPI_Comm_spawn(command, argv, maxprocs, info, root, comm, intercomm,
                       array_of_errcodes, ierror)
               CHARACTER(LEN=*), INTENT(IN) :: command, argv(*)
               INTEGER, INTENT(IN) :: maxprocs, root
               TYPE(MPI_Info), INTENT(IN) :: info
               TYPE(MPI_Comm), INTENT(IN) :: comm
               TYPE(MPI_Comm), INTENT(OUT) :: intercomm
               INTEGER :: array_of_errcodes(*)
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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 an empty string in Fortran (note that it is the
       MPI application’s responsibility to ensure that the last entry of the argv array is an empty string;  the
       compiler  will not automatically insert it). 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 and Fortran to indicate an empty argument list. In 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 and INTEGER in Fortran. It is a container for
       a  number  of  user-specified  (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.
          add-host               char *   Add the specified host to the list of
                                          hosts known to this job and use it for
                                          the associated process. 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.

       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.

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  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_errhandlerMPI_File_create_errhandler then MPI_File_set_errhandlerMPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_initMPI_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_multipleMPI_Comm_get_parentmpirun(1)

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                                 MPI_COMM_SPAWN(3)