MPI_COMM_SPAWN(3)				     Open MPI					 MPI_COMM_SPAWN(3)

MPI_Comm_spawn — Spawns a number of identical binaries.

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 <#mpi-init> has been  called
       in  the	children.   Similarly,	MPI_Init <#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  <#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	<#
       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  <#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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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 <#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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Comm_spawn_multiple <#mpi-comm-spawn-multiple>

	  • MPI_Comm_get_parent <#mpi-comm-get-parent>

	  • mpirun(1) <#man1-mpirun>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					 MPI_COMM_SPAWN(3)
