--- name: "MPI_Comm_spawn" section: "3" platform: "linux" package: "openmpi" version: "5.0.11-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/MPI_Comm_spawn/3" --- # MPI\_Comm\_spawn(3) *MPI\_Comm\_spawn* — Spawns a number of identical binaries. ## SYNTAX ### C Syntax ``` #include 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 - Important In Open MPI, this function only works properly when the MPI job is launched via *mpirun / mpiexec*. Using this function in other run-time environments is currently unsupported, and may result in undefined behavior (e.g., the MPI job may hang). 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\_multiple* - • *MPI\_Comm\_get\_parent* - • *[mpirun(1)](/mpirun/1.md)* ## COPYRIGHT 2003-2026, The Open MPI Community