const char* \fIargv\fP[], int \fImaxprocs\fP, const MPI::Info& \fIinfo\fP,
int \fIroot\fP) const
.SH INPUT PARAMETERS
.ft R
.TP 1i
command
Name of program to be spawned (string, significant only at \fIroot\fP).
.TP 1i
argv
Arguments to \fIcommand\fP (array of strings, significant only at \fIroot\fP).
.TP 1i
maxprocs
Maximum number of processes to start (integer, significant only at \fIroot\fP).
.TP 1i
info
A set of key-value pairs telling the runtime system where and how to start the processes (handle, significant only at \fIroot\fP).
.TP 1i
root
Rank of process in which previous arguments are examined (integer).
.TP 1i
comm
Intracommunicator containing group of spawning processes (handle).
.SH OUTPUT PARAMETER
.ft R
.TP 1i
intercomm
Intercommunicator between original group and the newly spawned group (handle).
.TP 1i
array_of_errcodes
One code per process (array of integers).
.TP 1i
IERROR
Fortran only: Error status (integer).
.SH DESCRIPTION
.ft R
MPI_Comm_spawn tries to start \fImaxprocs\fP identical copies of the MPI program specified by \fIcommand\fP, 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 \fIcomm\fP, 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 \fIcomm\fP 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.
.sp
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.
.sp
The \fIcommand\fP Argument
.sp
The \fIcommand\fP 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.
In C, the MPI_Comm_spawn argument \fIargv\fP differs from the \fIargv\fP argument of \fImain\fP in two respects. First, it is shifted by one element. Specifically, \fIargv\fP[0] of \fImain\fP contains the name of the program (given by \fIcommand\fP). \fIargv\fP[1] of \fImain\fP corresponds to \fIargv\fP[0] in MPI_Comm_spawn, \fIargv\fP[2] of \fImain\fP to \fIargv\fP[1] of MPI_Comm_spawn, and so on. Second, \fIargv\fP of MPI_Comm_spawn must be null-terminated, so that its length can be determined. Passing an \fIargv\fP of MPI_ARGV_NULL to MPI_Comm_spawn results in \fImain\fP receiving \fIargc\fP of 1 and an \fIargv\fP whose element 0 is the name of the program.
.sp
The \fImaxprocs\fP Argument
.sp
Open MPI tries to spawn \fImaxprocs\fP processes. If it is unable to spawn \fImaxprocs\fP 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, \fIm\fP, is given by the size of the remote group of \fIintercomm\fP.
.sp
A spawn call with the default behavior is called hard. A spawn call for which fewer than \fImaxprocs\fP processes may be returned is called soft.
.sp
The \fIinfo\fP Argument
.sp
The \fIinfo\fP 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 (\fIkey,value\fP) pairs. \fIkey\fP and \fIvalue\fP are strings (null-terminated char* in C, character*(*) in Fortran). Routines to create and manipulate the \fIinfo\fP argument are described in Section 4.10 of the MPI-2 standard.
.sp
For the SPAWN calls, \fIinfo\fP 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 \fIinfo\fP are recognized in Open MPI. (The reserved values mentioned in Section 5.3.4 of the MPI-2 standard are not implemented.)
All arguments before the \fIroot\fP argument are examined only on the process whose rank in \fIcomm\fP is equal to \fIroot\fP. The value of these arguments on other processes is ignored.
.sp
The \fIarray_of_errcodes\fP Argument
.sp
The \fIarray_of_errcodes\fP is an array of length \fImaxprocs\fP in which MPI reports the status of the processes that MPI was requested to start. If all \fImaxprocs\fP processes were spawned, \fIarray_of_errcodes\fP is filled in with the value MPI_SUCCESS. If anyof the processes are \fInot\fP spawned, \fIarray_of_errcodes\fP 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 \fIarray_of_errcodes\fP argument may be omitted from the argument list.
.SH NOTES
.ft R
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).
.SH 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.
.sp
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.
