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.TH "MPI_WAITANY" "3" "May 30, 2025" "" "Open MPI"
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.sp
\fI\%MPI_Waitany\fP — Waits for any specified send or receive to complete.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Waitany(int count, MPI_Request array_of_requests[],
     int *index, MPI_Status *status)
.ft P
.fi
.UNINDENT
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.SS Fortran Syntax
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.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
MPI_WAITANY(COUNT, ARRAY_OF_REQUESTS, INDEX, STATUS, IERROR)
     INTEGER COUNT, ARRAY_OF_REQUESTS(*), INDEX
     INTEGER STATUS(MPI_STATUS_SIZE), IERROR
.ft P
.fi
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.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE mpi_f08
MPI_Waitany(count, array_of_requests, index, status, ierror)
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Request), INTENT(INOUT) :: array_of_requests(count)
     INTEGER, INTENT(OUT) :: index
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBcount\fP: List length (integer).
.IP \(bu 2
\fBarray_of_requests\fP: Array of requests (array of handles).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBindex\fP: Index of handle for operation that completed (integer). In the range 0 to count\-1. In Fortran, the range is 1 to count.
.IP \(bu 2
\fBstatus\fP: Status object (status).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
A call to \fI\%MPI_Waitany\fP can be used to wait for the completion of one out
of several requests.
.sp
The array_of_requests list may contain null or inactive handles. If the
list contains no active handles (list has length zero or all entries are
null or inactive), then the call returns immediately with index =
\fBMPI_UNDEFINED\fP, and an empty status.
.sp
The execution of MPI_Waitany(count, array_of_requests, index, status)
has the same effect as the execution of \fBMPI_Wait(&array_of_requests[i], status)\fP,
where i is the value returned by index (unless the value of
index is \fBMPI_UNDEFINED\fP). \fI\%MPI_Waitany\fP with an array containing one active
entry is equivalent to \fI\%MPI_Wait\fP\&.
.sp
If your application does not need to examine the \fIstatus\fP field, you can
save resources by using the predefined constant \fBMPI_STATUS_IGNORE\fP as a
special value for the \fIstatus\fP argument.
.sp
\fBExample:\fP Client\-server code (starvation can occur).
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.INDENT 3.5
.sp
.nf
.ft C
CALL MPI_COMM_SIZE(comm, size, ierr)
CALL MPI_COMM_RANK(comm, rank, ierr)
IF(rank > 0) THEN         ! client code
    DO
       CALL MPI_ISEND(a, n, MPI_REAL, 0, tag, comm, request, ierr)
       CALL MPI_WAIT(request, status, ierr)
    END DO
ELSE         ! rank=0: server code
       DO i=1, size\-1
          CALL MPI_IRECV(a(1,i), n, MPI_REAL, i tag, &
                   comm, request_list(i), ierr)
       END DO
       DO
          CALL MPI_WAITANY(size\-1, request_list, index, status, ierr)
          CALL DO_SERVICE(a(1,index))  ! handle one message
          CALL MPI_IRECV(a(1, index), n, MPI_REAL, index, tag, &
                    comm, request_list(index), ierr)
       END DO
END IF
.ft P
.fi
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.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
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.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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.sp
MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
.UNINDENT
.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
Note that per the “Return Status” section in the “Point\-to\-Point
Communication” chapter in the \fI\%MPI Standard\fP, MPI errors on requests passed to
\fI\%MPI_Waitany\fP do not set the \fBstatus.MPI_ERROR\fP field in the
returned \fIstatus\fP\&.  The error code is always passed to the back\-end
error handler and may be passed back to the caller through the return
value of \fI\%MPI_Waitany\fP if the back\-end error handler returns it.
The pre\-defined MPI error handler \fBMPI_ERRORS_RETURN\fP exhibits this
behavior, for example.
.sp
\fBSEE ALSO:\fP
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.INDENT 3.5
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.IP \(bu 2
\fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Test\fP
.IP \(bu 2
\fI\%MPI_Testall\fP
.IP \(bu 2
\fI\%MPI_Testany\fP
.IP \(bu 2
\fI\%MPI_Testsome\fP
.IP \(bu 2
\fI\%MPI_Wait\fP
.IP \(bu 2
\fI\%MPI_Waitall\fP
.IP \(bu 2
\fI\%MPI_Waitsome\fP
.IP \(bu 2
\fI\%MPI_Win_set_errhandler\fP
.UNINDENT
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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