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.TH "MPI_GET_COUNT" "3" "Feb 06, 2024" "" "Open MPI"
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\fI\%MPI_Get_count\fP — Gets the number of top\-level elements received.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Get_count(const MPI_Status *status, MPI_Datatype datatype,
    int *count)
.ft P
.fi
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.SS Fortran Syntax
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.INDENT 3.5
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.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq

MPI_GET_COUNT(STATUS, DATATYPE, COUNT, IERROR)
    INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR
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.SS Fortran 2008 Syntax
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.INDENT 3.5
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.nf
.ft C
USE mpi_f08

MPI_Get_count(status, datatype, count, ierror)
    TYPE(MPI_Status), INTENT(IN) :: status
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    INTEGER, INTENT(OUT) :: count
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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.fi
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBstatus\fP : Return status of receive operation (status).
.IP \(bu 2
\fBdatatype\fP : Datatype of each receive buffer element (handle).
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.SH OUTPUT PARAMETERS
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.IP \(bu 2
\fBcount\fP : Number of received elements (integer).
.IP \(bu 2
\fBierror\fP : Fortran only: Error status (integer).
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.SH DESCRIPTION
.sp
Returns the number of entries received. (We count entries, each of type
datatype, not bytes.) The datatype argument should match the argument
provided by the receive call that set the status variable. As explained
in the “Use of General Datatypes in Communication” section of the \fI\%MPI
Standard\fP\&.
\fI\%MPI_Get_count\fP may, in certain situations, return the value
\fBMPI_UNDEFINED\fP\&.
.sp
The datatype argument is passed to \fI\%MPI_Get_count\fP to improve performance.
A message might be received without counting the number of elements it
contains, and the count value is often not needed. Also, this allows the
same function to be used after a call to \fI\%MPI_Probe\fP\&.
.SH NOTES
.sp
If the size of the datatype is zero, this routine will return a count of
zero. If the amount of data in status is not an exact multiple of the
size of datatype (so that count would not be integral), a count of
\fBMPI_UNDEFINED\fP is returned instead.
.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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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
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Note that MPI does not guarantee that an MPI program can continue past
an error.
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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
If the value to be returned is larger than can fit into the count
parameter, an \fBMPI_ERR_TRUNCATE\fP error is raised.
.sp
\fBSEE ALSO:\fP
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.INDENT 3.5
\fI\%MPI_Get_elements\fP
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.SH COPYRIGHT
2003-2024, The Open MPI Community
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