.\" Man page generated from reStructuredText
.\" by the Docutils 0.23 manpage writer.
.
.
.nr rst2man-indent-level 0
.
.de1 rstReportMargin
\\$1 \\n[an-margin]
level \\n[rst2man-indent-level]
level margin: \\n[rst2man-indent\\n[rst2man-indent-level]]
-
\\n[rst2man-indent0]
\\n[rst2man-indent1]
\\n[rst2man-indent2]
..
.de1 INDENT
.\" .rstReportMargin pre:
. RS \\$1
. nr rst2man-indent\\n[rst2man-indent-level] \\n[an-margin]
. nr rst2man-indent-level +1
.\" .rstReportMargin post:
..
.de UNINDENT
. RE
.\" indent \\n[an-margin]
.\" old: \\n[rst2man-indent\\n[rst2man-indent-level]]
.nr rst2man-indent-level -1
.\" new: \\n[rst2man-indent\\n[rst2man-indent-level]]
.in \\n[rst2man-indent\\n[rst2man-indent-level]]u
..
.TH "MPI_PACK" "3" "Oct 18, 2026" "" "Open MPI"
.SH NAME
MPI_Pack \- Packs data of a given datatype into contiguous memory.
.sp
MPI_Pack — Packs data of a given datatype into contiguous memory.
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBinbuf\fP: Input buffer start (choice).
.IP \(bu 2
\fBincount\fP: Number of input data items (integer).
.IP \(bu 2
\fBdatatype\fP: Datatype of each input data item (handle).
.IP \(bu 2
\fBoutsize\fP: Output buffer size, in bytes (integer).
.IP \(bu 2
\fBcomm\fP: Communicator for packed message (handle).
.UNINDENT
.SH INPUT/OUTPUT PARAMETER
.INDENT 0.0
.IP \(bu 2
\fBposition\fP: Current position in buffer, in bytes (integer).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBoutbuf\fP: Output buffer start (choice).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
Packs the message in the send buffer specified by \fIinbuf\fP, \fIincount\fP,
\fIdatatype\fP into the buffer space specified by \fIoutbuf\fP and \fIoutsize\fP\&.
The input buffer can be any communication buffer allowed in MPI_Send \%<#\:mpi-send>\&.
The output buffer is a contiguous storage area containing \fIoutsize\fP
bytes, starting at the address \fIoutbuf\fP (length is counted in bytes, not
elements, as if it were a communication buffer for a message of type
MPI_Packed).
.sp
The input value of \fIposition\fP is the first location in the output buffer
to be used for packing. \fIposition\fP is incremented by the size of the
packed message, and the output value of \fIposition\fP is the first location
in the output buffer following the locations occupied by the packed
message. The \fIcomm\fP argument is the communicator that will be
subsequently used for sending the packed message.
.sp
\fBExample:\fP An example using MPI_Pack:
.INDENT 0.0
.INDENT 3.5
.sp
.EX
int myrank, position, i, j, a[2];
char buff[1000];

// ...

MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
if (myrank == 0) {
  /* SENDER CODE */
  position = 0;
  MPI_Pack(&i, 1, MPI_INT, buff, 1000, &position, MPI_COMM_WORLD);
  MPI_Pack(&j, 1, MPI_INT, buff, 1000, &position, MPI_COMM_WORLD);
  MPI_Send(buff, position, MPI_PACKED, 1, 0, MPI_COMM_WORLD);
} else {
  /* RECEIVER CODE */
  MPI_Recv(a, 2, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
.EE
.UNINDENT
.UNINDENT
.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
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>/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.
.sp
Open MPI includes three predefined error handlers that can be used:
.INDENT 0.0
.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 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
.UNINDENT
.sp
MPI applications can also implement their own error handlers by calling:
.INDENT 0.0
.IP \(bu 2
MPI_Comm_create_errhandler \%<#\:mpi-comm-create-errhandler> then MPI_Comm_set_errhandler \%<#\:mpi-comm-set-errhandler>
.IP \(bu 2
MPI_File_create_errhandler \%<#\:mpi-file-create-errhandler> then MPI_File_set_errhandler \%<#\:mpi-file-set-errhandler>
.IP \(bu 2
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>
.IP \(bu 2
MPI_Win_create_errhandler \%<#\:mpi-win-create-errhandler> then MPI_Win_set_errhandler \%<#\:mpi-win-set-errhandler>
.UNINDENT
.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the MPI man page \%<#\:open-mpi> for a full list of MPI error codes \%<#\:open-mpi-errors>\&.
.sp
See the Error Handling section of the MPI\-5.0 standard for
more information.
.sp
\fBSee also:\fP
.INDENT 0.0
.INDENT 3.5
.INDENT 0.0
.IP \(bu 2
MPI_Unpack \%<#\:mpi-unpack>
.IP \(bu 2
MPI_Pack_size \%<#\:mpi-pack-size>
.UNINDENT
.UNINDENT
.UNINDENT
.SH Author
The Open MPI community
.SH Copyright
2003-2026, The Open MPI Community
.\" End of generated man page.