---
name: "MPI_Pack"
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_Pack"
---

# MPI\_Pack(3)

*MPI\_Pack* — Packs data of a given datatype into contiguous memory.

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Pack(const void *inbuf, int incount, MPI_Datatype datatype,
     void *outbuf, int outsize, int *position, MPI_Comm comm)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_PACK(INBUF, INCOUNT, DATATYPE, OUTBUF,OUTSIZE, POSITION,
             COMM, IERROR)
     <type>  INBUF(*), OUTBUF(*)
     INTEGER INCOUNT, DATATYPE, OUTSIZE, POSITION, COMM, IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Pack(inbuf, incount, datatype, outbuf, outsize, position, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
     TYPE(*), DIMENSION(..) :: outbuf
     INTEGER, INTENT(IN) :: incount, outsize
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     INTEGER, INTENT(INOUT) :: position
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **inbuf**: Input buffer start (choice).

- •

  **incount**: Number of input data items (integer).

- •

  **datatype**: Datatype of each input data item (handle).

- •

  **outsize**: Output buffer size, in bytes (integer).

- •

  **comm**: Communicator for packed message (handle).

## INPUT/OUTPUT PARAMETER

- •

  **position**: Current position in buffer, in bytes (integer).

## OUTPUT PARAMETERS

- •

  **outbuf**: Output buffer start (choice).

- •

  **ierror**: Fortran only: Error status (integer).

## DESCRIPTION

Packs the message in the send buffer specified by *inbuf*, *incount*, *datatype* into the buffer space specified by *outbuf* and *outsize*. The input buffer can be any communication buffer allowed in *MPI\_Send*. The output buffer is a contiguous storage area containing *outsize* bytes, starting at the address *outbuf* (length is counted in bytes, not elements, as if it were a communication buffer for a message of type MPI\_Packed).

The input value of *position* is the first location in the output buffer to be used for packing. *position* is incremented by the size of the packed message, and the output value of *position* is the first location in the output buffer following the locations occupied by the packed message. The *comm* argument is the communicator that will be subsequently used for sending the packed message.

**Example:** An example using *MPI\_Pack*:

```
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);
}
```

## ERRORS

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\_Unpack*

- •

  *MPI\_Pack\_size*

## COPYRIGHT

2003-2026, The Open MPI Community