---
name: "MPI_Pack_size_c"
section: "3"
platform: "linux"
package: "mpich-doc"
version: "4.0.2-3"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Pack_size_c"
---

# MPI\_Pack\_size\_c(3)

Returns the upper bound on the amount of space needed to pack a message

## NAME

MPI\_Pack\_size\_c -  Returns the upper bound on the amount of space needed to pack a message

## SYNOPSIS

```
int MPI_Pack_size_c(MPI_Count incount, MPI_Datatype datatype, MPI_Comm comm, MPI_Count *size)
```

## INPUT PARAMETERS

- **incount**

  \- count argument to packing call (non-negative integer) (non-negative integer)

- **datatype**

  \- datatype argument to packing call (handle) (handle)

- **comm**

  \- communicator argument to packing call (handle) (handle)

## OUTPUT PARAMETERS

- **size**

  \- upper bound on size of packed message, in bytes (non-negative integer) (non-negative integer)

## NOTES

The MPI standard document describes this in terms of  *MPI\_Pack* , but it applies to both  *MPI\_Pack* and  *MPI\_Unpack* . That is, the value  *size* is the maximum that is needed by either  *MPI\_Pack* or  *MPI\_Unpack* .

## THREAD AND INTERRUPT SAFETY

This routine is thread-safe.  This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks.  However, the routine is not interrupt safe.  Typically, this is due to the use of memory allocation routines such as  *malloc* or other non-MPICH runtime routines that are themselves not interrupt-safe.

## NOTES FOR FORTRAN

All MPI routines in Fortran (except for  *MPI\_WTIME* and  *MPI\_WTICK* ) have an additional argument  *ierr* at the end of the argument list.   *ierr* is an integer and has the same meaning as the return value of the routine in C.  In Fortran, MPI routines are subroutines, and are invoked with the *call* statement.

All MPI objects (e.g.,  *MPI\_Datatype* ,  *MPI\_Comm* ) are of type  *INTEGER* in Fortran.

## ERRORS

All MPI routines (except  *MPI\_Wtime* and  *MPI\_Wtick* ) return an error value; C routines as the value of the function and Fortran routines in the last argument.  Before the value is returned, the current MPI error handler is called.  By default, this error handler aborts the MPI job.  The error handler may be changed with  *MPI\_Comm\_set\_errhandler* (for communicators), *MPI\_File\_set\_errhandler* (for files), and  *MPI\_Win\_set\_errhandler* (for RMA windows).  The MPI-1 routine  *MPI\_Errhandler\_set* may be used but its use is deprecated.  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; however, MPI implementations will attempt to continue whenever possible.

- **MPI\_SUCCESS**

  \- No error; MPI routine completed successfully.

- **MPI\_ERR\_ARG**

  \- Invalid argument.  Some argument is invalid and is not identified by a specific error class (e.g.,  *MPI\_ERR\_RANK* ).

- **MPI\_ERR\_COMM**

  \- Invalid communicator.  A common error is to use a null communicator in a call (not even allowed in  *MPI\_Comm\_rank* ).

- **MPI\_ERR\_COUNT**

  \- Invalid count argument.  Count arguments must be  non-negative; a count of zero is often valid.

- **MPI\_ERR\_TYPE**

  \- Invalid datatype argument.  Additionally, this error can occur if an uncommitted MPI\_Datatype (see  *MPI\_Type\_commit* ) is used in a communication call.

- **MPI\_ERR\_OTHER**

  \- Other error; use  *MPI\_Error\_string* to get more information about this error code.