---
name: "MPI_Win_start"
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_Win_start"
---

# MPI\_Win\_start(3)

*MPI\_Win\_start* — Starts an RMA access epoch for *win*

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Win_start(MPI_Group group, int assert, MPI_Win win)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WIN_START(GROUP, ASSERT, WIN, IERROR)
     INTEGER GROUP, ASSERT, WIN, IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Win_start(group, assert, win, ierror)
     TYPE(MPI_Group), INTENT(IN) :: group
     INTEGER, INTENT(IN) :: assert
     TYPE(MPI_Win), INTENT(IN) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **group**: The group of target processes (handle).

- •

  **assert**: Program assertion (integer).

- •

  **win**: Window object (handle).

## OUTPUT PARAMETERS

- •

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

## DESCRIPTION

*MPI\_Win\_start* is a one-sided MPI communication synchronization call that starts an RMA access epoch for *win*. RMA calls issued on *win* during this epoch must access only windows at processes in *group*. Each process in *group* must issue a matching call to *MPI\_Win\_post*. *MPI\_Win\_start* is allowed to block until the corresponding *MPI\_Win\_post* calls have been executed, but is not required to.

The *assert* argument is used to provide assertions on the context of the call that may be used for various optimizations. (See Section 6.4.4 of the MPI-2 Standard.) A value of *assert* = 0 is always valid. The following assertion value is supported:

- **MPI\_MODE\_NOCHECK**

  When this value is passed in to this call, the library assumes that the post call on the target has been called and it is not necessary for the library to check to see if such a call has been made.

## 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\_Win\_post*

- •

  *MPI\_Win\_complete*

## COPYRIGHT

2003-2026, The Open MPI Community