# MPI_T_pvar_start

Start a performance variable

## Synopsis

    #ifdef ENABLE_QMPI
    #ifndef MPICH_MPI_FROM_PMPI
    int QMPI_T_pvar_start(QMPI_Context context, int tool_id, MPI_T_pvar_session session,
                          MPI_T_pvar_handle handle) MPICH_API_PUBLIC

## Input Parameters

session
  identifier of performance experiment session (handle)
handle
  handle of a performance variable (handle)

## Notes

If the constant MPI_T_PVAR_ALL_HANDLES is passed in handle, the MPI implementation attempts to start all variables within the session identified by the parameter session for which handles have been allocated. In this case, the routine returns MPI_SUCCESS if all variables are started successfully, otherwise MPI_T_ERR_PVAR_NO_STARTSTOP is returned. Continuous variables and variables that are already started are ignored when MPI_T_PVAR_ALL_HANDLES is specified.

## 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.

## 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_T_ERR_INVALID_HANDLE
  The handle is invalid.

MPI_T_ERR_INVALID_SESSION
  Session argument is not valid.

MPI_T_ERR_NOT_INITIALIZED
  The MPI tool information interface is not initialized.

MPI_T_ERR_PVAR_NO_STARTSTOP
  The performance variable can not be started or stopped.

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.