int MPI_Win_lock(int lock_type, int rank, int assert, MPI_Win win)
The name of this routine is misleading. In particular, this routine need not block, except when the target process is the calling process.
Implementations may restrict the use of RMA communication that is synchronized by lock calls to windows in memory allocated by MPI_Alloc_mem. Locks can be used portably only in such memory.
The assert argument is used to indicate special conditions for the fence that an implementation may use to optimize the MPI_Win_lock operation. The value zero is always correct. Other assertion values may be or'ed together. Assertions that are valid for MPI_Win_lock are:
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.
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.
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 guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.