# MPI reference

Quick reference for [[mpi|MPI]] functions, datatypes, and reduction operators.

## Functions

```c
// Initialisation
MPI_Init(&argc, &argv)                               // initialise MPI; must be first
MPI_Finalize()                                       // shut down MPI; must be last
MPI_Abort(comm, errorcode)                           // terminate all processes in comm

// Communicator queries
MPI_Comm_rank(comm, &rank)                           // rank of calling process in comm
MPI_Comm_size(comm, &size)                           // number of processes in comm
MPI_Comm_split(comm, color, key, &newcomm)           // partition into sub-communicators
MPI_Comm_free(&comm)                                 // release a communicator

// Point-to-point
MPI_Send(buf, count, type, dest, tag, comm)
MPI_Recv(buf, count, type, src, tag, comm, &status)
MPI_Sendrecv(sbuf, sc, st, dest, stag,
             rbuf, rc, rt, src,  rtag, comm, &status)
MPI_Isend(buf, count, type, dest, tag, comm, &req)
MPI_Irecv(buf, count, type, src,  tag, comm, &req)
MPI_Wait(&req, &status)
MPI_Test(&req, &flag, &status)
MPI_Waitall(count, reqs, statuses)

// Collectives
MPI_Barrier(comm)
MPI_Bcast(buf, count, type, root, comm)
MPI_Scatter(sbuf, sc, st, rbuf, rc, rt, root, comm)
MPI_Gather(sbuf, sc, st, rbuf, rc, rt, root, comm)
MPI_Scatterv(sbuf, scounts, displs, st, rbuf, rc, rt, root, comm)
MPI_Gatherv(sbuf, sc, st, rbuf, rcounts, displs, rt, root, comm)
MPI_Allgather(sbuf, sc, st, rbuf, rc, rt, comm)
MPI_Alltoall(sbuf, sc, st, rbuf, rc, rt, comm)
MPI_Reduce(sbuf, rbuf, count, type, op, root, comm)
MPI_Allreduce(sbuf, rbuf, count, type, op, comm)
MPI_Scan(sbuf, rbuf, count, type, op, comm)          // inclusive prefix reduction

// Derived datatypes
MPI_Type_contiguous(count, oldtype, &newtype)
MPI_Type_vector(count, blocklength, stride, oldtype, &newtype)
MPI_Type_create_struct(count, blocklengths, displs, types, &newtype)
MPI_Type_commit(&type)
MPI_Type_free(&type)

// One-sided
MPI_Win_create(base, size, disp_unit, info, comm, &win)
MPI_Put(obuf, oc, ot, target_rank, disp, tc, tt, win)
MPI_Get(obuf, oc, ot, target_rank, disp, tc, tt, win)
MPI_Accumulate(obuf, oc, ot, target_rank, disp, tc, tt, op, win)
MPI_Win_fence(assert, win)
MPI_Win_free(&win)

// Timing
MPI_Wtime()                                          // wall-clock time in seconds
MPI_Wtick()                                          // resolution of MPI_Wtime
```

## Built-in datatypes

^ C type ^ MPI type ^
| `int` | `MPI_INT` |
| `long` | `MPI_LONG` |
| `float` | `MPI_FLOAT` |
| `double` | `MPI_DOUBLE` |
| `char` | `MPI_CHAR` |
| `unsigned char` | `MPI_UNSIGNED_CHAR` |
| `long long` | `MPI_LONG_LONG` |

## Built-in reduction operators

^ Operator ^ Meaning ^
| `MPI_SUM` | sum |
| `MPI_PROD` | product |
| `MPI_MAX` | maximum |
| `MPI_MIN` | minimum |
| `MPI_LAND` | logical and |
| `MPI_LOR` | logical or |
| `MPI_BAND` | bitwise and |
| `MPI_BOR` | bitwise or |
| `MPI_MAXLOC` | maximum value and the rank that holds it |
| `MPI_MINLOC` | minimum value and the rank that holds it |