# MPI Send modes
**Send modes** in MPI make blocking behaviour explicit. Standard `MPI_Send` may block depending on message size; `MPI_Ssend` always synchronizes, `MPI_Bsend` always buffers, and `MPI_Rsend` requires a posted receive.

`MPI_Ssend` (synchronous send) never buffers. It blocks until the receiver has posted a matching `MPI_Recv`, which makes it the most useful tool for debugging deadlocks: replacing `MPI_Send` with `MPI_Ssend` removes buffering as a variable, so if the code hangs it definitely has a cyclic dependency rather than just exceeding buffer limits. `MPI_Bsend` (buffered send) always returns immediately by copying into a user-supplied buffer attached with `MPI_Buffer_attach`. `MPI_Rsend` (ready send) asserts that a matching receive is already posted; behaviour is undefined if it is not, but it can skip handshake overhead on implementations that exploit the guarantee.

```c
// synchronous: blocks until receiver has called MPI_Recv
MPI_Ssend(&x, 1, MPI_INT, dest, 0, MPI_COMM_WORLD);

// buffered: always returns immediately; user manages the buffer
char buf[MPI_BSEND_OVERHEAD + sizeof(int)];
MPI_Buffer_attach(buf, sizeof(buf));
MPI_Bsend(&x, 1, MPI_INT, dest, 0, MPI_COMM_WORLD);
MPI_Buffer_detach(&buf_ptr, &buf_size);  // blocks until buffer is drained
```

All four modes have non-blocking variants: `MPI_Issend`, `MPI_Ibsend`, `MPI_Irsend`. They follow the same request/wait pattern as `MPI_Isend`.