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.

// 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.