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# Blocking and non-blocking (MPI)
A `read()` call on a file descriptor blocks: it doesn't return until the kernel has data to hand back. That is convenient but ties up the thread for the duration of the wait. POSIX offers `aio_read()` as an alternative: issue the request, do other work, and check completion later with `aio_suspend()`. MPI has the same split. `MPI_Send` and `MPI_Recv` are blocking: `MPI_Send` does not return until the send buffer is safe to reuse, and `MPI_Recv` does not return until the receive buffer holds the data. `MPI_Isend` and `MPI_Irecv` are the non-blocking counterparts — they return immediately with a request handle, and `MPI_Wait` blocks until the transfer is complete.

```c
MPI_Request req;
int x = 42;
MPI_Isend(&x, 1, MPI_INT, 1, 0, MPI_COMM_WORLD, &req);
do_other_work();            // overlap computation with communication
MPI_Wait(&req, MPI_STATUS_IGNORE);  // x must not change before this point
```

`MPI_Test` checks whether a request is complete without blocking, returning a flag. `MPI_Waitall` and `MPI_Testall` handle arrays of requests, which is common when a process has posted non-blocking sends to multiple neighbors. Non-blocking communication is the primary tool for hiding communication latency behind useful computation.