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# Probing for messages (MPI)
A standard `recv()` on a socket requires a pre-allocated buffer. If the incoming message is larger than the buffer, data gets truncated or the call fails. The POSIX escape hatch is `MSG_PEEK`: pass it as a flag and `recv()` fills your buffer but leaves the data in the socket queue, letting you inspect the size before committing to a real receive. `ioctl(fd, FIONREAD, &n)` goes further and tells you exactly how many bytes are waiting.

MPI's `MPI_Probe` does the same thing: it matches an incoming message and fills an `MPI_Status` struct with metadata — including the element count — without consuming the message from the queue.

```c
MPI_Status status;
MPI_Probe(src, tag, MPI_COMM_WORLD, &status);

int count;
MPI_Get_count(&status, MPI_DOUBLE, &count);
double *buf = malloc(count * sizeof(double));

MPI_Recv(buf, count, MPI_DOUBLE, src, tag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
```

`MPI_Iprobe` is the non-blocking variant: it sets a flag and returns immediately whether or not a matching message is available. Both functions accept `MPI_ANY_SOURCE` and `MPI_ANY_TAG`. When wildcards are used, the source rank from the `MPI_Status` must be passed verbatim to the following `MPI_Recv` to ensure the same message is consumed, not a different one that arrived in between.