# MPI Probing for messages
**Probing** checks for incoming messages without consuming them. `MPI_Probe` returns metadata in `MPI_Status` (including element count) so you can allocate the right buffer before calling `MPI_Recv`.

```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.