This solves highly theoretical race conditions of file systems being
unmounted in an application while a shell `df` runs, fixes the previous
weird behavior that `/mountpoint/non-existant-path` could be df'd and
would even report that non-existant path as a file name, but more
practically ensures that an example of vfs_iter_mount_dirs is around.
The RSSI values reported by LoRa transceiver can be less than -127.
Therefore, `int8_t` is not enough. This commit defines the RSSI of
`netdev_lora_rx_info` as `int16_t` and adapt the drivers accordingly
(sx126x, sx127x).
`msg_avail()` will return -1 if the thread has no message queue.
Casting this to unsigned will result in the `ping` command to try
receiving 4294967295 messages, which hangs the shell.
Drop the cast to `unsigned` and the loop behaves as intended.
But then it's still wrong: If new messages become available, they
would be ignored.
So change the `for` loop to a `while` loop. The index variable is
not used at all.
Currently a valid netif name must be passed to show the usage
instructions:
```
> ifconfig help
error: invalid interface given
> ifconfig 6 help
usage: ifconfig
usage: ifconfig <if_id> [up|down]
[...]
```
`ifconfig --help` is also accepted.
The Linux ping utility has the nice feature that fills the ICMPv6 echo
request payload with a pattern `payload_index & 0xFF`.
Then the ICMPv6 echo response payload is checked to verify that the pattern
is still intact.
This way corrupted messages can be detected.
In the past that revealed some 6lo-fragmentation bugs in Linux when
corrupted replies arrived.
This feature is also useful for RIOT, so implement it in RIOTs `ping`
command.
The `udp` command is a valuable debugging tool that is also useful
outside of the gnrc_networking example.
To enable easy sending of udp messages in other applications during
development, move the `udp` command to the shell module and introduce
the `gnrc_udp_cmd` pseudo-module to enable it.
