hi,

the parameter validation for the optional "start" and "end" arguments to
"recvfrom!" are off by one if "end" is passed. From libguile/socket.c
(master commit 64c89458e6):

  ...
  if (SCM_UNBNDP (end))
    cend = SCM_BYTEVECTOR_LENGTH (buf);
  else
    {
      cend = scm_to_size_t (end);
      if (SCM_UNLIKELY (cend >= SCM_BYTEVECTOR_LENGTH (buf)
                        || cend < offset))
        scm_out_of_range (FUNC_NAME, end);
    }
  ...

"end" is the optional end argument, "offset" is 0 or "start" if start
was given. The check must be:

  cend > SCM_BYTEVECTOR_LENGTH (buf) || cend <= offset

to allow filling the last byte in the buffer and verify that start is
not equal to end. A workaround to skip the validation is to not pass
end. But i think a better way would be to always validate start (and
end), if one (or both) of them are passed. A potentional fix is
attached.

If you need any additional information, please let me know.

Thank you for your great work!

-  d4ryus