By default, a read operation on a socket will block until data becomes available. The procedure Peek enables you to test for the presence of data on a socket. This is useful for avoiding the use of timeouts on calls to Sockets[Read], Sockets[ReadLine], or Sockets[ReadBinary].

The procedure Peek never blocks; either the value false or a string is returned immediately.

When Peek is called, it "samples" the data available on the socket without reading it. A string containing the data "sample" is returned. The same data will be available to the next call to Sockets[Read], Sockets[ReadLine], or Sockets[ReadBinary]. If no data is available on the socket, then the value false is returned.

$\mathrm{with}\left(\mathrm{Sockets}\right)\:$

$\mathrm{sid}≔\mathrm{Open}\left(''localhost''\,''echo''\right)\:$

$\mathrm{Write}\left(\mathrm{sid}\,''Sample\; data''\right)\:$

$\mathrm{Peek}\left(\mathrm{sid}\right)$

$''Sample\; data''$

$\mathrm{Read}\left(\mathrm{sid}\right)$

$''Sample\; data''$

$\mathrm{Peek}\left(\mathrm{sid}\right)$

$\mathrm{false}$

$\mathrm{Write}\left(\mathrm{sid}\,''Sample\; data''\right)\:$

$$\begin{gathered} \mathbf{while}\mathrm{Peek}\left(\mathrm{sid}\right)\ne \mathrm{false}\mathbf{do} \\ \mathrm{Read}\left(\mathrm{sid}\right) \\ \mathbf{end}\mathbf{do} \end{gathered}$$

$''Sample\; data''$

$\mathrm{Close}\left(\mathrm{sid}\right)$

$\mathrm{true}$