Next: Writing to Streams, Previous: Stream IO Spaces, Up: Filters [Contents][Index]
Unlike normal Unix file descriptors, read-only poke streams are buffered. Let’s supposed we open the standard input:
var stdin = open ("<stdin>");
Then we map some data (one byte) at some particular offset:
var b = byte @ stdin : 28#B;
The mapping operation above causes poke to read 28 bytes from the
standard input and to set the variable b
to the 28-th byte.
The bytes read from the stream are still available (buffered) and the
offsets are still relative from the “beginning” of the standard
input:
var c = byte @stdin : 0#B;
The byte in c
is the byte that appeared in the standard input
27 bytes before the byte in b
.
This buffering is nice, and it is what allows us to access the standard input like if it were a random oriented device. However, it is obvious we would be in trouble if we filter big amounts of data, like in a network interface: we would likely use all available memory.
To allow filtering big amount of data, poke allows to flush the read-only streams. Flushing means that the buffered data in the read-only stream is “forgotten”, and trying to access it will result in an exception:
var stdin = open ("<stdin>"); var b = byte @ stdin : 28#B; var c = byte @ stdin : 29#B; flush (stdin, 29#B); var d = byte @ stdin : 30#B; var e = byte @ stdin : 20#B; /* Exception. */