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4.3.6 The NOTREADY condition
The condition NOTREADY is a condition of mixed origin. It is
raised as a result of problems with stream I/O. Exactly what
causes it, may vary between implementations, but some of the more
probable causes are: waiting for more I/O on transient streams;
access to streams not allowed; I/O operation would block if
attempted; etc. See the chapter; Stream Input and Output for more

Differences between NOTREADY and the standard condition are:

    It will be ignored rather than queued if condition trap is in
  state DELAY.

    This condition differs from the rest in that it can be raised
  during execution of a clause, but can still be trapped by method

The descriptive text returned by CONDITION() when called with the
Description option for condition NOTREADY, is the name of the
stream which caused the problem. This is probably the same string
that you used as the first parameter to the functions that
operates on stream I/O. For the default streams (default input and
output stream), the string returned by CONDITION() will be

Note that if the NOTREADY trap is in state DELAY, then all I/O for
files which has tried to raise NOTREADY within the current clause
will be simulated as if operation had succeeded.