Copyright (C) 1994, Digital Equipment Corp.An
Rd.T (or ``reader'') is a character input stream. The basic
operation on a reader is GetChar, which returns the source
character at the ``current position'' and advances the current
position by one. Some readers are ``seekable'', which means that
they also allow setting the current position anywhere in the
source. For example, readers from random access files are
seekable; readers from terminals and sequential files are not.
\index{character input stream}
\index{input stream}
\index{stream!input}
\index{reader}
Some readers are ``intermittent'', which means that the source of the reader trickles in rather than being available to the implementation all at once. For example, the input stream from an interactive terminal is intermittent. An intermittent reader is never seekable.
Abstractly, a reader rd consists of
len(rd) the number of source characters
src(rd) a sequence of length len(rd)+1
cur(rd) an integer in the range [0..len(rd)]
avail(rd) an integer in the range [cur(rd)..len(rd)+1]
closed(rd) a boolean
seekable(rd) a boolean
intermittent(rd) a boolean
These values are not necessarily directly represented in the data
fields of a reader object. In particular, for an intermittent
reader, len(rd) may be unknown to the implementation. But in
principle the values determine the state of the reader.
The sequence src(rd) is zero-based: src(rd)[i] is valid for i
from 0 to len(rd). The first len(rd) elements of src are the
characters that are the source of the reader. The final element is
a special value eof used to represent end-of-file. The value
eof is not a character.
The value of cur(rd) is the index in src(rd) of the next
character to be returned by GetChar, unless cur(rd) = len(rd),
in which case a call to GetChar will raise the exception
EndOfFile.
The value of avail(rd) is important for intermittent readers: the
elements whose indexes in src(rd) are in the range
[cur(rd)..avail(rd)-1] are available to the implementation and
can be read by clients without blocking. If the client tries to
read further, the implementation will block waiting for the other
characters. If rd is not intermittent, then avail(rd) is equal
to len(rd)+1. If rd is intermittent, then avail(rd) can
increase asynchronously, although the procedures in this interface
are atomic with respect to such increases.
The definitions above encompass readers with infinite sources. If
rd is such a reader, then len(rd) and len(rd)+1 are both
infinity, and there is no final eof value.
Every reader is a monitor; that is, it contains an internal lock
that is acquired and held for each operation in this interface, so
that concurrent operations will appear atomic. For faster,
unmonitored access, see the UnsafeRd interface.
If you are implementing a long-lived reader class, such as a pipe or TCP stream, the index of the reader may eventually overflow, causing the program to crash with a bounds fault. We recommend that you provide an operation to reset the reader index, which the client can call periodically.
INTERFACESince there are many classes of readers, there are many ways that a reader can break---for example, the connection to a terminal can be broken, the disk can signal a read error, etc. All problems of this sort are reported by raising the exceptionRd ; IMPORT AtomList; FROM Thread IMPORT Alerted; TYPE T <: ROOT; EXCEPTION EndOfFile; Failure(AtomList.T);
Failure. The
documentation of a reader class should specify what failures the
class can raise and how they are encoded in the argument to
Failure.
Illegal operations cause a checked runtime error.
PROCEDURE GetChar(rd: T): CHAR
RAISES {EndOfFile, Failure, Alerted};
Return the next character fromrd. More precisely, this is equivalent to the following, in whichresis a local variable of typeCHAR:
IF closed(rd) THEN Cause checked runtime error END;
Block until avail(rd) > cur(rd);
IF cur(rd) = len(rd) THEN
RAISE EndOfFile
ELSE
res := src(rd)[cur(rd)]; INC(cur(rd)); RETURN res
END
Many operations on a reader can wait indefinitely. For example,
GetChar can wait if the user is not typing. In general these waits
are alertable, so each procedure that might wait includes
Thread.Alerted in its RAISES clause.
PROCEDURE EOF(rd: T): BOOLEAN RAISES {Failure, Alerted};
ReturnTRUEiffrdis at end-of-file. More precisely, this is equivalent to:
IF closed(rd) THEN Cause checked runtime error END;
Block until avail(rd) > cur(rd);
RETURN cur(rd) = len(rd)
Notice that on an intermittent reader, EOF can block. For example, if
there are no characters buffered in a terminal reader, EOF must wait
until the user types one before it can determine whether he typed the
special key signalling end-of-file. If you are using EOF in an
interactive input loop, the right sequence of operations is:
\begin{enumerate}
\item prompt the user;
\item call EOF, which probably waits on user input;
\item presuming that EOF returned FALSE, read the user's input.
\end{enumerate}
PROCEDURE UnGetChar(rd: T) RAISES {};
``Push back'' the last character read fromrd, so that the next call toGetCharwill read it again. More precisely, this is equivalent to the following:
IF closed(rd) THEN Cause checked runtime error END;
IF cur(rd) > 0 THEN DEC(cur(rd)) END
except there is a special rule: UnGetChar(rd) is guaranteed to
work only if GetChar(rd) was the last operation on rd. Thus
UnGetChar cannot be called twice in a row, or after Seek or
EOF. If this rule is violated, the implementation is allowed (but
not required) to cause a checked runtime error.
PROCEDURE CharsReady(rd: T): CARDINAL RAISES {Failure};
Return some number of characters that can be read without indefinite waiting. The ``end of file marker'' counts as one character for this purpose, soCharsReadywill return 1, not 0, ifEOF(rd)is true. More precisely, this is equivalent to the following:
IF closed(rd) THEN Cause checked runtime error END;
IF avail(rd) = cur(rd) THEN
RETURN 0
ELSE
RETURN some number in the range [1~..~avail(rd) - cur(rd)]
END;
Warning: CharsReady can return a result less than avail(rd) -
cur(rd); also, more characters might trickle in just as
CharsReady returns. So the code to flush buffered input without
blocking requires a loop:
LOOP
n := Rd.CharsReady(rd);
IF n = 0 THEN EXIT END;
FOR i := 1 TO n DO EVAL Rd.GetChar(rd) END
END;
PROCEDURE GetSub(rd: T; VAR (*OUT*) str: ARRAY OF CHAR)
: CARDINAL RAISES {Failure, Alerted};
Read from rd into str until rd is exhausted or str is
filled. More precisely, this is equivalent to the following, in
which i is a local variable:
i := 0;
WHILE i # NUMBER(str) AND NOT EOF(rd) DO
str[i] := GetChar(rd); INC(i)
END;
RETURN i
PROCEDURE GetSubLine(rd: T; VAR (*OUT*) str: ARRAY OF CHAR)
: CARDINAL RAISES {Failure, Alerted};
Read from rd into str until a newline is read, rd is
exhausted, or str is filled. More precisely, this is equivalent
to the following, in which i is a local variable:
i := 0;
WHILE
i # NUMBER(str) AND
(i = 0 OR str[i-1] # '\n') AND
NOT EOF(rd)
DO
str[i] := GetChar(rd); INC(i)
END;
RETURN i
Note that GetLine strips the terminating line break, while
GetSubLine does not.
PROCEDURE GetText(rd: T; len: CARDINAL): TEXT
RAISES {Failure, Alerted};
Read from rd until it is exhausted or len characters have been
read, and return the result as a TEXT. More precisely, this is
equivalent to the following, in which i and res are local
variables:
res := ""; i := 0;
WHILE i # len AND NOT EOF(rd) DO
res := res & Text.FromChar(GetChar(rd));
INC(i)
END;
RETURN res
PROCEDURE GetLine(rd: T): TEXT
RAISES {EndOfFile, Failure, Alerted};
If EOF(rd) then raise EndOfFile. Otherwise, read characters
until a line break is read or rd is exhausted, and return the
result as a TEXT---but discard the line break if it is present.
A line break is either {\tt "\n"} or {\tt
"\r\n"} More precisely, this is
equivalent to the following, in which ch and res are local
variables:
IF EOF(rd) THEN RAISE EndOfFile END;
res := ""; ch := '\000'; (* any char but newline
| WHILE ch # '\n' AND NOT EOF(rd) DO
| ch := GetChar(rd);
| IF ch = '\n' THEN
| IF NOT Text.Empty(res) AND
| Text.GetChar(res, Text.Length(res)-1) = '\r' THEN
| res := Text.Sub(res, 0, Text.Length(res)-1)
| END
| ELSE
| res := res & Text.FromChar(ch)
| END
| RETURN res
*)
PROCEDURE Seek(rd: T; n: CARDINAL) RAISES {Failure, Alerted};
This is equivalent to:
IF closed(rd) OR NOT seekable(rd) THEN
Cause checked runtime error
END;
cur(rd) := MIN(n, len(rd))
PROCEDURE Close(rd: T) RAISES {Failure, Alerted};
Release any resources associated with rd and set closed(rd) :=
TRUE. The documentation of a procedure that creates a reader
should specify what resources are released when the reader is
closed. This leaves rd closed even if it raises an exception,
and is a no-op if rd is closed.
PROCEDURE Index(rd: T): CARDINAL RAISES {};
This is equivalent to:
IF closed(rd) THEN Cause checked runtime error END;
RETURN cur(rd)
PROCEDURE Length(rd: T): INTEGER RAISES {Failure, Alerted};
This is equivalent to:
IF closed(rd) THEN
Cause checked runtime error
END;
RETURN len(rd)
If len(rd) is unknown to the implementation of an intermittent
reader, Length(rd) returns -1.
PROCEDURE Intermittent(rd: T): BOOLEAN RAISES {};
PROCEDURE Seekable(rd: T): BOOLEAN RAISES {};
PROCEDURE Closed(rd: T): BOOLEAN RAISES {};
Return intermittent(rd), seekable(rd), and closed(rd),
respectively. These can be applied to closed readers.
END Rd.