gm2-libs-iso/IOChan (The GNU Modula-2 Compiler)

#### 4.4.13 gm2-libs-iso/IOChan ¶

    DEFINITION MODULE IOChan;

      (* Types and procedures forming the interface to channels for
         device-independent data transfer modules
      *)

    IMPORT IOConsts, ChanConsts, SYSTEM;

    TYPE
      ChanId; (* Values of this type are used to identify channels *)

      (* There is one pre-defined value identifying an invalid channel
         on which no data transfer operations are available.  It may
         be used to initialize variables of type ChanId.
      *)

    PROCEDURE InvalidChan (): ChanId;
      (* Returns the value identifying the invalid channel. *)

      (* For each of the following operations, if the device supports
         the operation on the channel, the behaviour of the procedure
         conforms with the description below.  The full behaviour is
         defined for each device module.  If the device does not
         support the operation on the channel, the behaviour of the
         procedure is to raise the exception notAvailable.
      *)

      (* Text operations - these perform any required translation between the
         internal and external representation of text.
      *)

    PROCEDURE Look (cid: ChanId; VAR ch: CHAR; VAR res: IOConsts.ReadResults);
      (* If there is a character as the next item in the input stream
         cid, assigns its value to ch without removing it from the stream;
         otherwise the value of ch is not defined.  res (and the stored
         read result) are set to the value allRight, endOfLine, or endOfInput.
      *)

    PROCEDURE Skip (cid: ChanId);
      (* If the input stream cid has ended, the exception skipAtEnd
         is raised; otherwise the next character or line mark in cid is
         removed, and the stored read result is set to the value
         allRight.
      *)

    PROCEDURE SkipLook (cid: ChanId; VAR ch: CHAR; VAR res: IOConsts.ReadResults);
      (* If the input stream cid has ended, the exception skipAtEnd is
         raised; otherwise the next character or line mark in cid is
         removed.  If there is a character as the next item in cid
         stream, assigns its value to ch without removing it from the
         stream.  Otherwise, the value of ch is not defined.  res
         (and the stored read result) are set to the value allRight,
         endOfLine, or endOfInput.
      *)

    PROCEDURE WriteLn (cid: ChanId);
      (* Writes a line mark over the channel cid. *)

    PROCEDURE TextRead (cid: ChanId; to: SYSTEM.ADDRESS; maxChars: CARDINAL;
                        VAR charsRead: CARDINAL);
      (* Reads at most maxChars characters from the current line in cid,
         and assigns corresponding values to successive components of
         an ARRAY OF CHAR variable for which the address of the first
         component is to. The number of characters read is assigned to charsRead.
         The stored read result is set to allRight, endOfLine, or endOfInput.
      *)

    PROCEDURE TextWrite (cid: ChanId; from: SYSTEM.ADDRESS;
                         charsToWrite: CARDINAL);
      (* Writes a number of characters given by the value of charsToWrite,
         from successive components of an ARRAY OF CHAR variable for which
         the address of the first component is from, to the channel cid.
      *)

      (* Direct raw operations  - these do not effect translation between
         the internal and external representation of data
      *)

    PROCEDURE RawRead (cid: ChanId; to: SYSTEM.ADDRESS; maxLocs: CARDINAL;
                       VAR locsRead: CARDINAL);
      (* Reads at most maxLocs items from cid, and assigns corresponding
         values to successive components of an ARRAY OF LOC variable for
         which the address of the first component is to. The number of
         characters read is assigned to charsRead. The stored read result
         is set to the value allRight, or endOfInput.
      *)

    PROCEDURE RawWrite (cid: ChanId; from: SYSTEM.ADDRESS; locsToWrite: CARDINAL);
      (* Writes a number of items given by the value of charsToWrite,
         from successive components of an ARRAY OF LOC variable for
         which the address of the first component is from, to the channel cid.
      *)

      (* Common operations *)

    PROCEDURE GetName (cid: ChanId; VAR s: ARRAY OF CHAR);
      (* Copies to s a name associated with the channel cid, possibly truncated
         (depending on the capacity of s).
      *)

    PROCEDURE Reset (cid: ChanId);
      (* Resets the channel cid to a state defined by the device module. *)

    PROCEDURE Flush (cid: ChanId);
      (* Flushes any data buffered by the device module out to the channel cid. *)

      (* Access to read results *)

    PROCEDURE SetReadResult (cid: ChanId; res: IOConsts.ReadResults);
      (* Sets the read result value for the channel cid to the value res. *)

    PROCEDURE ReadResult (cid: ChanId): IOConsts.ReadResults;
      (* Returns the stored read result value for the channel cid.
         (This is initially the value notKnown).
      *)

      (* Users can discover which flags actually apply to a channel *)

    PROCEDURE CurrentFlags (cid: ChanId): ChanConsts.FlagSet;
      (* Returns the set of flags that currently apply to the channel cid. *)

      (* The following exceptions are defined for this module and its clients *)

    TYPE
      ChanExceptions =
        (wrongDevice,      (* device specific operation on wrong device *)
         notAvailable,     (* operation attempted that is not available on that
                              channel *)
         skipAtEnd,        (* attempt to skip data from a stream that has ended *)
         softDeviceError,  (* device specific recoverable error *)
         hardDeviceError,  (* device specific non-recoverable error *)
         textParseError,   (* input data does not correspond to a character or
                              line mark - optional detection *)
         notAChannel       (* given value does not identify a channel -
                              optional detection *)
        );

    PROCEDURE IsChanException (): BOOLEAN;
      (* Returns TRUE if the current coroutine is in the exceptional
         execution state because of the raising of an exception from
         ChanExceptions; otherwise returns FALSE.
      *)

    PROCEDURE ChanException (): ChanExceptions;
      (* If the current coroutine is in the exceptional execution state
         because of the raising of an exception from ChanExceptions,
         returns the corresponding enumeration value, and otherwise
         raises an exception.
      *)

      (* When a device procedure detects a device error, it raises the
         exception softDeviceError or hardDeviceError.  If these
         exceptions are handled, the following facilities may be
         used to discover an implementation-defined error number for
         the channel.
      *)

    TYPE
      DeviceErrNum = INTEGER;

    PROCEDURE DeviceError (cid: ChanId): DeviceErrNum;
      (* If a device error exception has been raised for the channel cid,
         returns the error number stored by the device module.
      *)

    END IOChan.