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5.13.7 Stream I/O in Regina 0.07a
Regina implements stream I/O in a fashion that closely resembles
how it is described in TRL2. The following list gives the relevant
system-specific information.

[Names for standard streams.]
     Regina uses <stdout> and <stdin> as names for the standard
     output and input streams. Note that the angle brackets are
     part of the names.  You may also access the standard error
     stream (on systems supporting this stream) under the name
     <stderr>.  In addition, the nullstring is taken to be
     equivalent to an empty first parameter in the I/O-related
     built-in functions.

[Implicit opening.]
     Regina implicitly opens any file whenever it is first used.

     If the first operation is a read, it will be opened in read-
     only mode. If the first operation is a write, it is opened in
     read-write mode. In this case if the read-write opening does
     not succeed, the file is opened in write-only mode. If the
     file exists, the opening is non-destructive, i.e. that the
     file is not truncated or overwritten when opened, else it is
     created if opened in read-write mode.

     If you name a file currently open in read-only mode in a
     write operation, Regina closes the file, and reopens it in
     read-write mode. The only exception is when you call
     LINEOUT() with both second and third arguments unspecified,
     which always closes a file, both for reading and writing.
     Similarly, if the file was opened in write-only mode, and you
     use it in a read operation, Regina closes and reopens in read-
     write mode.

     This implicit reopening is enabled by default. You can turn
     it off by unsetting the extension ExplicitOpen.

[Separate current positions.]
     The environment in which Regina operates (ANSI C and POSIX)
     does not allow separate read and write positions, but only
     supplies one position for both operations. Regina handles
     this by maintaining the two positions internally, and move
     the “real” current position back and forth depending on
     whether a read or write operation is next.

[Swapping out file descriptors.]
     In order to defend itself against “open-many-close-none”
     programming, Regina tries to “swap out” files that have been
     unused for some time. Assume that your operating system
     limits Regina to 100 simultaneously open files; when your try
     to open your 101st file, Regina closes the least recently
     used stream, and recycles its descriptor for the new file.
     You can enable or disable this recycling with the SwapFilePtr
     extension.

     During this recycling, Regina only closes the file in the
     operating system, but retains all vital information about the
     file itself. If you re-access the file later, Regina reopens
     it, and positions the current read and write positions at the
     correct (i.e. previous) positions.  This introduces some
     uncertainties into stream processing. Renaming a file affects
     it only if it gets swapped out. Since the swap operation is
     something the users do not see, it can cause some strange
     effects.

     Regina will not allow a transient stream to be swapped out,
     since they often are connected to some sort of active partner
     in the other end, and closing the file might kill the partner
     or make it impossible to reestablish the stream.  So only
     persistent files are swapped out.  Thus, you can still fill
     the file table in Regina.

[Explicit opening and closing.]
     Regina allows streams to be explicitly opened or closed
     through the use of the built-in function STREAM().  The exact
     syntax of this function is described in section stream. Old
     versions of Regina supported two non-standard built-in
     functions OPEN() and CLOSE() for these operations. These
     functions are still supported for compatibility reasons, but
     might be removed in future releases.  Their availability is
     controlled by the OpenBif and CloseBif extensions.

[Truncation after writing lines.]
     If you reposition line-wise the current write position to the
     middle of a file, Regina truncates the file at the new
     position. This happens whether data is written during the
     LINEOUT() or not.  If not, the file might contain half a
     line, some lines might disappear, and the linecount would in
     general be disrupted.  The availability of this behavior is
     controlled by LineOutTrunc, which is turned on by default.

     Unfortunately, the operation of truncating a file is not part
     of POSIX, and it might not exist on all systems, so on some
     rare systems, this truncating will not occur.  In order to be
     able to truncate a file, your machine must have the
     ftruncate() system call in C. If you don’t have this, the
     truncating functionality is not available.

[Caching info on lines left.]
     When Regina executes the built-in function LINES() for a
     persistent stream, it caches the number of lines left as an
     attribute to the stream. In subsequent calls to LINEIN(),
     this number is updated, so that subsequent calls to LINES()
     can retrieve the cached number instead of having to re-scan
     the rest of the stream, provided that the number is still
     valid. Some operations will invalidate the count:
     repositioning the current read position; reading using the
     character oriented I/O, i.e. CHARIN(); and any write
     operation by the same interpreter on the stream. Ideally, any
     write operation should invalidate the count, but that might
     require a large overhead before any operation, in order to
     check whether the file has been written to by other programs.

     This functionality can be controlled by the extension called
     CacheLineNo, which is turned on by default. Note that if you
     turn that off, you can experience a serious decrease in
     performance.

The following extra built-in functions relating to stream I/O are
defined in Regina. They are provided for extra support and
compatibility with other systems. Their support may be
discontinued in later versions, and they are likely to be moved to
a library of extra support.


CLOSE(streamid)

Closes the stream named by streamid. This stream must have been
opened by implicit open or by the OPEN function call earlier.  The
function returns 1 if there was any file to close, and 0 if the
file was not opened. Note that the return value does not indicate
whether the closing was successful. You can use the extension
named CloseBif with the OPTIONS instruction to select or remove
this function.  This function is now obsolete, instead you should
use:

     STREAM( streamid, ‘Command’, ‘CLOSE’ )

               CLOSE(myfile)     1 if stream was open
               CLOSE(‘NOSUCHFIL  0 if stream didn’t
               E’)                 exist



OPEN(streamid,access)

Opens the stream named streamid with the access access. If access
is not specified, the access R will be used. access may be the
following characters. Only the first character of the access is
needed.

[R]
     (Read) Open for read access. The file pointer will be
     positioned at the start of the file, and only read operations
     are allowed.

[W]
     (Write) Open for write access and position the current write
     position at the end of the file.  An error is returned if it
     was not possible to get appropriate access.

The return value from this function is either 1 or 0, depending on
whether the named stream is in opened state after the operation
has been performed.

Note that if you open the files “foobar” and “./foobar” they will
point to the same physical file, but Regina interprets them as two
different streams, and will open a internal file descriptor for
each one.  If you try to open an already open stream, using the
same name, it will have no effect.

You can use the extension OpenBif with the OPTIONS instruction to
control the availability of this function.  This function is now
obsolete, but is still kept for compatibility with other
interpreters and older versions of Regina. Instead, with Regina
you should use:

     STREAM( streamid, ‘C’, ‘READ’|’WRITE’|’APPEND’|’UPDATE’ )

               OPEN(myfile,     1 maybe, if successful
               ‘write’)
               OPEN(passwd,     0 maybe, if no write
               ‘Write’)           access
               OPEN(‘DATA’,     0 maybe, if successful
               ‘READ’)
The return value from this function is either 1 or 0, depending on
whether the named stream is in opened state after the operation
has been performed.



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