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Jarkko Hietaniemi


open - perl pragma to set default disciplines for input and output


    use open IN => ":crlf", OUT => ":raw";


Full-fledged support for I/O disciplines is now implemented provided perl is configured to use PerlIO as its IO system (which is now the default).

The open pragma serves as one of the interfaces to declare default "layers" (aka disciplines) for all I/O.

The open pragma is used to declare one or more default layers for I/O operations. Any open(), readpipe() (aka qx//) and similar operators found within the lexical scope of this pragma will use the declared defaults.

When open() is given an explicit list of layers they are appended to the list declared using this pragma.

Directory handles may also support disciplines in future.


If perl is not built to use PerlIO as its IO system then only the two pseudo-disciplines ":raw" and ":crlf" are available.

The ":raw" discipline corresponds to "binary mode" and the ":crlf" discipline corresponds to "text mode" on platforms that distinguish between the two modes when opening files (which is many DOS-like platforms, including Windows). These two disciplines are no-ops on platforms where binmode() is a no-op, but perform their functions everywhere if PerlIO is enabled.


There are two package variables %layers and @layers which are mainly manipulated by C code in perlio.c, but are visible to the nosy:

  print "Have ",join(',',keys %open::layers),"\n";
  print "Using ",join(',',@open::layers),"\n";

The %open::layers hash is a record of the available "layers" that may be pushed onto a PerlIO stream. The values of the hash are perl objects, of class PerlIO::Layer which are created by the C code in perlio.c. As yet there is nothing useful you can do with the objects at the perl level.

The @open::layers array is the current set of layers and their arguments. The array consists of layer => argument pairs and must always have even number of entries and the even entries must be PerlIO::Layer objects or perl will "die" when it attempts to open a filehandle. In most cases the odd entry will be undef, but in the case of (say) ":encoding(iso-8859-1)" it will be 'iso-8859-1'. These argument entries are currently restricted to being strings.

When a new PerlIO stream is opened, the C code looks at the array to determine the default layers to be pushed. So with care it is possible to manipulate the default layer "stack":

    push(@PerlIO::layers,$PerlIO::layers{'stdio'} => undef);


"binmode" in perlfunc, "open" in perlfunc, perlunicode, PerlIO