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diff --git a/pod/perliol.pod b/pod/perliol.pod index 9abc72ec59..8a41917224 100644 --- a/pod/perliol.pod +++ b/pod/perliol.pod @@ -24,6 +24,51 @@ The aim of the implementation is to provide the PerlIO API in a flexible and platform neutral manner. It is also a trial of an "Object Oriented C, with vtables" approach which may be applied to perl6. +=head2 Basic Structure + +PerlIO is as a stack of layers. + +The low levels of the stack work with the low-level operating system +calls (file descriptors in C) getting bytes in and out, the higher +layers of the stack buffer, filter, and otherwise manipulate the I/O. +Terms I<above> and I<below> are used to refer to the relative +positioning of the stack layers. + +A layer contains a "vtable", the table of I/O operations (at C level +a table of function pointers), and status flags. The functions in the +vtable implement operations like "open", "read", and "write". + +When I/O, for example "read", is requested, the request goes from Perl +first down the stack using "read" functions of each layer, then at the +bottom the input is requested from the operating system services, then +the result is returned up the stack, finally being interpreted as Perl +data. + +When you do an open() and specify extra PerlIO layers to be deployed, +the layers you specify are "pushed" on top of the already existing +default stack. What exact layers are in this default stack depends on +a lot of things: your operating system, Perl version, Perl compile +time configuration, and Perl runtime configuration. See L<PerlIO>, +L<perlrun/PERLIO>, and L<open> for more information. + +binmode() operates similarly to open(): by default the specified +layers are pushed on top of the existing stack. + +However, note that even as the specified layers are "pushed on top" +for open() and binmode(), this doesn't mean that the effects are +limited to the "top": PerlIO layers can be very 'active' and inspect +and affect layers also deeper in the stack. As an example there +is a layer called "raw" which repeatedly "pops" layers until +it reaches the first layer that has declared itself capable of +handling binary data. The "pushed" layers are processed in left-to-right +order. + +sysopen() operates (unsurprisingly) at a lower level in the stack than +open(). For example in UNIX or UNIX-like systems sysopen() operates +directly at the level of file descriptors: in the terms of PerlIO +layers, it uses only the "unix" layer, which is a rather thin wrapper +on top of the UNIX file descriptors. + =head2 Layers vs Disciplines Initial discussion of the ability to modify IO streams behaviour used |