perlio.3

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

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.\" ========================================================================
.\"
.IX Title "PerlIO 3"
.TH PerlIO 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
PerlIO \- On demand loader for PerlIO layers and root of PerlIO::* name space
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  open($fh,"<:crlf", "my.txt"); # support platform\-native and CRLF text files
\&
\&  open($fh,"<","his.jpg");      # portably open a binary file for reading
\&  binmode($fh);
\&
\&  Shell:
\&    PERLIO=perlio perl ....
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
When an undefined layer 'foo' is encountered in an \f(CW\*(C`open\*(C'\fR or
\&\f(CW\*(C`binmode\*(C'\fR layer specification then C code performs the equivalent of:
.PP
.Vb 1
\&  use PerlIO \*(Aqfoo\*(Aq;
.Ve
.PP
The perl code in PerlIO.pm then attempts to locate a layer by doing
.PP
.Vb 1
\&  require PerlIO::foo;
.Ve
.PP
Otherwise the \f(CW\*(C`PerlIO\*(C'\fR package is a place holder for additional
PerlIO related functions.
.PP
The following layers are currently defined:
.IP ":unix" 4
.IX Item ":unix"
Lowest level layer which provides basic PerlIO operations in terms of
\&\s-1UNIX/POSIX\s0 numeric file descriptor calls
(\fIopen()\fR, \fIread()\fR, \fIwrite()\fR, \fIlseek()\fR, \fIclose()\fR).
.IP ":stdio" 4
.IX Item ":stdio"
Layer which calls \f(CW\*(C`fread\*(C'\fR, \f(CW\*(C`fwrite\*(C'\fR and \f(CW\*(C`fseek\*(C'\fR/\f(CW\*(C`ftell\*(C'\fR etc.  Note
that as this is \*(L"real\*(R" stdio it will ignore any layers beneath it and
got straight to the operating system via the C library as usual.
.IP ":perlio" 4
.IX Item ":perlio"
A from scratch implementation of buffering for PerlIO. Provides fast
access to the buffer for \f(CW\*(C`sv_gets\*(C'\fR which implements perl's readline/<>
and in general attempts to minimize data copying.
.Sp
\&\f(CW\*(C`:perlio\*(C'\fR will insert a \f(CW\*(C`:unix\*(C'\fR layer below itself to do low level \s-1IO\s0.
.IP ":crlf" 4
.IX Item ":crlf"
A layer that implements DOS/Windows like \s-1CRLF\s0 line endings.  On read
converts pairs of \s-1CR\s0,LF to a single \*(L"\en\*(R" newline character.  On write
converts each \*(L"\en\*(R" to a \s-1CR\s0,LF pair.  Note that this layer likes to be
one of its kind: it silently ignores attempts to be pushed into the
layer stack more than once.
.Sp
It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.
.Sp
(Gory details follow) To be more exact what happens is this: after
pushing itself to the stack, the \f(CW\*(C`:crlf\*(C'\fR layer checks all the layers
below itself to find the first layer that is capable of being a \s-1CRLF\s0
layer but is not yet enabled to be a \s-1CRLF\s0 layer.  If it finds such a
layer, it enables the CRLFness of that other deeper layer, and then
pops itself off the stack.  If not, fine, use the one we just pushed.
.Sp
The end result is that a \f(CW\*(C`:crlf\*(C'\fR means \*(L"please enable the first \s-1CRLF\s0
layer you can find, and if you can't find one, here would be a good
spot to place a new one.\*(R"
.Sp
Based on the \f(CW\*(C`:perlio\*(C'\fR layer.
.IP ":mmap" 4
.IX Item ":mmap"
A layer which implements \*(L"reading\*(R" of files by using \f(CW\*(C`mmap()\*(C'\fR to
make (whole) file appear in the process's address space, and then
using that as PerlIO's \*(L"buffer\*(R". This \fImay\fR be faster in certain
circumstances for large files, and may result in less physical memory
use when multiple processes are reading the same file.
.Sp
Files which are not \f(CW\*(C`mmap()\*(C'\fR\-able revert to behaving like the \f(CW\*(C`:perlio\*(C'\fR
layer. Writes also behave like \f(CW\*(C`:perlio\*(C'\fR layer as \f(CW\*(C`mmap()\*(C'\fR for write
needs extra house-keeping (to extend the file) which negates any advantage.
.Sp
The \f(CW\*(C`:mmap\*(C'\fR layer will not exist if platform does not support \f(CW\*(C`mmap()\*(C'\fR.
.IP ":utf8" 4
.IX Item ":utf8"
Declares that the stream accepts perl's \fIinternal\fR encoding of