zlibc.3

zlib压缩原码

.PP
 
.nf
.ft 3
.in +0.3i
> gzip tstfil
> ls -l
total 1
pr--r--r--   1 alknaff  sirac          15 Feb 25 19:40 tstfil
lrwxrwxrwx   1 alknaff  sirac           8 Feb 25 19:40 x -> tstfil
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
\&\fR\&\f(CW\(ifTstfil\(is\fR is now shown as a pipe by zlibc in order to warn programs
that they cannot seek in it.  Zlibc still shows it with its old name,
and you can directly look at its contents:
 
.nf
.ft 3
.in +0.3i
> cat tstfil
this is a test
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
However, \fR\&\f(CW\(iftstfil\(is\fR is not yet accessible using the symbolic link:
 
.nf
.ft 3
.in +0.3i
> cat x
cat: x: No such file or directory
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
In order to make \fR\&\f(CW\(iftstfil\(is\fR accessible using the link, you have to
destroy the link, and remake it:
 
.nf
.ft 3
.in +0.3i
> rm x
/bin/rm: remove `x'? y
> ln -s tstfil x
> ls -l
total 1
pr--r--r--   1 alknaff  sirac          15 Feb 25 19:40 tstfil
lrwxrwxrwx   1 alknaff  sirac           8 Feb 25 19:44 x -> tstfil
> cat x
this is a test
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
.SS \ \ Hard\ links
.PP
If you compress datafiles with hard links pointing to them, gzip refuses
to compress them. 
.PP
 
.nf
.ft 3
.in +0.3i
> echo 'this is a test' >tstfil
> ln tstfil x
> ls -li
total 2
    166 -rw-r--r--   2 alknaff  sirac          15 Feb 25 19:46 tstfil
    166 -rw-r--r--   2 alknaff  sirac          15 Feb 25 19:46 x
> gzip tstfil
gzip: tstfil has 1 other link  -- unchanged
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
Thus you need to remove these hard links first, and remake them after
compressing the file.
.PP
 
.nf
.ft 3
.in +0.3i
> rm x
/bin/rm: remove `x'? y
> gzip tstfil
> ln tstfil x
> ls -li
total 2
    167 pr--r--r--   2 alknaff  sirac          15 Feb 25 19:46 tstfil
    167 pr--r--r--   2 alknaff  sirac          15 Feb 25 19:46 x
> cat x
this is a test
.fi
.in -0.3i
.ft R
.lp
 
\&\fR
.PP
.SH How\ it\ works
.PP
Usually, programs don't make system calls directly, but instead call a
library function which performs the actual system calls. For instance,
to open a file, the program first calls the \fR\&\f(CWopen\fR library
function, and then this function makes the actual syscall.  Zlibc
overrides the \fR\&\f(CWopen\fR function and other related functions in order
to do the uncompression on the fly.
.PP
If the \fR\&\f(CWopen\fR system call fails because the file doesn't exist,
zlibc constructs the filename of a compressed file by appending
\&\fR\&\f(CW.gz\fR to the filename supplied by the user program. If this
compressed file exists, it is opened and piped trough \fR\&\f(CWgunzip\fR, and
the descriptor of the read end of this pipe is returned to the caller.
.PP
In some cases, the compressed file is first uncompressed into a
temporary file, and a read descriptor for this file is passed to the
caller.  This is necessary if the caller wants to call \fR\&\f(CWlseek\fR on
the file or \fR\&\f(CWmmap\fR it. A description of data files for which using
temporary is necessary can be given in the configuration files
\&\fR\&\f(CW\(if/usr/local/etc/zlibc.conf\(is\fR (\fR\&\f(CW\(if/etc/zlibc.conf\(is\fR on
Linux)\fR\fR\fR and
\&\fR\&\f(CW\(if~/.zlibrc\(is\fR. See section Configuration files, for a detailed description
of their syntax.
.PP
Many user programs try to check the existence of a given file by other
system calls before actually opening it. That's why zlibc also
overrides these system calls. If for example the user program tries to
stat a file, this call is also intercepted.
.PP
The compressed file, which exists physically on the disk, is also called
\&'the real file', and the uncompressed file, whose existence is only
simulated by zlibc is called 'the virtual file'.
.PP
.SH Customization
.iX "c Customization"
.PP
The behavior of zlibc can be tailored using configuration files or
environment variables. This customization should normally not be needed,
as the compiled-in defaults are already pretty complete.
.PP
.SS Environmental\ variables
.PP
Environmental variables come in two kinds: \fIswitch\fR variables have
a boolean value and can only be turned on or off, whereas \fIstring\fR
variables can have arbitrary strings as values.
.PP
.SS \ \ Switch\ variables
.iX "v LD_ZLIB_VERBOSE"
.iX "c debugging zlibc"
.iX "v LD_ZLIB_UNLINK"
.iX "c removing files"
.iX "c preventing accidental removal of compressed files (env.var.)"
.iX "v LD_ZLIB_DISABLE"
.iX "v LD_ZLIB_READDIR_COMPR"
.iX "c showing compressed files in a directory listing (env.var.)"
.PP
These variables represent a flag which can be turned on or off.  If
their value is \fR\&\f(CWon\fR or \fR\&\f(CW1\fR they are turned on, if their value
is \fR\&\f(CWoff\fR or \fR\&\f(CW0\fR they are turned off. All other values are
ignored. If the same flag can be turned on or off using config files,
the environmental variable always has the priority.
.TP
\&\fR\&\f(CWLD_ZLIB_VERBOSE\ \fR\ 
If this variable is turned on, informational messages are printed on
many operations of zlibc.  Moreover, error messages are printed in order
to point out errors in the configuration files, if any. If this variable
is turned off, errors are silently ignored.
.TP
\&\fR\&\f(CWLD_ZLIB_UNLINK\fR\ 
If this variable is turned on, and if the user program tries to unlink a
virtual (uncompressed) file, zlibc translates this call into unlinking
the real file. If this variable is turned off, unlink calls on virtual
files are ignored.
.TP
\&\fR\&\f(CWLD_ZLIB_DISABLE\fR\ 
If this variable is turned on, zlibc is switched off.
.TP
\&\fR\&\f(CWLD_ZLIB_READDIR_COMPR\fR\ 
If this variable is turned on, the readdir function shows the real
(compressed) files instead of the virtual (uncompressed) files.
.PP
.SS \ \ String\ variables
.iX "v LD_ZLIB_TMP"
.iX "c directory for temporary files (env.var.)"
.iX "v LD_ZLIB_EXT"
.iX "c filename extension for compressed files (env.var.)"
.iX "v LD_ZLIB_UNCOMPRESSOR"
.iX "c command line for the uncompressing program (env.var.)"
.iX "v LD_ZLIB_CONFFILE"
.iX "c location of the configuration file (env.var.)"
.PP
These variables have a string value, which represent a file, a directory
or a command.
.TP
\&\fR\&\f(CWLD_ZLIB_TMP\fR\ 
This is the name of the directory where the temporary uncompressed files are 
put. The default is /tmp.
.TP
\&\fR\&\f(CWLD_ZLIB_EXT\fR\ 
This is the extension which is appended to a virtual file name in order
to obtain the real (compressed) file name. The default is \fR\&\f(CW.gz\fR.
.TP
\&\fR\&\f(CWLD_ZLIB_UNCOMPRESSOR\fR\ 
This is the name of the program to be invoked to uncompress the data. Default
is \fR\&\f(CWgzip -dc\fR.
.TP
\&\fR\&\f(CWLD_ZLIB_CONFFILE\ \fR\ 
This is the name of an additional configuration file. If this variable
is defined and if the corresponding file exists, the configuration
described in this file overrides the configurations in \fR\&\f(CW\(if~/.zlibrc\(is\fR
and in \fR\&\f(CW\(if/usr/local/etc/zlibc.conf\(is\fR (\fR\&\f(CW\(if/etc/zlibc.conf\(is\fR on
Linux).
.PP
.SS Compiled-in\ defaults
.PP
It is possible to operate zlibc entirely without configuration files.
In this case, it uses the \fIcompiled-in defaults\fR.  These are
generated at compile-time from the \fR\&\f(CW\(ifzlibrc.sample\(is\fR file.  This file
has the same syntax as the configuration files described above
(see section  Configuration files).  If you want to change the compiled-in
defaults of zlibc, edit that file, and remake.
.PP
.SS Compile-time\ configuration\ via\ GNU\ autoconf
.iX "c configure options"
.iX "c temporary file directory"
.iX "c directory for temporary files"
.iX "c compile-time configuration"
.PP
Before it can be compiled, zlibc must be configured using the GNU
autoconf script \fR\&\f(CW./configure\fR.  In most circumstances, running
\&\fR\&\f(CW./configure\fR without any parameters is enough. However, you may
customize zlibc using various options to \fR\&\f(CW./configure\fR. The
following options are supported:
.TP
\&\fR\&\f(CW--prefix\ \fIdirectory\fR\&\f(CW\fR\ 
Prefix used for any directories used by zlibc.  By default, this is
\&\fR\&\f(CW\(if/usr/local\(is\fR.  Zlibc is installed in \fR\&\f(CW\(if$prefix/lib\(is\fR, looks for
its system wide configuration file in \fR\&\f(CW\(if$prefix/etc\(is\fR.  Man pages are
installed in \fR\&\f(CW\(if$prefix/man\(is\fR, info pages in \fR\&\f(CW\(if$prefix/info\(is\fR etc.
On Linux, if you use zlibc via \fR\&\f(CW\(if/etc/ld.so.preload\(is\fR, you should use
\&\fR\&\f(CW\(if/\(is\fR as the prefix instead of the default \fR\&\f(CW\(if$prefix/lib\(is\fR.
.TP
\&\fR\&\f(CW--sysconfdir\ \fIdirectory\fR\&\f(CW\fR\ 
Directory containing the system-wide configuration file
\&\fR\&\f(CW\(ifzlibc.conf\(is\fR.  By default, this is derived from \fR\&\f(CWprefix\fR (see
above).
.TP
\&\fR\&\f(CW--disable-runtime-conf\fR\ 
Disables run time configuration via environmental variables and via the
configuration files.  This may be needed in hyper secure environments.
.TP
\&\fR\&\f(CW--disable-env-conf\fR\ 
Disables run time configuration via environmental variables
.TP
\&\fR\&\f(CW--disable-have-proc\fR\ 
Tells zlibc not to use the /proc filesystem to find out the commandline
of the programs for which it runs, even if a working /proc is detected.
.TP
\&\fR\&\f(CW--disable-have-proc\fR\ 
Tells zlibc to use the /proc filesystem to find out the commandline of
the programs for which it runs, even if no working /proc is detected.
.TP
\&\fR\&\f(CW--with-compr-ext=\fIextension\fR\&\f(CW\fR\ 
Uses \fIextension\fR as the filename extension of compressed files.  By
default, is \fR\&\f(CW.gz\fR
.TP
\&\fR\&\f(CW--with-extlen=\fIlength\fR\&\f(CW\fR\ 
Allows to configure compressed filename extensions with at most
\&\fIlength\fR character via runtime configuration.  By default is 5.
.TP
\&\fR\&\f(CW--with-tmpdir=\fIdirectory\fR\&\f(CW\fR\ 
Uses \fIdirectory\fR to store the uncompressed files.  By default is
\&\fR\&\f(CW/tmp\fR.  
.TP
\&\fR\&\f(CW--with-uncompressor=\fIuncompressor-command-line\fR\&\f(CW\fR\ 
Defines how the program for uncompressing files should be invoked.  This
command should read the compressed file from stdin, and output the
uncompressed data to stdout By default is \fR\&\f(CWgzip -dc\fR.
.PP
In addition to the above-listed options, the standard GNU autoconf
options apply.  Type \fR\&\f(CW./configure --help\fR to get a complete list of
these.
.PP
.SS \ \ See\ also
zlibc.conf(5)
.PP