Zlibc
Transparently access compressed files.
Edition 0.9h, for Zlibc version 0.9h
January 1998
by Alain Knaff
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Copyright (C) 1993, 1994, 1995, 1996, 1997 Alain Knaff
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Permission is granted to copy and distribute translations of this manual
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except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
Introduction
The zlibc package allows transparent on the fly uncompression of gzipped
files. Your programs will be able to access any compressed file, just as if
they were uncompressed. Zlibc will transparently uncompresses the data from
these files as soon as they are read, just as a compressed filesystem would
do. No kernel patch, no recompilation of these executables and no
recompilation of the libraries is needed.
It is not (yet) possible execute compressed files with zlibc. However,
there is another package, called tcx, which is able to uncompress
executables on the fly. On the other hand tcx isn't able to uncompress data
files on the fly. Fortunately, the both zlibc and tcx may coexist on the
same machine without problems.
This documentation looks most pretty when printed or as html. Indeed, in
the info version certain examples are difficult to read due to the
confusing quoting conventions of info.
Where to get zlibc
Zlibc can be found at the following places (and their mirrors):
ftp://prep.ai.mit.edu/pub/gnu/zlibc-0.9h.tar.gz
ftp://www.tux.org/pub/knaff/zlibc/zlibc-0.9h.tar.gz
ftp://sunsite.unc.edu/pub/Linux/libc/zlibc-0.9h.tar.gz
ftp://tsx-11.mit.edu/pub/linux/sources/libc/zlibc-0.9h.tar.gz
Before reporting a bug, make sure that it has not yet been fixed in the
Alpha patches which can be found at:
http://www.poboxes.com/Alain.Knaff/zlibc
ftp://www.tux.org/pub/knaff/zlibc
Note: It is highly probable that the innet address will cease to exist
after November 1997. The wauug address is permanent.
These patches are named zlibc-version-ddmm.taz, where version stands for
the base version, dd for the day and mm for the month. Due to a lack of
space, I usually leave only the most recent patch.
There is an zlibc mailing list at zlibc @ www.tux.org . Please send all bug
reports to this list. You may subscribe to the list by sending a message
with 'subscribe zlibc @ www.tux.org' in its body to majordomo @ www.tux.org
. (N.B. Please remove the spaces around the "@" both times. I left them
there in order to fool spambots.) Announcements of new zlibc versions will
also be sent to the list, in addition to the linux announce newsgroups. The
mailing list is archived at http://www.tux.org/hypermail/zlibc/latest
Installing zlibc
1. If you install zlibc on Linux, make sure that your shared loader
(ld-linux.so.1/ld.so) understands LD_PRELOAD. (Best if ld.so-1.8.5 or
more recent)
2. Type ./configure. This runs the GNU autoconfigure script which
configures the `Makefile' and the `config.h' file. You may compile
time configuration options to ./configure, see for details.
3. Type make to compile zlibc.
4. Type make install to install zlibc and associated programs to its
final target.
5. To use this module, set the environment variable LD_PRELOAD to point
to the object. Example (sh syntax):
LD_PRELOAD=/usr/local/lib/uncompress.o
export LD_PRELOAD
or (csh syntax):
setenv LD_PRELOAD /usr/local/lib/uncompress.o
On linux, use /lib/uncompress.o instead of /usr/local/lib/uncompress.o
. You might want to put these lines in your `.profile' or `.cshrc' in
order to have the uncompressing functions available all the time.
6. Compress your files using gzip and enjoy
For security reasons, the dynamic loader disregards environmental variables
such as LD_PRELOAD when executing set uid programs.
However, on Linux, you can use zlibc with set uid programs too, by using
one of the two methods described below:
1. You may ing the path to `uncompress.o' into `/etc/ld.so.preload'
instead of using LD_PRELOAD. WARNING: If you use `/etc/ld.so.preload',
be sure to install `uncompress.o' on your root filesystem, for
instance in /lib, as is done by the default configuration. Using a
directory which is not available at boot time, such as /usr/local/lib
will cause trouble at the next reboot! It is also careful to remove
zlibc from `/etc/ld.so.preload' when installing a new version. First
test it out using LD_PRELOAD, and only if everything is ok, put it
back into `/etc/ld.so.preload'. The zlibc package also supplies four
statically linked programs srm, smv, sln and ssln, which are equivalen
to rm, mv, ln and ln -s. These can be used in case anything goes wrong
with the installation.
2. If you have a version of ld.so which is more recent than 1.9.0, you
can set LD_PRELOAD to just contain the basename of `uncompress.o'
without the directory. In that case, the file is found as long as it
is in the shared library path (which usually contains `/lib' and
`/usr/lib')). Because the search is restricted to the library search
path, this also works for set-uid programs. Example (sh syntax):
LD_PRELOAD=uncompress.o
export LD_PRELOAD
or (csh syntax):
setenv LD_PRELOAD uncompress.o
The advantage of this approach over `ld.so.preload' is that zlibc can
more easily be switched off in case something goes wrong.
Using zlibc
Once zlibc is installed, simply compress your biggest datafiles using gzip.
Your programs are now able to uncompress these files on the fly whenever
they need them.
Zlibc and links
Symbolic links
After compressing your datafiles, you also need to change any potential
symbolic links pointing to them. Let's suppose that `x' is a symlink to
`tstfil':
> echo 'this is a test' >tstfil
> ln -s tstfil x
> ls -l
total 1
-rw-r--r-- 1 alknaff sirac 15 Feb 25 19:40 tstfil
lrwxrwxrwx 1 alknaff sirac 8 Feb 25 19:40 x -> tstfil
After compressing it, you'll see the following listing:
> 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
`Tstfil' 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:
> cat tstfil
this is a test
However, `tstfil' is not yet accessible using the symbolic link:
> cat x
cat: x: No such file or directory
In order to make `tstfil' accessible using the link, you have to destroy
the link, and remake it:
> 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
Hard links
If you compress datafiles with hard links pointing to them, gzip refuses to
compress them.
> 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
Thus you need to remove these hard links first, and remake them after
compressing the file.
> 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
How it works
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 open library function, and then
this function makes the actual syscall. Zlibc overrides the open function
and other related functions in order to do the uncompression on the fly.
If the open system call fails because the file doesn't exist, zlibc
constructs the filename of a compressed file by appending .gz to the
filename supplied by the user program. If this compressed file exists, it
is opened and piped trough gunzip, and the descriptor of the read end of
this pipe is returned to the caller.
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 lseek on the file or mmap it. A
description of data files for which using temporary is necessary can be
given in the configuration files `/usr/local/etc/zlibc.conf'
(`/etc/zlibc.conf' on Linux)(1) and `~/.zlibrc'. See section Configuration
files for a detailed description of their syntax.
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.
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'.
Customization
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.
Environmental variables
Environmental variables come in two kinds: switch variables have a boolean
value and can only be turned on or off, whereas string variables can have
arbitrary strings as values.
Switch variables
These variables represent a flag which can be turned on or off. If their
value is on or 1 they are turned on, if their value is off or 0 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.
LD_ZLIB_VERBOSE
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.
LD_ZLIB_UNLINK
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.
LD_ZLIB_DISABLE
If this variable is turned on, zlibc is switched off.
LD_ZLIB_READDIR_COMPR
If this variable is turned on, the readdir function shows the real
(compressed) files instead of the virtual (uncompressed) files.
String variables
These variables have a string value, which represent a file, a directory or
a command.
LD_ZLIB_TMP
This is the name of the directory where the temporary uncompressed
files are put. The default is /tmp.
LD_ZLIB_EXT
This is the extension which is appended to a virtual file name in
order to obtain the real (compressed) file name. The default is .gz.
LD_ZLIB_UNCOMPRESSOR
This is the name of the program to be invoked to uncompress the data.
Default is gzip -dc.
LD_ZLIB_CONFFILE
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 `~/.zlibrc' and
in `/usr/local/etc/zlibc.conf' (`/etc/zlibc.conf' on Linux).
Configuration files
To get its configuration, zlibc first looks into the file described by the
environment variable LD_ZLIB_CONFFILE (if any), then in `~/.zlibrc' and
finally in `/usr/local/etc/zlibc.conf' (`/etc/zlibc.conf' on Linux (2)). If
the desired information is found in neither of these files, the compiled-in
defaults are used. It is possible to supply only part of the needed
information in the configuration files. In that case, the missing
information is retrieved from the compiled-in defaults. This allows you to
have really small runtime configuration files, which only list the
differences between the desired configuration and the compiled-in
configuration.
If an error occurs while parsing one of the configuration files, the
offending file is skipped, and the search continues with the next file.
However, no error message is printed unless the environmental variable
LD_ZLIB_VERBOSE is turned on (i.e. set to 1 or to on ).
If two files contain contradictory information, the information in the file
which is scanned first is retained (usually `~/.zlibrc'). If any flags have
been set or unset using environmental variables, these settings override
the flags specified in the configuration files.
The configuration files are read by each process. For each process, they
are read at most once, at the time when zlibc is first used (attempt to
access a compressed file). Afterwards they are cached in the process's
virtual memory. Thus, changing zlibc configuration files doesn't generally
have any effect on already running processes.
Overall structure
The zlibrc files consist of two sections: A commands section (section
Available commands line flags) and a class definition section (section
Class section).
The commands section describes how zlibc should behave depending on the
executable that it was called from. Several commands are grouped together
into command classe.
The class definition section describes how zlibc should behave depending on
the class of the command and the datafile opened.
The configuration file may contain comments in both sections: a comment
starts with a hash (#) and stops at the end of the line.
Dashes (-) and underbars (_) may be used indifferently in all keywords.
Commands section
General syntax of the commands section
The format for a line in the "commands" section is as follows:
commands "cmd1" [ ... "cmdn" ] use flags "class"
In this line, the cmd1 ... cmdn are the basenames of the programs
(commands) for which this line should apply. The basename is the name
without the path, i.e. ls instead of /usr/bin/ls. The command names should
be enclosed in quotes. You may also use the keyword default (without
quotes) to match all commands.
The flags describe those aspects of zlibc's behaviour that are independant
from the datafile which is being accessed. These flags come in pairs. The
flags need not to be listed on a single commands line, they may occur in
several places, even in several different configuration files (for instance
one in /etc/zlibc.conf and another one in ~/.zlibrc.
If two contradictory flags are found in the configuration files, the one
which is seen first is taken.
If on the other hand a certain flag is not found at all in the
configuration files, the compiled-in default for this flag is used. This is
usually the second flag of each pair, described below (see section
Available commands line flags).
These flags can all be overridden by environmental variables. When the
corresponding environmental variable is set to 1 or to on, the first flag
is used, when it is set to 0 or to off. If the environmental variables is
set to neither of these 4 values, it is ignored).
The class names the commands class that these commands belong to. If, for a
given command, two commands lines give different classes, the one which is
seen first is taken. No union of classes is made, the classes are always
treated as a whole. Thus, if you want to make a change to a command class,
you need to describe it in its entirety.
The following example says that the tar, cpio, pax, cp and mv show
compressed files in a directory listing (readdir_compr flag), and are of
class generic_safe.
commands "tar" "cpio" "pax" "cp" "mv" use readdir_compr "generic_safe"
The class generic_safe would then need to be described further in the class
section.
Available commands line flags
This section describes the flags which can be used on a commands line. All
these flags come in pairs.
The table below describes each of these pairs. The first word in the header
of each item is the non-default flag, the second word is the default flag,
and the third word is the environmental variable by which you can override
the settings from the configuration files. If this environmental variable
is turned on (set to 1), the non-default (first) flag is taken, if it is
turned off (set to 0), the default (second) flag is taken.
disable / enable / LD_ZLIB_DISAB
The disable flag disables zlibc for the programs on this commands
line. This is useful for compression and uncompression utilities.
Without this flag, gunzip would not work anymore, because it would
think that the uncompressed file exists already, and it would refuse
to overwrite this file.
readdir_compr / readdir_uncompr / LD_ZLIB_READDIR
The readdir function shows the uncompressed files (i.e. with their
trailing .gz extension) when the readdir_compr flag is set, and the
compressed files otherwise.
verbose / silent / LD_ZLIB_VERBOSE
When verbose is set, zlibc prints informational messages.
unlink / no_unlink / LD_ZLIB_UNLINK
If the unlink flag is set, and if the user program tries to unlink a
virtual (uncompressed) file, the package translates this call into
unlinking the real file. If the no_unlink flag is set, requests to
unlink virtual (uncompressed) files are silently ignored.
Class section
A command class(3) definition describes those aspects of zlibc's behavior
that depend on the name of the datafile which is being accessed. Command
classes are identified by a name which is matched against the class
parameter from the commands. The class name should be enclosed between
quotes both in the commands line and in the class definition.
The class section contains descriptions of different command classes (i.e
descriptions how datafiles should be uncompressed). Each class definition
begins with a line of the following format:
class "id"
The class id is the same string as the one used in the commands line. The
remaining lines of a class definition are as follows:
[ [ criterion ] [ "name" ]] mode
The following example shows the definition of the class used for X-servers.
# X uses tmp files in its own directories.
class "X"
subdir "/usr/X11R6" usetmpfile
subdir "/usr/X386" usetmpfile
subdir "/usr/lib/X11" usetmpfile
showpipe
This says that all compressed files who are in a subdirectory below
`/usr/X11R6', `/usr/X386' or `/usr/lib/X11' are decompressed using
temporary files (usetmpfile), and that files from other directories are
decompressed using pipes (showpipe)
The following examples illustrates a command class, named nopipe, which
always uses temporary files for decompression
# generic class which uses temp files for all files.
class "nopipe"
usetmpfile
File selection Criteria
The criterion describes what parts of the filename should match:
filename
The entire filename of the target data file must match name.
basename
The basename (filename without directory) of the target data file must
match name.
directory
The data file must live in the directory name. If the user program
opens the file with an absolute pathname, that filename is used as is.
If on the other hand the user program uses a relative pathname, zlibc
uses the most direct path to the file (i.e. without symlinks).
subdirectory
The data file must live in the directory name or in one of its
subdirectories. If the user program opens the file with an absolute
pathname, that filename is used as is. If on the other hand the user
program uses a relative pathname, zlibc uses the most direct path to
the file (i.e. without symlinks).
suffix
The data file's name must end in name. This is useful for selecting
files according to their extension.
filesystem
The data file must live on the same filesystem as name. This criterion
can for example be used for example to disable zlibc on a doublespaced
filesystem (where zlibc would be redundant), or to switch off
uncompressed size reporting on an ftpfs filesystems (4).
default
All data files match. All class definitions must have a default line,
and this default line must be the last line of the definition. The
default criterion needs no 'name' parameter.
all All data files match. Unlike default, this line needs not to be the
last line of the class definitions. Thus it is possible to specify
several all lines for different aspects of zlibc behaviour.
The lines of each class definition are scanned in sequence, and, for each
aspect(5), the first matching line is adopted. The class may be defined in
another initialisation file, if this file is scanned later. The criterion
parameter may be omitted if it can be deduced from the name. In that case,
the following heuristics are used:
* Names beginning with a dot are considered as suffix.
* Names beginning with a slash, but not ending with a slash are
considered as filename.
* Names beginning and ending with a slash are considered as
subdirectory.
* A missing name is considered as default.
* All the other names are considered as basename.
filesystem criteria, because these tend to be more predictable.
Mode
The mode describes the behavior of zlibc in certain situations. These are:
1. What to do when a file is accessed in readonly mode.
2. What to do when a user program tries to append data to an existing
file.
3. What to do when a user program tries to create a new file.
4. What to do when a user program tries to access an existing file and
none of the preceding situations apply (see section Other write
requests).
5. Whether to show the size of the uncompressed file or the size of the
compressed file as a response to stat (see section Size shown for
compressed files).
The mode for each off these 5 situations has to be described separately.
For each of these 5 situations, the scan through the class description is
done until the reaction of zlibc for this situation is defined.
Reaction to a readonly request
The reaction to a readonly request may be one of the following:
show-pipe
The uncompressed data file is sent to the user program using a pipe.
This consumes very few resources, and it allows the decompression to
run in paralell with the user process, but it has the disadvantage
that the user program cannot use lseek. To warn the user program of
this, the data file is shown as a named pipe (FIFO) when it is
stat'ed.
use-tmp-file
This is the default setting. The data is uncompressed and put into a
temporary file. The user programs then reads its data from the
temporary file. This has the advantage that the user program may
lseek, and the disadvantage that more disk space is consumed, while
the programming is accessing the file.
hide-pipe
The data is sent through a pipe, but the file is shown as a regular
file ("hidden") to the user program when it stats it. This might be
needed for programs which are picky about a file type, but who
actually don't need lseek.
leave-compressed
The virtual (uncompressed) file is shown as non-existent to stat, and
readdir shows the physical (compressed) file. For certain programs,
this is enough to disable zlibc on a file per file basis. This is
useful, for example, to make emacs use its own compression support
(crypt.el) instead of zlibc. Crypt.el is able to compress files when
writing them back, whereas zlibc isn't able to do so. The
leave-compressed doesn't work correctly with the directory and
subdirectory criteria. Use the filesystem criterion instead.
dir-leave-compressed
The virtual (uncompressed) file is shown to stat, but readdir shows
the physical (compressed) file. This is useful to tell zlibc that we
prefer working on the physical file, but without making access to the
virtual file impossible. The dir-leave-compressed doesn't work with
the dir and subdir criteria. Use the filesystem criterion instead.
Reaction to an append request
The reaction to an append request may be one of the following:
append-compressed
When a user program tries to append data to a non-existant file, but
the corresponding compressed file exists, zlibc translates this
request into appending the compressed data to the compressed file.
WARNING-1: This works with gzip, and might not work with other
(un)compressors! This relies on gzip's feature to consider a
concatenation of compressed files as a compression of concatenated
files. WARNING-2: This is only reliable if you can guarantee that the
file is not accessed by some other program while the first one has it
still open, or that it won't be opened again (even by the same
program) shortly after it has been closed. Delays longer than a second
should be ok. Don't enable append-compressed if you expect the file to
be written to by several programs at once.
no-append-compressed
Don't append to a compressed file (the user program will get a file
not found error). This is the default behavior.
Reaction to a file creation request
The reaction to a create request may be one of the following:
create-compressed
When a user program tries to create a new file whose name matches the
pattern, zlibc translates this request into creating a compressed
file. A file is considered to be created, if the O_TRUNC flag is set,
or if both O_CREAT and O_EXCL are set. This only applies to files
opened write-only. WARNING-1: This is only reliable if you can
guarantee that the newly created file is not accessed by some other
program while the first one has it still open, or that it won't be
opened again (even by the same program) shortly after it has been
closed. WARNING-2: This should not be used if you expect the program
to seek in this file.
no-create-compressed
Newly created files are created uncompressed. This is the default.
Other write requests
The reaction to other write requests may be one of the following:
uncompress-before-write
When a user program tries to write to a non-existing file, and when a
compressed file with a corresponding name exists, this file is
uncompressed in place (i.e. having the same name, but without the .gz
extension) WARNING-1: This is only safe when you can guarantee that
the file is not opened by several programs at once. However, once the
call to the open function has returned, other programs may open this
file safely. If a second program tries to open the file during the
open call of the first, this second program gets a permission error.
WARNING-2: When using this option, be careful when opening files
belonging to another user, or files living in a directory where you
have no write access to. Using this option in a directory without
write access will result in a permission error. Using this option in a
directory where you do have write access will change the ownership of
the file to you, even if it belonged to another user initially.
no-uncompress-before-write
Compressed files are not uncompressed before writing to them, and
zlibc returns a "file not found" error. This is the default.
If several of these options apply for the same file, create-compressed has
priority over append-compressed which has priority over
uncompress-before-write.
Size shown for compressed files
When an application calls stat to fetch the attributes of a file (such as
its permissions, size, type, etc.), zlibc stats the corresponding
compressed file instead. After doing so, zlibc has to adjust some of the
values returned by stat, such as the type and the size. The type has to be
adjusted for those files that should be shown as pipes. The size has to be
adjusted because user programs are usually interested in the amount of data
that they can actually read from the file (i.e. the size of the
uncompressed file) rather than the size of the physical file (i.e. the size
of the compressed file). However, in order to find out the size of the
uncompressed file, zlibc has to read some data of the file, which may
impact performance in situations where many files are stat'ed. This is for
instance the case for find, or for ls on an ftpfs filesystem. The following
two behaviors of the stat call may be specified:
show-compressed-size
Stat returns the size of the compressed file. This is less useful for
the application, but more efficient. If you mount any ftpfs
filesystems, you may switch on show-compressed-size just for that
filesystem by using the filesystem criterion (see section File
selection Criteria).
show-uncompressed-size
Stat returns the size of the uncompressed file. This is more useful
for the application, but less efficient. This is the default behavior.
Compiled-in defaults
It is possible to operate zlibc entirely without configuration files. In
this case, it uses the compiled-in defaults. These are generated at
compile-time from the `zlibrc.sample' 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.
Compile-time configuration via GNU autoconf
Before it can be compiled, zlibc must be configured using the GNU autoconf
script ./configure. In most circumstances, running ./configure without any
parameters is enough. However, you may customize zlibc using various
options to ./configure. The following options are supported:
--prefix directory
Prefix used for any directories used by zlibc. By default, this is
`/usr/local'. Zlibc is installed in `$prefix/lib', looks for its
system wide configuration file in `$prefix/etc'. Man pages are
installed in `$prefix/man', info pages in `$prefix/info' etc. On
Linux, if you use zlibc via `/etc/ld.so.preload', you should use `/'
as the prefix instead of the default `$prefix/lib'.
--sysconfdir directory
Directory containing the system-wide configuration file `zlibc.conf'.
By default, this is derived from prefix (see above).
--disable-runtime-conf
Disables run time configuration via environmental variables and via
the configuration files. This may be needed in hyper secure
environments.
--disable-env-conf
Disables run time configuration via environmental variables
--disable-have-proc
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.
--disable-have-proc
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.
--with-compr-ext=extension
Uses extension as the filename extension of compressed files. By
default, is .gz
--with-extlen=length
Allows to configure compressed filename extensions with at most length
character via runtime configuration. By default is 5.
--with-tmpdir=directory
Uses directory to store the uncompressed files. By default is /tmp.
--with-uncompressor=uncompressor-command-line
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 gzip -dc.
In addition to the above-listed options, the standard GNU autoconf options
apply. Type ./configure --help to get a complete list of these.
Porting zlibc
Zlibc has been tested on three variants of Unix so far: Linux, SunOs,
Solaris. On all three platforms, zlibc has been compiled using GNU make and
gcc. However, porting it to other platforms should be straightforward as
long as they verify both of the following conditions:
1. The target platform should have shared library support. The default
Makefile links zlibc by supplying the flags -nostdlib -shared to gcc
in addition to the usual CFLAGS. If your target platform needs
different flags to make shared libraries, change the definition of the
variable SHAREDCFLAGS in the Makefile.
2. The target platform should supply a way to perform system calls
directly without going through the library functions. Indeed, this is
needed because we are redefining the library functions and thus the
default functions become unavailable. Usually direct syscalls done by
calling the syscall function. For instance, in order to perform
open("test", O_RDONLY) directly, sycall(SYS_open, "test", O_RDONLY) is
called. If your target platform uses a different way to perform direct
system calls, you need to supply different definitions for the
real_xxx functions in direct_sys.h. This is the case for AIX.
3. The target platform should provide a way to instruct the dynamic
linker to preload programs with extra object files not present in the
shared library. Any symbols defined in that object file override
equivalent symbols from libc. Usually this is done by pointing the
LD_PRELOAD variable to the object file to be used. Unfortunately, I
have found no way to achieve this on AIX. Any ideas about how to do
this are welcome.
Many platforms, including Solaris, provide several aliases for their
syscall stubs. It would be interesting to leave one of them alone, and use
it as a "direct syscall" stub, but unfortunately, it turns out that in
sometimes both stubs are used by the library! Thus, what would be an
advantage, becomes actually a disadvantage, because we will need to
override these aliases in addition to the canonical names. This is done in
altnames.c. Change it as needed. In order to know the names of these
aliases, try the following command: nm /lib/libc.so | grep lstat. This
lists any library symbols with unlink in their name, which could be
aliases.
If you have successfully ported zlibc to a new platform, could you drop me
a note, so that I can include support for that platform in my next release.
Thanks
Variable index
d
* disable
e
* enable
l
* LD_PRELOAD
* LD_ZLIB_CONFFILE
* LD_ZLIB_DISABLE
* LD_ZLIB_EXT
* LD_ZLIB_READDIR_COMPR
* LD_ZLIB_TMP
* LD_ZLIB_UNCOMPRESSOR
* LD_ZLIB_UNLINK
* LD_ZLIB_VERBOSE
r
* readdir_compr
* readdir_uncompr
s
* show-compressed-size
* silent
v
* verbose
Concept index
a
* ALPHA patches
b
* basename (selection criterion)
* Boot problems
* bugs
c
* class section
* command line for the uncompressing program (env.var.)
* commands section, commands section
* compile-time configuration
* configure options
* Customization
d
* debugging zlibc, debugging zlibc
* diffs
* directory (selection criterion)
* directory for temporary files
* directory for temporary files (env.var.)
* disabling zlibc (conf. file)
* doublespaced DOS filesystem
e
* extension (selection criterion)
f
* filename (selection criterion)
* filename extension for compressed files (env.var.)
* find
* ftpfs
h
* hanging (of ls, on an ftpfs filesystem)
i
* Installation
l
* location of the configuration file (env.var.)
* ls slowness (on an ftpfs filesystem)
m
* mailing list
p
* Parsing order of configuration files.
* patches
* preventing accidental removal of compressed files (env.var.)
* Processing order of configuration file.
r
* removing files
s
* Set-uid programs
* showing compressed files in a directory listing (env.var.)
* showing compressed files on a directory listing (conf. file)
* size of compressed files
* slowness (of find due to reading the size)
* slowness (of ls, on an ftpfs filesystem)
* suffix (selection criterion)
* Symbolic links
t
* temporary file directory
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