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Home | Switchboard | Unix Administration | Red Hat | TCP/IP Networks | Neoliberalism | Toxic Managers |
(slightly skeptical) Educational society promoting "Back to basics" movement against IT overcomplexity and bastardization of classic Unix |
(working notes)
It looks like the best documentation about this topic is for Java. There are also some e-mail post that point out possible problems. TCL implementation can be used for learning tricks too.
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Shared libraries use version numbers to allow for upgrades to the libraries used by applications while preserving compatibility for older applications. This article reviews what's really going on under the book jacket and why there are so many symbolic links in /usr/lib on a normal Linux™ system. Shared libraries are a fundamental component for the efficient use of space and resources on a modern UNIX® system. The C library on a SUSE 9.1 system is made up of about 1.3 MB. A copy of that library for every program in /usr/bin (and I have 2,569!) would take up a couple of gigabytes of space.
Of course this number is inflated -- statically linked programs would incorporate only those parts of the library that they use. Nonetheless, the amount of space tied up by all those duplicate copies of
printf()
would give the system a very bloated feel.Shared libraries can save memory, not just disk space. The kernel can keep a single copy of a shared library in memory, sharing it among multiple applications. So, not only do we only have one copy of
printf()
on the disk, we only have one in memory. That has a pretty noticeable effect on performance.In this article, we'll review the underlying technology used for shared libraries and the way in which shared library versioning helps prevent the compatibility nightmares that naive shared library implementations have had in the past. First, a look at how shared libraries work.
How shared libraries work
The concept is easy enough to understand. You have a library; you share the library. But what actually happens when your program tries to callprintf()
-- the real way this works -- is a bit more complex.It is a simpler process in a static linking system than in a dynamically linked system. In a static linked system, the generated code possesses a reference to a function. The linker replaces that reference with the actual address at which it had loaded the function, so that the resulting binary code has the right address in place. Then, when the code is run, it simply jumps to the relevant address. This is a simple task to administer because it lets you to link in only those objects that are actually referred to at some point in the program.
But most shared libraries are dynamically linked. That has several further implications. One is that you can't predict in advance at which address a function will really be when it's called! (There have also been statically linked shared library schemes, such as the one in BSD/OS, but they are beyond the scope of this article.)
The dynamic linker can do a fair amount of work for each function linked, so most linkers are lazy. They only actually finish that work when the function is called. With more than a thousand externally visible symbols in the C library and nearly three thousand more local ones, this idea could save a noticeable amount of time.
The magic trick here that makes it work is a chunk of data called a Procedure Linkage Table (PLT), a table in the program that lists every function that a program calls. When the program is started, the PLT contains code for each function to query the runtime linker for the address at which it has loaded a function. It then fills in that entry in the table and jumps there. As each function is called, its entry in the PLT is simplified into a direct jump to the loaded function.
However, it's important to notice that this still leaves an extra layer of indirection -- each function call is resolved through a jump into a table.
Compatibility's not just for relationships
This means that the library you end up being linked to had better be compatible with the code that's calling it. With a statically linked executable, there is some guarantee that nothing will change on you. With dynamic linking, you don't have that guarantee.What happens if a new version of the library comes out? Especially, what happens if the new version changes the calling sequence for a given function?
Version numbers to the rescue -- a shared library will have a version. When a program is linked against a library, it has the version number it's designed for stored in it. The dynamic linker can check for a matching version number. If the library has changed, the version number won't match, and the program won't be linked to the newer version of library.
One of the potential advantages of dynamic linking, however, is in fixing bugs. It'd be nice if you could fix a bug in the library and not have to recompile a thousand programs to take advantage of that fix. So sometimes, you want to link to a newer version.
Unfortunately, that creates some cases where you want to link to the newer version and some cases where you'd rather stick with an older version. There is a solution, though -- two kinds of version numbers:
- A major number indicates a potential incompatibility between library versions.
- A minor number indicates only bug fixes.
So under most circumstances, it is safe to load a library with the same major number and a higher minor number; consider it an unsafe practice to load a library with a higher major number.
To prevent users (and programmers) from needing to track library numbers and updates, the system comes with a large number of symbolic links. In general, the pattern is that
libexample.so
will be a link to
libexample.so.N
in which N is the highest major version number found on the system.
For every major version number supported,
libexample.so.N
will be a link in turn to
libexample.so.N.M
in which M is the largest minor version number.
Thus, if you specify
-lexample
to the linker, it looks forlibexample.so
which is a symbolic link to a symbolic link to the most recent version. On the other hand, when an existing program is loaded, it will try to loadlibexample.so.N
in which N is the version to which it was originally linked. Everyone wins!To debug, first you must know how to compile
To debug problems with shared libraries, it's useful to know a little more about how they're compiled.In a traditional static library, the code generated is usually bound together into a library file with a name ending in
.a
and then it's passed to the linker. In a dynamic library, the library file's name generally ends in.so
. The file structures are somewhat different.A normal static library is in a format created by the
ar
utility, which is basically a very simple-minded archive program, similar totar
but simpler. In contrast, shared libraries are generally stored in more complicated file formats.On modern Linux systems, this generally means the ELF binary format (Executable and Linkable Format). In ELF, each file is made up of one ELF header followed by zero or some segments and zero or some sections. The segments contain information necessary for runtime execution of the file, while sections contain important data for linking and relocation. Each byte in the entire file is taken by no more than one section at a time, but there can be orphan bytes that are not covered by a section. Normally in a UNIX executable, one or more sections are enclosed in one segment.
The ELF format has specifications for applications and libraries. The library format is a lot more complicated than just a simple archive of object modules, though.
The linker sorts through references to symbols, making notes about in which libraries they were found. Symbols from static libraries are added to the final executable; symbols from shared libraries are put into the PLT, and references to the PLT are created. Once those tasks are done, the resulting executable has a list of symbols it plans to look up from libraries it will load at runtime.
At runtime, the application loads the dynamic linker. In fact, the dynamic linker itself uses the same kind of versioning as the shared libraries. On SUSE Linux 9.1, for instance, the file
/lib/ld-linux.so.2
is a symbolic link to/lib/ld-linux.so.2.3.3
. On the other hand, a program looking for/lib/ld-linux.so.1
won't try to use the new version.The dynamic linker then gets to do all the fun work. It looks to see which libraries (and which versions) a program was originally linked to and then loads them. Loading a library consists of:
- Finding it (and it may be in any of several directories on a system)
- Mapping it into the program's address space
- Allocating blocks of zero-filled memory the library may need
- Attaching the library's symbol table
Debugging this process can be difficult. There are a few kinds of problems you can encounter. For example, if the dynamic linker can't find a given library, it will abort loading the program. If it finds all the libraries it wants but can't find a symbol, it can abort for that too (but it may not act until the actual attempt to reference that symbol occurs) -- this is rare case though because normally, if the symbol isn't there, it will be noticed during the initial link.
Modifying the dynamic linker search path
When linking a program, you can specify additional paths to search at runtime. Ingcc
the syntax is-Wl,-R/path
. If the program is already linked, you can also change this behavior by setting the environment variableLD_LIBRARY_PATH
. Usually this is needed only if your application wants to search paths that aren't part of the system-wide default, a rare case for most Linux systems. In theory, the Mozilla people could have distributed a binary compiled with that path set, but they preferred to distribute a wrapper script that sets the library path appropriately before launching the executable.Setting the library path can provide a workaround in the rare case where two applications require incompatible versions of a library. A wrapper script can be used to have one application search in a directory using the special version of the library it requires. Hardly an elegant solution, but in some cases it's the best you can do.
If you have a compelling reason to add a path to many programs, you can also change the system's default search path. The dynamic linker is controlled through
/etc/ld.so.conf
, which contains a list of directories to search by default. Any paths specified inLD_LIBRARY_PATH
will be searched before the paths listed inld.so.conf
, so users can override these settings.Most users have no reason to change the system default library search paths; generally the environment variable is a better match for likely reasons to change the search path, such as linking with libraries in a toolkit or testing programs against a newer version of a library.
Using ldd
One useful tool for debugging shared library problems isldd
. The name derives from list dynamic dependencies. This program looks at a given executable or shared library and figures out what shared libraries it needs to load and which versions would be used. The output looks like this:Listing 1. Dependencies of /bin/sh
$ ldd /bin/sh linux-gate.so.1 => (0xffffe000) libreadline.so.4 => /lib/libreadline.so.4 (0x40036000) libhistory.so.4 => /lib/libhistory.so.4 (0x40062000) libncurses.so.5 => /lib/libncurses.so.5 (0x40069000) libdl.so.2 => /lib/libdl.so.2 (0x400af000) libc.so.6 => /lib/tls/libc.so.6 (0x400b2000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
It can be a little surprising to find out how many libraries a "simple" program uses. It's probably the case that
libhistory
is the one calling forlibncurses
. To find out, we can just run anotherldd
command:Listing 2. Dependencies of libhistory
$ ldd /lib/libhistory.so.4 linux-gate.so.1 => (0xffffe000) libncurses.so.5 => /lib/libncurses.so.5 (0x40026000) libc.so.6 => /lib/tls/libc.so.6 (0x4006b000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
In some cases, an application may need extra library paths specified. For instance, the first few lines of an attempt to run
ldd
on the Mozilla binary came out like this:Listing 3. Result of ldd for items not in search path
$ ldd /opt/mozilla/lib/mozilla-bin linux-gate.so.1 => (0xffffe000) libmozjs.so => not found libplds4.so => not found libplc4.so => not found libnspr4.so => not found libpthread.so.0 => /lib/tls/libpthread.so.0 (0x40037000)
Why aren't these libraries found? Because they're not in the usual search path for libraries. In fact, they're found in
/opt/mozilla/lib
, so one solution would be to add that directory toLD_LIBRARY_PATH
.Another option is to set the path to
.
and runldd
from that directory, although this is a little more dangerous -- putting the current directory in your library path is just as potentially treacherous as putting it in your executable path.In this case, it's pretty clear that adding the directory these are in to the system-wide search path would be a bad idea. Nothing but Mozilla needs these libraries.
Linking Mozilla
And speaking of Mozilla, in case you were thinking that you'd never see more than a few lines of libraries, here's a somewhat more typical large application. Now you can see why Mozilla takes so long to launch!Listing 4. Dependencies of mozilla-bin
linux-gate.so.1 => (0xffffe000) libmozjs.so => ./libmozjs.so (0x40018000) libplds4.so => ./libplds4.so (0x40099000) libplc4.so => ./libplc4.so (0x4009d000) libnspr4.so => ./libnspr4.so (0x400a2000) libpthread.so.0 => /lib/tls/libpthread.so.0 (0x400f5000) libdl.so.2 => /lib/libdl.so.2 (0x40105000) libgtk-x11-2.0.so.0 => /opt/gnome/lib/libgtk-x11-2.0.so.0 (0x40108000) libgdk-x11-2.0.so.0 => /opt/gnome/lib/libgdk-x11-2.0.so.0 (0x40358000) libatk-1.0.so.0 => /opt/gnome/lib/libatk-1.0.so.0 (0x403c5000) libgdk_pixbuf-2.0.so.0 => /opt/gnome/lib/libgdk_pixbuf-2.0.so.0 (0x403df000) libpangoxft-1.0.so.0 => /opt/gnome/lib/libpangoxft-1.0.so.0 (0x403f1000) libpangox-1.0.so.0 => /opt/gnome/lib/libpangox-1.0.so.0 (0x40412000) libpango-1.0.so.0 => /opt/gnome/lib/libpango-1.0.so.0 (0x4041f000) libgobject-2.0.so.0 => /opt/gnome/lib/libgobject-2.0.so.0 (0x40451000) libgmodule-2.0.so.0 => /opt/gnome/lib/libgmodule-2.0.so.0 (0x40487000) libglib-2.0.so.0 => /opt/gnome/lib/libglib-2.0.so.0 (0x4048b000) libm.so.6 => /lib/tls/libm.so.6 (0x404f7000) libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x40519000) libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x405d5000) libc.so.6 => /lib/tls/libc.so.6 (0x405dd000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000) libX11.so.6 => /usr/X11R6/lib/libX11.so.6 (0x406f3000) libXrandr.so.2 => /usr/X11R6/lib/libXrandr.so.2 (0x407ef000) libXi.so.6 => /usr/X11R6/lib/libXi.so.6 (0x407f3000) libXext.so.6 => /usr/X11R6/lib/libXext.so.6 (0x407fb000) libXft.so.2 => /usr/X11R6/lib/libXft.so.2 (0x4080a000) libXrender.so.1 => /usr/X11R6/lib/libXrender.so.1 (0x4081e000) libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x40826000) libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x40850000) libexpat.so.0 => /usr/lib/libexpat.so.0 (0x408b9000)
Learning more about shared libraries
Users interested in learning more about dynamic linking on Linux have a broad field of options. The GNU compiler and linker tool chain documentation is excellent, although the guts of it are stored in theinfo
format and not mentioned in the standard man pages.The manual page for
ld.so
contains a fairly comprehensive list of variables that modify the behavior of the dynamic linker, as well as explanations of the different versions of the dynamic linker that have been used in the past.Most Linux documentation assumes that all shared libraries are dynamically linked because on Linux systems, they generally are. The work needed to make statically linked shared libraries is substantial and most users don't gain any benefit from it, although the performance difference is noticeable on systems that support the feature.
If you're using a pre-packaged system off the shelf, you probably won't run into very many shared library versions -- the system probably just ships with the ones it was linked against. On the other hand, if you do a lot of updates and source builds, you can end up with many versions of a shared library since old versions get left around "just in case."
As always, if you want to know more, experiment. Remember that nearly everything on a system refers back to those same few shared libraries, so if you break one of the system's core shared libraries, you're going to get to play with some kind of system recovery tool.
- Linkers and Loaders by John Levine (Morgan Kauffman, October 1999) is an authoritative source devoted to compile-time and run-time processes. (Some manuscript chapters are available online.)
- Try this source for more information on the ELF binary format.
- Read this communique if you ever wondered why a versioning scheme for shared libraries is important.
- Override the GNU C library -- painlessly (developerWorks, April 2002) shows how to use dynamic linking to override individual library functions without root privileges and without rebuilding the entire library.
- Writing DLLs for Linux apps (developerWorks, October 2001) demonstrates how dynamically linked libraries are often a great way to add functionality without writing a whole new Linux application.
- Shared objects for the object disoriented! (developerWorks, April 2001) explains how to write dynamically loadable libraries and suggests tools to use in the process.
- Use shared objects on Linux (developerWorks, May 2004) demonstrates how to make shared memory processes work.
Creating and Loading Shared Libraries
dev-misc-c++java - How to integrate C++ and Java code
Google Search shared library for Solaris tutorial
The World-Wide Web Virtual Library Tcl and Tk
Shared Library Interposer for SPARC Solaris 2.x -- code example
Re how to create shared library under solaris
> can anyone point me to some good documentation on and/or
> outline the salient
> points of creating a shared library using GNU C++.
>
> the man pages do not go beyond listing -fpic and -shared
> as compile and link
> options and i would like to understand whats required.
gcc -fPIC -c source.c
gcc -o library.so -shared source.o
Should work. You can give the shared lib an
soname by using
gcc -o library.so -Wl,-h,library.so -shared source.o
Hope this helps,
Martin.
Shared library problems on Solaris
Library Interface Versioning in Solaris and Linux
[email protected] from October to December 2000 Libwww Shared Li
Date: Fri, 29 Sep 2000 15:52:48 -0400 (EDT)
Message-ID: <[email protected]>
From: Prasanta Sahoo <[email protected]>
To: "'[email protected]'" <[email protected]>, "M.Murali" <[email protected]>, Sridhar Natarajan <[email protected]>
Subject: Libwww Shared Library Problem on Solaris 2.7
Hi,
I have problem with LIBWWW on Solaris 2.7. This is related with Shared
Object library file.
I am getting __eprintf symbol not found when I am trying to
start my applications.Can you tell me how do I solve this problem?.
file my.so: symbol __eprintf: referenced symbol not found
The symbol __eprintf is referenced in libwwwhtml.a (SGML.o):
# nm -A ./Library/src/.libs/* | grep eprint
./Library/src/.libs/libwwwhtml.a:SGML.o: U __eprintf
#
Is this a linker problem?
Who knows, where __eprintf is defined, I can't found it?
Also what do I need to set up before installing LIBWWW on Solaris?Is it
necessary
to install autoconf/aclocal/automake/autoheader/makeinfo ??
When I start with ./configure, it is generating config.log file.I have
listed portion of config.log file below.Please check it and let me know what
to do?
Regards
prasanta
650-620-5204
Message-ID: <11A760648FE3D211BE5200A0C9F2A194888F61@NL-ENS-MAIL01>
From: Sander Alberink <[email protected]>
To: [email protected]
Date: Thu, 5 Oct 2000 11:48:53 +0200
Subject: FW: Libwww Shared Library Problem on Solaris 2.7
-----Original Message-----
From: Sander Alberink
Sent: Thursday, October 05, 2000 11:18
To: 'Prasanta Sahoo'
Subject: RE: Libwww Shared Library Problem on Solaris 2.7
Hi!
> I have problem with LIBWWW on Solaris 2.7. This is related
> with Shared Object library file.
> I am getting __eprintf symbol not found when I am trying to
> start my applications.Can you tell me how do I solve this problem?.
>
> file my.so: symbol __eprintf: referenced symbol not found
>
> The symbol __eprintf is referenced in libwwwhtml.a (SGML.o):
>
> # nm -A ./Library/src/.libs/* | grep eprint
> ./Library/src/.libs/libwwwhtml.a:SGML.o: U __eprintf
> #
>
> Is this a linker problem?
> Who knows, where __eprintf is defined, I can't found it?
Apparently, you tried to link libwww (which has been compiled with gcc) to a
program compiled with the Sun compiler. __eprintf is an internal function of
the GCC compiler and is linked to every executable you compile with it (it
is used for the assert() macro, for example). As the SUN compiler does not
include that symbol, you get a linking error. Try compiling your program
with the gcc compiler, that should work.
HTH,
--
The woods are lovely, dark and deep,
But I have lines to code before I sleep, lines
to code before I sleep.
Shared library tricks
Notes
(mail to ilug (http://www.linux.ie))
on the topic of shared libraries, i thought id just mention an idea close to my heart.
there are linux and solaris calls dlopen and friends that allow you to at runtime from inside your own code open explictly a shared library and execute the code found there, theres an example in the dlopen manpage. so for instance you could have a widget library that consists of a scrollbar shared library, a button shared library etc, and the program rather than being linked to these libraries actually expliclty opens the button shared lib and runs the button stuff, so from a menu you could select which of multiple button libs, each with a utterly different look and feel that you want your program to use. pretty snazzy eh ?
the point of this mail though is that its bitch difficult to do this with c++, when you open the lib with dlopen in c you basically search for a function named for instance foo and then run it, with c++ if you had a function named foo, then compiled to a lib foo becomes due to name mangling foo_int23qwe_ug_addh or some other nonsense, so you cant just find the function you want with dlsym when c++ does its bizz to it, and if you want to get a handle to an object etc you might as well piss against the breeze, but there is a way out of the problem if you have one c call protected with extern C in the shared lib that returns a pointer the the object. now the name doesnt get mangled so you can find it with dlsym, and you get a handle to the pointer so you're off. of course as the program you call dysym from isnt actually linked to the program it wont actually have any knowledge of the object being passed to it, so you cant use the object unless you have a common parent class that the shared lib inherits from and that is visible to the dlopen code. so what you have now is a parent class definition that the code using dlopen has included, and that the object that the extern C protected call in the shared lib will return is based on so you can use the baby. from the dlopen calling code the only methods you can call directly on the returned objects are ones that had definitions in the base class, this is a big limitation, but it still allows a mechanism where a program can at run time with c++, swap in and out widget modules, change the style of buttons its using at the drop of a hat, any program using such a lib would be fully configurable. Im not too sure whether something like this is of any use, but its a damn cool idea. It all works pretty well in theory, and a sample chunk code that demonstrates the basic code for the concept is at http://www.csn.ul.ie/~caolan/pub/c++dl ive also gotten it to work with cygnus's windows library as well, so the same thing is possible under windows.
Caolan McNamara (1998) <[email protected]>
Download Package
Compiling the Version 2 computer player
The version 2 computer player is packaged as a java native method, and is written in C++. So you will need the header files (jni.h, and others) distributed with the JDK. You will also need a C++ compiler.
Then you must create a Makefile (or equivalent). Makefile.linux is an example file, which I use for compiling for Linux, running on Intel i686 processors (PentiumPro or Pentium II/III). Make a copy of it, and edit it according to your O/S and compiler requirements, naming it appropriately. If your file system supports links, then make a link from Makefile to Makefile.whatever.
I have included a file linking.html, which I have culled from the Java Tutorial, which shows how to link a shared library under Solaris with the Solaris C++ compiler, or Windows with Visual C++. Hopefully, this may be of some assistance.
Finally, write some installation instructions for downloading and installing the shared library/dynamic link library, and send the instructions and your makefile to me, so I can include it in future distributions.
Setting up the Restart Database
If you are intending to play games across the Internet, then you may want to set up a restart database. See for guidelines on how to do this.
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Last modified: March 12, 2019