lf6, SoftwareDevelopment: Making Shared Libraries

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by Luis Colorado
<luis(at)ctv2247.ctv.es>

Content:

1.- The process of generating a program. Introduction.
2.- Brief history of the process of program generation.
3.- What is a Library?
4.- Types of Libraries.
5.- Linking Operation under Linux.
6.- Dynamic Linking and Loading Shared Libraries
7.- soname. Versions of Shared Libraries. Compatibility.
8.- ldconfig(8)
9.- I Want to Make a Dynamic Library.
10.- Making a static library
11.- Static versus Dynamic Linking

Making Shared Libraries

Abstract:

Shared Libraries allow you to efficently re-use code between different applications.

_________________ _________________ _________________

1.- The process of generating a program. Introduction.

The process of generating a program nowadays in development environments is the fruit of an evolution in the habits and experiences suffered by programmers and designers.

This process consists of the following steps:

Creation of the source code in a high level language with a text editor. Very large programs can be hard to handle if we try to make them fit in a single file. For this reason, the source code is divided into functional modules, which are formed by one or more files of source code. The source code in these modules do not have to be written in the same language necessarily since some languages appear to be more appropriate to solve a given task than others.
After creating the files of source code for the program, they must be translated into segments of code executable by the machine. This code is usually referred to as object code. This code performs the same operations as the source code except that it is in a special language that is directly executable by the machine. The process of translating the source code into object code is known as compilation. A compilation is carried out by units and a compilation session usually will include (depending on the compiler) part of the program and in general, only one or a few files. The compiled object code contains a program, a subroutine, variables, etc. -- in general, the parts of a program that have already been translated and that can be delivered to the next stage.
After all the files with machine code for the program are generated, one proceeds to put them together or link them through a process performed by a special utility known as the linker. In this process, all the references that the code of a module makes to code belonging to another module are "resolved" (such as calls to subroutines or references to variables belonging to or defined in other modules). The resulting product is a program that normally can be loaded and run directly.
The execution of a program is performed by a special piece of software that is essential part of the operating system, which in the case of Linux is the system call exec(). This function finds the file, assigns memory to the process, loads specific parts of the file content (those containing the code and the initial values of the variables) and transfers the control to the CPU at a point in the program 'text' that is usually indicated in the executable file itself.

2.- Brief history of the process of program generation.

The process of program generation has suffered a constant evolution in order to always achieve the most efficient execution or the best use of system resources.

Initially, programs were written directly in machine code. Later, it was realized that writing a program in a higher level language and its subsequent translation to machine code could be automated due to the systematic nature of the translation. This increased the productivity of software.

Upon achievement of the compilation of programs (I have simplified the evolution of compilation, actually this was a very difficult step to take because it is a very complex process), the process of program generation consisted of generating a file with the program source, compiling it and executing it as a final step.

It was soon noticed, however, that the process of compilation was very expensive and took too many resources, including CPU time, and that many functions included by those programs were used over and over in various programs. Moreover, when somebody modified part of a program, this meant compiling the whole source again, including translating once again a whole bunch of identical code in order to compile the new code inserted.

That was the reason for introducing the compilation by modules. This consists of separating to one side the main program, and to the other side, those functions that are frequently used over and over, and which were already compiled and archived in a special place (we will call it the precursor of a library).

One could then develop programs supported by those functions without expending additional effort introducing their code again and again. Even then, the process was complex because when linking the program it was necessary to join all the pieces and these had to be identified by the programmer (this added the additional cost of perhaps using a known function that uses/needs other unknown functions)

3.- What is a Library?

The above problems lead to the creation of Libraries. This is nothing but a special type of file (more precisely an archive, type tar(1) or cpio(1)) with the peculiar characteristic that the linker understands its format and when we specified a library archive, THE LINKER SELECTS ONLY THOSE MODULES THAT THE PROGRAM NEEDS, and excludes everything else. A new advantage came into game. Now we could develop programs that used large libraries of functions and the programmer did not have to know all the dependencies of the functions in the library.

The library such as we have discussed so far, has not evolved much more that this. It has only acquired a new special file, that often appears at the beginning of the archive and that contains a description of the modules and the identifiers that the linker has to resolve without having to read the whole library (and thus removing the need to read the library several times). This process (adding the table of symbols to the library archive) is performed under Linux by the command ranlib(1). The libraries described thus far are known as STATIC LIBRARIES.

An advancement occurred after the introduction of the first multitasking systems: the sharing of code. If, in the same system, two copies of the same code were launched, it appeared interesting that two processes could share code because normally a program does not modify its own code. This idea eliminates the need for having multiple copies in memory which saves large amounts of memory on huge multi-user systems.

Taking this last innovation one step further, someone (I do not know who he/she was but the idea was great ;-) thought that quite often many programs used the same library, but being different programs, the portion of the library used by a program did not have to be the same as the portion used in other program. Moreover the main code was not the same (they were different programs), so their text were not shared. Well, our person had thought that if different programs using the same library could share the code of such a library we could save some memory. Now different programs share the library code, without having identical program text.

However, now the process is more complex. The executable program is not fully linked, but the referencing to identifiers of the library are postponed for the process of program loading. The linker (in the case of Linux is ld(1)) recognizes that it is dealing with a shared library and does not include its code in the program. The system itself, the kernel, when executing exec() recognizes that it is launching a program using shared libraries and it runs a special code for loading the shared libraries (assigning shared memory for its text, assigning private memory for the values of the library, etc.). This process is performed now when loading an executable and the whole procedure is much more complex.

Off course, when the linker is faced with a normal library it continues to behave as before.

The shared library is not an archive of files containing object code, but more like a file containing object code by itself. During linking of a program with a shared library, the linker does not inquire inside the library for which modules must be added to the program and which not It only makes sure that the unresolved references get resolved and detects which must be added to the list by the inclusion of the library. One could make an archive ar(1) library of all the shared libraries, but this is not often done because a shared library is often the result of linking various modules so that the library is necessary later, during run-time. Perhaps, the name shared library is not the most appropriate and it would be more clear to call it shared object (nevertheless, we will not use this other term in order to be understood).

4.- Types of Libraries.

As we already mentioned, under Linux there are two types of libraries: static and shared. The static libraries are collections of modules included in an archive with the utility ar(1) and indexed with the utility ranlib(1). These modules are often stored in a file whose name terminates in .a by convention (I will not use the term extension because under Linux the concept of extension of a file does not apply). The linker recognizes the termination .a in a file and starts the search for the modules as if it was a static library, selecting and adding to the program those modules that resolve the unresolved references.

The shared libraries, by contrast, are not archives but reallocable objects, marked by a special code (that identifies them as shared libraries). The linker ld(1), as mentioned, does not add the modules to the program code, but selects the identifiers provided by the library as resolved, adds those needed by the library itself and continues without adding any code, pretending the code in question has been added already to the main code. The linker ld(1) recognized a shared library by having the termination .so (not .so.xxx.yyy, and we will come back to this point).

5.- Linking Operation under Linux.

Every program consists of object modules linked to form an executable. This operation is performed by ld(1), which is the Linux linker.

ld(1) supports several options that modifies its behavior, but we will restrict ourselves here to those options related with the use of libraries in general. ld(1) is not invoked directly by the user but by the compiler itself gcc(1) in its final stage. A superficial knowledge about its modus operandis helps will help us understand the use of libraries under Linux.

ld(1) requires for its proper functioning the list of objects that are going to be linked to the program. These objects can be given and called in any order(*) as long as we follow the previous convention, as mentioned, that a shared library is indicated by a termination .so (not .so.xx.yy) and a static library by .a (and of course, simple object files are those whose names terminate in .o).

(*) This is not completely true. ld(1) includes only those modules that resolve the references at the moment of including the library, then there could still be a reference originated by a module included later that, since it does not appear yet in the moment of including this library, can cause the order of inclusion of the libraries to be significant.

On the other hand, ld(1) allows the inclusion of standard libraries thanks to the options -l and -L.

But... What do we understand by a standard library, what is the difference? None. Only that ld(1) searches for the standard libraries in predetermined locations while those appearing as object in the list of parameters are searched using their filename.

The libraries are searched by default in the directories /lib and /usr/lib (although I have heard that according to the version/ implementation of ld(1) there could be additional places). -L allows us to add directories to those used for the normal search of libraries. It is used by writing one -L directory for each directory we want to add. The standard libraries are specified with the option -l Name (where Name specifies the library to be loaded) and ld(1) will search, in order, in the corresponding directories, a filename libName.so. If not found it will try for libName.a., its static version

If ld(1) finds a libName.so file, it links it as if it was a shared library, while if it finds a file libName.a, it will link the modules obtained from this if they resolved any of the unresolved references.

6.- Dynamic Linking and Loading Shared Libraries

The dynamic linking is performed at the moment of loading the executable by a special module (in fact, this special module is a shared library itself), called /lib/ld-linux.so

Actually the are two modules for linking with dynamic libraries: /lib/ld.so (for libraries using the old a.out format) and /lib/ld-linux.so (for libraries using the new ELF format).

These modules are special, in that they must be loaded each time a program is linked dynamically. Their names are standard ( the reason they are not to be moved from the directory /lib, nor are their names to be modified). If we changed the name of /etc/ld-linux.so, we would automatically halt the use of any program using shared libraries because this module takes charge of resolving all the references not yet resolved at run-time.

The last module is helped by the existence of a file, /etc/ld.so.cache, who indicates, for every library, the most appropriate executable file that contains the library. We will return to this issue later.

7.- soname. Versions of Shared Libraries. Compatibility.

We now enter the most treacherous issue related to shared libraries: their versions

A message often received is 'library libX11.so.3 not found,' leaving us with the frustration of having the library libX11.so.6 and incapable of doing anything. How is it possible that ld.so(8) recognizes as interchangeable the libraries libpepe.so.45.0.1 and libpepe.so.45.22.3 and does not recognize libpepe.so.46.22.3?

Under Linux (and all the operating systems implementing the ELF format) the libraries are identified by a sequence of characters that distinguish them: the soname.

The soname is included inside the library itself and the sequence is determined when linking the objects forming the library. When the shared library is created, one has to pass to ld(1) an option (-soname <name_of_the_library>), to give a value to this character string.

This sequence of characters is used by the dynamic loader to identify the shared library that must be loaded and to identify the executable. The process is like this:
Ld-linux.so detects that the program requires a library and determines its soname. Then comes /etc/ld.so.cache with such a name and obtains the name of the file containing it. Next it compares the soname requested with the name existing in the library and if they are identical that's it! If they are not, it will continue searching until it finds it or if it cannot, it reports an error.

The soname can detect if a library is the appropriate one to be loaded because ld-linux.so makes sure that the soname requested coincides with the file requested. In case of disagreement we obtain the famous 'libXXX.so.Y' not found. What it is looking for is the soname and the error given refers to the soname.

This can cause a lot of confusion when we change the name of a library and the problem persists. But it is not a good idea to access the soname and change it because there is a convention in the Linux community for assigning soname:

The soname of a library, by convention, must identify the appropriate library and the INTERFACE of such library. If we perform modifications of a library that only affect their internal functioning, but the whole interface is intact (number of functions, variables, parameters of the functions) then the two libraries will be interchangeable and in general, we will say that the modifications introduced are minor (both libraries are compatible and we can replace one for the other. When this happens the minor number is often modified (which does not appear in the soname) and the library can be replaced without mayor problems.

However, when we add functions, remove functions, and in general, MODIFY THE INTERFACE of the library, then is not possible to maintain that the library as interchangeable with the previous one (for example substituting libX11.so.3 with libX11.so.6 is part of the upgrade from X11R5 to X11R6 which defines new functions and therefore modifies the interface). The change from X11R6-v3.1.2 to X11R6-v3.1.3 probably will not include changes in the interface and the library will have the same soname--although in order to preserve the old one we must give it a different name (for this reason the version number appears complete in the name of the library while only the major number shows in the soname).

8.- ldconfig(8)

As we mentioned earlier /etc/ld.so.cache allows tt>ld-linux.so to convert the soname of the file contained in the library. This is a binary file for more efficiency and is created with the utility ldconfig(8).
ldconfig(8) generates for each dynamic library found in the directories specified by /etc/ld.so.conf a symbolic link called by the soname of the library. It does this such that when ld.so is going to obtain the name of the file, what it really does is to select in the directory list a file with the soname sought, and in this fashion there is no need to execute ldconfig(8) each time we add a library. We run ldconfig only when we add a directory to the list.

9.- I Want to Make a Dynamic Library.

Before making a dynamic library we must consider if it is really going to be useful. The dynamic libraries cause an overload in the system due to several reasons:

The loading of a program is performed in several stages; one for loading the main program, and another for each dynamic library that the program uses (we will see that this is for appropriate the dynamic library, as this last item ceases to be inconvenient and starts to be an advantage).
The dynamic libraries must contain rellocable code, since the address allocated within the space of virtual addresses for the process will not be known until loading time. The compiler is then forced to reserved a register to maintain the loading position of the library and as a result we have one register less for the optimization of code. This case is a minor ill since the overload introduced by this case does not represent more than 5% of an overload in most cases.

For a dynamic library to be appropriate it must be used most of the time by some program (this avoids the problem of reloading the text of the library after the death of the process that started it. While other processes are still using modules of the library it remains in memory).

The shared library is fully loaded in memory (not only the modules needed) therefore, to be useful, it must be useful in its totality. The worse example of a dynamic library is where only a function is used and 90% of the library is hardly ever used.

A good example of dynamic library is the C standard library (it is used by all the programs written in C). On average all the functions are used here and there.

In a static library it is unnecessary to include functions whose usage is infrequent; as long as those functions are contained in their own module, they will not be linked in to those programs that do not required them.

9.1.- Compiling the Sources

The compilation of the sources is carried out in the same fashion as in the case of a normal source, except for that we will use the option '-f PIC' (Position Independent Code) to generate code that can be loaded in different positions within the space of virtual addresses of a process.

This step is fundamental because in a statically linked program, the position of the library objects are resolved at link-time, therefore at a fixed time. In the old a.out executables, it was impossible to performed this step, resulting in each shared library getting placed at a fixed position in the space of virtual addresses. As a consequence, there were conflicts anytime a program wanted to use two libraries and loaded them in overlapping regions of virtual memory. This meant you were forced to maintain a list, where whenever someone wanted to make a library dynamic, one would declare the range of addresses used so that nobody else would use it.

Well, as we mentioned, registering a dynamic library in an official list is not necessary because when the library is loaded, it goes to positions determined at that given instant, despite that fact that the code must be rellocable.

9.2.- Linking Objects in the Library

After compiling all the objects, it is necessary to link them with a special option which generates an object which is dynamically loadable.

gcc -shared -o libName.so.xxx.yyy.zzz
-Wl,-soname,libName.so.xxx

As the reader can appreciate, it looks like a normal link operation, except for the introduction of a series of options that will lead to the generation of a shared library. Let us explain them one by one:

-shared.
This phrase tells the linker that at the end it must generate a shared library, and therefore there will be a type of executable in the output file corresponding to the library.
-o libName.so.xxx.yyy.zzz.
Is the name of the final file. It is not necessary to follow the name convention, but if we want this library to be standard for future developments, it is convenient to follow it.
-Wl,-soname,libName.so.xxx.
The option -Wl tells gcc(1) that the next options (separated by comma) are for the linker. This is the mechanism used by gcc(1) to pass options to ld(1). Above we are passing the following options to the linker:
```
   -soname libName.so.xxx
```
This option fixes the soname of the library so that it can only be invoked by those programs that require a library with the soname specified.

9.3.- Installing the Library

Well we already have the corresponding executable. Now we must install it in the appropriate place in order to be able to use it.

To compile a program that requires our new library, one would use the following line:

gcc -o program libName.so.xxx.yyy.zzz

or if the library has been installed in the appropriate place (/usr/lib), it would be sufficient with:

gcc -o program -lName

(were the library in /usr/local/lib instead then it would have been sufficient to add the option '-L/usr/local/lib'). To install the library, do the following:

Copy the library to the directory /lib or /usr/lib. If you decide to copy it to a different location (for example /usr/local/lib), you cannot be certainty that the linker ld(1) will find it automatically when linking programs.
Execute ldconfig(1) to make the symbolic link from libName.so.xxx.yyy.zzz to libName.so.xxx. This step will tell us if we have completed all the previous steps correctly and the library is recognize as a dynamic library. The way programs get linked is not effected by this step, only the loading of the libraries at run-time are effected.
Make a symbolic link from libName.so.xxx.yyy.zzz (or from libName.so.xxx, the soname) to libName.so, in order to allow the linker to find the library with the -l option. For this mechanism to work it is necessary that the name of the library fits the pattern libName.so

10.- Making a static library

If on the other hand, one would like to make a static library (or two versions are needed to be able to offer statically linked copies) then one has to proceed as follows:

Note: The linker, in its search for libraries, looks first a file called libName.so, followed by libName.a. If we call the two libraries (the static and dynamic versions) by the same name, it will not be possible to determine, in general, which of the two will get linked in each case (the dynamic always gets linked first when it is found first).

For this reason it is always recommended that if the two versions of the same library are needed, the static one be named as libName_s.a, while the dynamic is named libName.so. When linking, therefore, one would do:

gcc -o program -lName_s

to link with the static version, while in the case of the dynamic one:

gcc -o program -lName

10.1.- Compiling the Sources

To compile the sources, we will not take any special measures. In the same way as the positions of the objects are decided during the linking step, it is not necessary to compile with the -f PIC option (although it is possible to continue using it).

10.2.- Linking the Objects in the Library

In the case of static libraries, there is no linking step. All the objects are archived in the library file by the command ar(1). Next, in order to resolve the symbols quickly, one is advised to execute the command ranlib(1) on the library. Although it is not necessary, not executing this command may unlink modules in the executable because when the module get processed by the linker during library construction not all the indirect dependencies between modules are resolved immediately: say the module is required by another module later on in the archive, which leads to having to pass several times through the same library until all the references get resolved).

10.3.- Installing the Library

The static libraries will be named the format libName.a if one is only interested on maintaining a static library. In the case of two types of libraries, I would recommend naming them libName_s.a, so that it will be easier to control whether to load a static or dynamic library.

The process of linking allows the introduction of the option -static. This option controls the loading of the module /lib/ld-linux.so, and does not affect the search order of the libraries, so that if one writes -static and ld(1) finds a dynamic library, it will continue to work with it (instead of continuing looking for its static counterpart). This leads to errors at run-time due to the invocation of routines in a library that do not belong to the executable -- the module for automatic dynamic loading is not linked and therefore this process can not be carried out.

11.- Static versus Dynamic Linking

Let us supposed we wish to distribute a program that makes use of a library which we are authorized to distribute only if included statically in a program and not in any other form (A example of this case are the applications developed with Motif).

To produce this kind of software there are two options. The first is making an executable statically linked (using only .a libraries and avoiding the use of the dynamic loader). These kinds of programs are loaded only once and do not require having any library installed in the system (not even /lib/ld-linux.so). However they have the disadvantage of a carrying all the software necessary within the binary file and therefore they are usually huge files. The second option is to make a dynamically linked program, meaning that the environment where our application will run should provide all the corresponding dynamic libraries. The executable may be very small although some times it is not possible to have all the libraries available (for example, there are people who do not have Motif).

There is a third option, a mixed distribution, in which some libraries are linked dynamically and others statically. In this case, logically one would choose the conflictive library in its static form and all the others in their dynamic form. This option is a very convenient form for software distribution.

For example, one could compile three different versions of a program as follows:

gcc -static -o program.static
program.o -lm_s -lXm_s -lXt_s -lX11_s\
-lXmu_s -lXpm_s

gcc -o program.dynamic program.o
-lm -lXm -lXt -lX11 -lXmu -lXpm

gcc -o program.mixed program.o
-lm -lXm_s -lXt -lX11 -lXmu -lXpm

In the third case, only the Motif library Motif (-lXm_s) gets linked statically, and all the others are linked dynamically. The environment where the program runs must provide the appropriate versions of the libraries libm.so.xx libXt.so.xx libX11.so.xx libXmu.so.xx y libXpm.so.xx

For more information:

Consult ELF-HOWTO.

Translation information:

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