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An Introduction to GCC - for the GNU compilers gcc and g++
by Brian J. Gough, foreword by Richard M. Stallman
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ISBN 0954161793
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3.2 Shared libraries and static libraries

Although the example program above has been successfully compiled and linked, a final step is needed before being able to load and run the executable file.

If an attempt is made to start the executable directly, the following error will occur on most systems:

$ ./a.out
./a.out: error while loading shared libraries:
libgdbm.so.3: cannot open shared object file:
No such file or directory

This is because the GDBM package provides a shared library. This type of library requires special treatment--it must be loaded from disk before the executable will run.

External libraries are usually provided in two forms: static libraries and shared libraries. Static libraries are the ‘.a’ files seen earlier. When a program is linked against a static library, the machine code from the object files for any external functions used by the program is copied from the library into the final executable.

Shared libraries are handled with a more advanced form of linking, which makes the executable file smaller. They use the extension ‘.so’, which stands for shared object.

An executable file linked against a shared library contains only a small table of the functions it requires, instead of the complete machine code from the object files for the external functions. Before the executable file starts running, the machine code for the external functions is copied into memory from the shared library file on disk by the operating system--a process referred to as dynamic linking.

Dynamic linking makes executable files smaller and saves disk space, because one copy of a library can be shared between multiple programs. Most operating systems also provide a virtual memory mechanism which allows one copy of a shared library in physical memory to be used by all running programs, saving memory as well as disk space.

Furthermore, shared libraries make it possible to update a library without recompiling the programs which use it (provided the interface to the library does not change).

Because of these advantages gcc compiles programs to use shared libraries by default on most systems, if they are available. Whenever a static library ‘libNAME.a’ would be used for linking with the option -lNAME the compiler first checks for an alternative shared library with the same name and a ‘.so’ extension.

In this case, when the compiler searches for the ‘libgdbm’ library in the link path, it finds the following two files in the directory ‘/opt/gdbm-1.8.3/lib’:

$ cd /opt/gdbm-1.8.3/lib
$ ls libgdbm.*
libgdbm.a  libgdbm.so

Consequently, the ‘libgdbm.so’ shared object file is used in preference to the ‘libgdbm.a’ static library.

However, when the executable file is started its loader function must find the shared library in order to load it into memory. By default the loader searches for shared libraries only in a predefined set of system directories, such as ‘/usr/local/lib’ and ‘/usr/lib’. If the library is not located in one of these directories it must be added to the load path.(10)

The simplest way to set the load path is through the environment variable LD_LIBRARY_PATH. For example, the following commands set the load path to ‘/opt/gdbm-1.8.3/lib’ so that ‘libgdbm.so’ can be found:

$ LD_LIBRARY_PATH=/opt/gdbm-1.8.3/lib
$ export LD_LIBRARY_PATH
$ ./a.out
Storing key-value pair... done.

The executable now runs successfully, prints its message and creates a DBM file called ‘test’ containing the key-value pair ‘testkey’ and ‘testvalue’.

To save typing, the LD_LIBRARY_PATH environment variable can be set automatically for each session using the appropriate login file, such as ‘.bash_profile’ for the GNU Bash shell.

Several shared library directories can be placed in the load path, as a colon separated list DIR1:DIR2:DIR3:...:DIRN. For example, the following command sets the load path to use the ‘lib’ directories under ‘/opt/gdbm-1.8.3’ and ‘/opt/gtk-1.4’:

$ LD_LIBRARY_PATH=/opt/gdbm-1.8.3/lib:/opt/gtk-1.4/lib
$ export LD_LIBRARY_PATH

If the load path contains existing entries, it can be extended using the syntax LD_LIBRARY_PATH=NEWDIRS:$LD_LIBRARY_PATH. For example, the following command adds the directory ‘/opt/gsl-1.5/lib’ to the load path shown above:

$ LD_LIBRARY_PATH=/opt/gsl-1.5/lib:$LD_LIBRARY_PATH
$ echo $LD_LIBRARY_PATH
/opt/gsl-1.5/lib:/opt/gdbm-1.8.3/lib:/opt/gtk-1.4/lib

It is possible for the system administrator to set the LD_LIBRARY_PATH variable for all users, by adding it to a default login script, such as ‘/etc/profile’. On GNU systems, a system-wide path can also be defined in the loader configuration file ‘/etc/ld.so.conf’.

Alternatively, static linking can be forced with the -static option to gcc to avoid the use of shared libraries:

$ gcc -Wall -static -I/opt/gdbm-1.8.3/include/ 
    -L/opt/gdbm-1.8.3/lib/ dbmain.c -lgdbm

This creates an executable linked with the static library ‘libgdbm.a’ which can be run without setting the environment variable LD_LIBRARY_PATH or putting shared libraries in the default directories:

$ ./a.out
Storing key-value pair... done.

As noted earlier, it is also possible to link directly with individual library files by specifying the full path to the library on the command line. For example, the following command will link directly with the static library ‘libgdbm.a’,

$ gcc -Wall -I/opt/gdbm-1.8.3/include 
    dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.a

and the command below will link with the shared library file ‘libgdbm.so’:

$ gcc -Wall -I/opt/gdbm-1.8.3/include 
    dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.so

In the latter case it is still necessary to set the library load path when running the executable.

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