Chapter 8. Shared libraries

54

at once (so that you don't get filename clashes if you try to install different versions of the

combined shared libraries package).

The package should install the shared libraries under their normal names. For example, the

libgdbm3

package should install

libgdbm.so.3.0.0

as

/usr/lib/libgdbm.so.3.0.0

.

The files should not be renamed or re linked by any

prerm

or

postrm

scripts;

dpkg

will take

care of renaming things safely without affecting running programs, and attempts to interfere

with this are likely to lead to problems.

Shared libraries should not be installed executable, since the dynamic linker does not require

this and trying to execute a shared library usually results in a core dump.

The run time library package should include the symbolic link that

ldconfig

would create

for the shared libraries. For example, the

libgdbm3

package should include a symbolic link

from

/usr/lib/libgdbm.so.3

to

libgdbm.so.3.0.0

. This is needed so that the dynamic

linker (for example

ld.so

or

ld linux.so.*

) can find the library between the time that

dpkg

installs it and the time that

ldconfig

is run in the

postinst

script.

3

8.1.1

ldconfig

Any package installing shared libraries in one of the default library directories of the dy 

namic linker (which are currently

/usr/lib

and

/lib

) or a directory that is listed in

/etc

/ld.so.conf

4

must use

ldconfig

to update the shared library system.

The package must call

ldconfig

in the

postinst

script if the first argument is

configure

;

the

postinst

script may optionally invoke

ldconfig

at other times. The package should call

ldconfig

in the

postrm

script if the first argument is

remove

. The maintainer scripts must

not invoke

ldconfig

under any circumstances other than those described in this paragraph.

5

3

The package management system requires the library to be placed before the symbolic link pointing to it in

the

.deb

file. This is so that when

dpkg

comes to install the symlink (overwriting the previous symlink pointing at

an older version of the library), the new shared library is already in place. In the past, this was achieved by creating

the library in the temporary packaging directory before creating the symlink. Unfortunately, this was not always

effective, since the building of the tar file in the

.deb

depended on the behavior of the underlying file system. Some

file systems (such as reiserfs) reorder the files so that the order of creation is forgotten. Since version 1.7.0,

dpkg

reorders the files itself as necessary when building a package. Thus it is no longer important to concern oneself

with the order of file creation.

4

These are currently

  /usr/X11R6/lib/Xaw3d

  /usr/local/lib

  /usr/lib/libc5 compat

  /lib/libc5 compat

  /usr/X11R6/lib

5

During install or upgrade, the preinst is called before the new files are installed, so calling  ldconfig  is point 

less. The preinst of an existing package can also be called if an upgrade fails. However, this happens during the

critical time when a shared libs may exist on disk under a temporary name. Thus, it is dangerous and forbidden by

current policy to call  ldconfig  at this time. When a package is installed or upgraded,  postinst configure  runs

after the new files are safely on disk. Since it is perfectly safe to invoke ldconfig unconditionally in a postinst, it is

OK for a package to simply put ldconfig in its postinst without checking the argument. The postinst can also be

called to recover from a failed upgrade. This happens before any new files are unpacked, so there is no reason to