stupid_idiot
Group: Members
Posts: 344
Joined: Oct. 2006 |
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Posted: Jan. 06 2008,03:22 |
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This is how I understand LD_LIBRARY_PATH works:
Linking of programs during compilation is done by the linker ('ld').
Loading of shared libraries at runtime is done by the loader ('/lib/ld-linux.so.2').
Note: '/lib/ld-linux.so.2' is typically a symlink to
'/lib/ld-[glibc_version].so'.
e.g. '/lib/ld-linux.so.2' -> '/lib/ld-2.3.2.so' (DSL uses glibc 2.3.2.)
(1) For example: We have an application called 'my_program' that needs some libraries in '/opt/compile-3.3.5/lib/'.
We can add LD_LIBRARY_PATH to the environment, and then run the application:| Code Sample | export LD_LIBRARY_PATH=/opt/compile-3.3.5/lib
my_program |
Or, we can add LD_LIBRARY_PATH to the command line; it will take effect only for this command; it does not persist in the environment:| Code Sample | | LD_LIBRARY_PATH=/opt/compile-3.3.5/lib my_program |
or| Code Sample | | my_program LD_LIBRARY_PATH=/opt/compile-3.3.5/lib |
(3) Another method, that does not involve LD_LIBRARY_PATH, is to add '/opt/compile-3.3.5/lib/' to the ld.so cache file ('/etc/ld.so.cache'):| Code Sample | # Edit /etc/ld.so.conf:
editor /etc/ld.so.conf
# Run 'ldconfig' to rebuild '/etc/ld.so.cache' using the directories specified in '/etc/ld.so.conf':
ldconfig
# Run application:
my_program |
Note: 'ldconfig' does not make use of LD_LIBRARY_PATH.
To test this, we could do the following, for example:| Code Sample |
export LD_LIBRARY_PATH=/opt/compile-3.3.5/lib
ldconfig
# Then unset LD_LIBRARY_PATH:
unset LD_LIBRARY_PATH
# Then run the application:
my_program
(You will get an error.) |
As this shows, in line 2, 'ldconfig' does NOT add '/whatever/' to '/etc/ld.so.cache'.
Thus, once we unset LD_LIBRARY_PATH in line 4, the library in '/whatever/' can no longer be found.
Conclusion: The only way to add a directory to '/etc/ld.so.cache' is to edit '/etc/ld.so.conf'.
Re: Linking
'ld' can use LD_LIBRARY_PATH to find libraries at link-time (such as '-liconv') in the same fashion as above.
(1) We can export LD_LIBRARY_PATH before running 'configure':| Code Sample | export LD_LIBRARY_PATH=/opt/compile-3.3.5/lib
./configure
make |
LD_LIBRARY_PATH must not be unset until compilation is finished; otherwise, linking errors will occur because the linker cannot find needed libraries.
(1) Or, we can specify LDFLAGS when running 'configure':| Code Sample | | LDFLAGS=-L/opt/compile-3.3.5/lib ./configure |
We don't need to use LD_LIBRARY_PATH if we use LDFLAGS (but I am not sure about this...).
This TLDP page has relevant information (look for "LD_LIBRARY_PATH").
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