Resolving symbols error
Introduction
In this wiki, you will get a brief explanation of dynamic libraries as well as how to resolve errors related to symbols.
What is dynamic linking?
Dynamic linking lets the program find the libraries required to execute the program. When a program is compiled and the headers are in another location, you can include it by providing the location of this library.
It is important to notice that when the LD_LIBRARY_PATH changes, the application will search the libraries in this location even though using the embed linking it's stated to search in other paths.
To check if the binary is finding the required libraries you can use the tool "ldd".
With this tool, you can know if it can find or not the libraries to execute the program.
Why does the order of the libraries affect when doing static linking?
On the contrary side of dynamic linking where the compiler will search for the libraries that satisfy each symbol, static linking will require looking first at the dependent libraries and finally the libraries that satisfy these symbols.
For example, the next .c will require the math library to handle the square function if you provide the following order in the compiler it will fail cause it won't find the library.
$ gcc -Wall -lm calc.c -o calc (incorrect order) main.o: In function `main': main.o(.text+0xf): undefined reference to `sqrt'
While the correct order would be:
$ gcc -Wall calc.c -lm -o calc (correct order)
LD_PRELOAD Trick
Using the LD_PRELOAD you can force the system to find shared libraries when it can't find them do to an error with the linking.
By this LD_PRELOAD is possible to change the order of the linking search of the application.
Take the next example.
ldd /usr/bin/pigz linux-vdso.so.1 (0x00007ffd559d7000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fa48bcc1000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa48bc9e000) libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fa48bc82000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa48ba90000) /lib64/ld-linux-x86-64.so.2 (0x00007fa48c065000)
After using LD_PRELOAD
LD_PRELOAD=/data/preload/lib/libz.so.1.2.7 ldd /usr/bin/pigz linux-vdso.so.1 (0x00007ffc1d9c4000) /data/preload/lib/libz.so.1.2.7 (0x00007f33877d9000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f3387674000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f3387651000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f338745f000) /lib64/ld-linux-x86-64.so.2 (0x00007f3387a32000)
You can see the change of the dependency for libz.so.1.2.7 after using the LD_PRELOAD, where it was after the libpthread.so.0 before.
Symbol not found
When you find this message, the problem could be related to the order of the libraries, so if you want to check this error you can proceed first with the next command.
ldd
If you do not find the library then check the specific symbol in your library
nm -D
Finally, check if all the dependencies are resolved in the correct order and then change the linker order as explained before.
Further Reading
- Library order in static linking
- LDD manual
- What is dynamic linking?
- How do linkers resolve symbols? Systems Programming CS Lecture
References
What is LDD_PRELOAD_TRICK: https://www.baeldung.com/linux/ld_preload-trick-what-is
Linking order of libraries: https://www.linuxtopia.org/online_books/an_introduction_to_gcc/gccintro_18.html
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