/etc/ld.so.conf is the library where you set up the list of libraries to be included in the dynamic link libraries. But /etc/ld.so.conf is not the list that the kernel actually uses for a dynamic link library list. The kernel uses /etc/ld.so.cache. ldconfig is the program which takes /etc/ld.so.conf and builds /etc/ld.so.cache

When you install a new library, you need to run ldconfig to reconfigure the cache

ldconfig -vvvv

Reference

https://www.linuxquestions.org/questions/linux-software-2/what-is-ldconfig-369493/