The program ld.so handles a.out binaries, a format used long ago; ld-linux.so* ( /lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for glibc2) handles ELF, which everybody has been using for years now. Otherwise, both have the same behavior, and use the same support files and programs ldd(1), ldconfig(8), and /etc/ld.so.conf.
In the default path /lib, and then /usr/lib. (On some 64-bit architectures, the default paths for 64-bit shared objects are /lib64, and then /usr/lib64.) If the binary was linked with the -z nodeflib linker option, this step is skipped.
Will help to avoid many headaches like this ...
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