The system call uselib() serves to load a shared library to be used by the calling process. It is given a pathname. The address where to load is found in the library itself. The library can have any recognized binary format.
This obsolete system call is not supported by glibc. No declaration is provided in glibc headers, but, through a quirk of history, glibc versions before 2.23 did export an ABI for this system call. Therefore, in order to employ this system call, it was sufficient to manually declare the interface in your code; alternatively, you could invoke the system call using syscall(2).
In ancient libc versions, uselib() was used to load the shared libraries with names found in an array of names in the binary.
Since libc 4.3.2, startup code tries to prefix these names with "/usr/lib", "/lib" and "" before giving up. In libc 4.3.4 and later these names are looked for in the directories found in LD_LIBRARY_PATH, and if not found there, prefixes "/usr/lib", "/lib" and "/" are tried.
From libc 4.4.4 on only the library "/lib/ld.so" is loaded, so that this dynamic library can load the remaining libraries needed (again using this call). This is also the state of affairs in libc5.
glibc2 does not use this call.
Since Linux 3.15, this system call is available only when the kernel is configured with the CONFIG_USELIB option.