quickselect library source code is located in lib/libmedian/src/quickselect.c

A related header file is located at lib/libmedian/include/quickselect.h

A man page is in lib/libmedian/man/quickselect.3

A test framework program, median_test, (so named for historical reasons)
has source code in src/median_test.c. (it's not pretty)

Header files are located in include and lib/libmedian/include.

The test framework also uses a number of libraries, located under
lib, and is built using makefiles and a few shell scripts.  See
README.makefiles and README.ident.

Building the test framework with quickselect available to it for testing
should be as simple as
   make depend
   make median_test

That ought to work on Solaris, Linux, and UNIX (e.g. NetBSD) systems.
Median_test also incorporates Bentley & McIlroy's vanilla qsort code,
modified code from NetBSD, further modifications that improve performance,
and glibc qsort code; it can also run tests on the local qsort on the system
on which it is built.

There's no man page for median_test, running it with a -? arg will print
a usage synopsis.  It is advisable to build w/o optimization to run the
operations cost tests; otherwise optimization is recommended.

A pre-peer-reviewed document describing quickselect and its development
and testing may be found at lib/libmedian/doc/pub/generic/paper.pdf.

A separate document detailing internals is in lib/libmedian/doc/pub/design.pdf.