  The Fortran LHAPDF library has been a long-term workhorse in particle
physics, providing standardised access to parton density functions for
experimental and phenomenological purposes alike, following on from the
venerable PDFLIB package. During Run 1 of the LHC, however, several fundamental
limitations in LHAPDF's design have became deeply problematic, restricting the
usability of the library for important physics-study procedures and providing
dangerous avenues by which to silently obtain incorrect results. In this paper
we present the LHAPDF 6 library, a ground-up re-engineering of the
PDFLIB/LHAPDF paradigm for PDF access which removes all limits on use of
concurrent PDF sets, massively reduces static memory requirements, offers
improved CPU performance, and fixes fundamental bugs in multi-set access to PDF
metadata. The new design, restricted for now to interpolated PDFs, uses
centralised numerical routines and a powerful cascading metadata system to
decouple software releases from provision of new PDF data and allow completely
general parton content. More than 200 PDF sets have been migrated from LHAPDF 5
to the new universal data format, via a stringent quality control procedure.
LHAPDF 6 is supported by many Monte Carlo generators and other physics
programs, in some cases via a full set of compatibility routines, and is
recommended for the demanding PDF access needs of LHC Run 2 and beyond.
