JLib is the free collection of libraries and utility code from Julian Smith.
Whilst programming for RISC OS, and a stint at Acorn, Julian produced some of the most incredible C code to augment programs written in C. His commericial products, MemCheck and HeirProf, aimed a C developers, provided memory checking and speed analysis for programmers to debug and optimise their programs.
Alongside the commericial products, Julian also produced command line utilities like Makatic and Every that provided very flexible conditional execution of programs, or inclusion of files in Makefiles.
JLib, provides some of the building blocks found in the aforementioned utilities. The library is very extensive as it combines multiple sub-libraries into one project, but the benefit is having them all under one roof. There are several examples dotted throughout each component, as well as tests and examples to show off the code and tools that make use of the diagnostic facilities presented.