Scientific software for performing large computations is typically managed using textual control files that specify the parameters of the computation. Historically, these control files have typically consisted of long, inflexible collections of numbers whose meaning and format is hard-coded into the program. With libctl, we make it easy for programmers to support a greatly superior control file structure, and with less effort than was required for traditional input formats. The "ctl" in "libctl" stands for Control Language (by convention, libctl control files end with ".ctl" and are referred to as ctl files). Thus, libctl is the Control Language Library (where the "lib" prefix follows the Unix idiom).