FITSIO is a large library of subroutines that make it easier for programmers to read and write data files in FITS format. FITS stands for the Flexible Image Transport System and is the standard data format used within (and to some extent, outside) the astronomical community for archiving and transporting data files. When the FITS format was first developed in the late 1970s it was only used for simple n-dimensional arrays of data (such as 2-D images), so it was relatively easy for programmers to write software to directly read and write data in FITS format. The FITS format has grown in complexity, however, with the addition of various `extensions' to store tabular data, so that it has become increasingly difficult and time consuming for programmers to support software to read or write all the various possible FITS formats. For this reason, the FITSIO subroutine library was developed in the early 1990s to provide a simple, machine-independent interface which programmers can use to read and write FITS data files. Most of the internal details of the FITS file are hidden by this interface, so programmers can concentrate on simply accessing the information they need, without having to worry about all the complicated FITS formatting rules.
The basic FITSIO library is written in ANSI standard Fortran-77 and is supported on all commonly used computers and operating systems. There are also available sets of C macro and SPP wrapper routines for the convenience of programmers of those languages. (SPP is the programming language used in the IRAF data analysis system).