SKYMAP is an astronomical mapping program written in Fortran and C for Unix workstations by Doug Mink of the Smithsonian Astrophysical Observatory Telescope Data Center. If you just need positions, several other options are available. The WCSTools package contains C programs which can search the GSC, USNO, and SAO catalogs, among others. The obsolete programs, rgsc for the Guide Star Catalog and star and its variants for other catalogs, use the same Fortran and C code as skymap.