When linking applications software with the CFITSIO library, several system libraries usually need to be specified on the link command line. On Unix systems, the most reliable way to determine what libraries are required is to type 'make testprog' and see what libraries the configure script has added. The typical libraries that need to be added are -lm (the math library) and -lnsl and -lsocket (needed only for FTP and HTTP file access). These latter 2 libraries are not needed on VMS and Windows platforms, because FTP file access is not currently supported on those platforms.
Note that when upgrading to a newer version of CFITSIO it is usually necessary to recompile, as well as relink, the programs that use CFITSIO, because the definitions in fitsio.h often change.