CFITSIO can be used either with the POSIX pthreads interface or the OpenMP interface for multi-threaded parallel programs. When used in a multi-threaded environment, the CFITSIO library *must* be built using the -D_REENTRANT compiler directive. This can be done using the following build commands:
>./configure --enable-reentrant > makeA function called fits_is_reentrant is available to test whether or not CFITSIO was compiled with the -D_REENTRANT directive. When this feature is enabled, multiple threads can call any of the CFITSIO routines to simultaneously read or write separate FITS files. Multiple threads can also read data from the same FITS file simultaneously, as long as the file was opened independently by each thread. This relies on the operating system to correctly deal with reading the same file by multiple processes. Different threads should not share the same 'fitsfile' pointer to read an opened FITS file, unless locks are placed around the calls to the CFITSIO reading routines. Different threads should never try to write to the same FITS file.