When the HDF5 library is employed in a C application, the library is automatically initialized when the first HDf5 function call is issued. If one finds that an HDF5 library function is failing inexplicably, H5_OPEN can be called first. It is safe to call H5_OPEN before an application issues any other function calls to the HDF5 library as there are no damaging side effects in calling it more than once.
For Fortran Developers:
When the HDF5 library is employed in a Fortran application, H5_OPEN initializes global variables (for example, predefined types) and performs other tasks required to initialize the HDF5 Fortran library. H5_OPEN and H5_CLOSE are required calls in HDF5 Fortran applications.
Returns a non-negative value if successful; otherwise returns a negative value.