Each image slot has an associated buffer area, from which header information can be retrieved. This buffer, named HEADER(*,IS) for slot IS, is an array of size is 128 words, containing an copy of the file header, but converted to the machine-dependent number format. Subroutines are available to read (or write) the header section from (to) these internal buffers: GDF_RHSEC and GDF_WHSEC.
To provide pre-defined data structures with the appropriate header item type, 3 named commons called X, Y or Z are described in include files inc:apar.inc, where ``a'' is the common ``name''. Routines are available to copy headers from one common to another (GDF_CHab), and to read header from (or copy header to) the internal array HEADER (GDF_READa, GDF_WRITa).
Each named common contains a variable to define one memory slot, a_MSLO, but more memory slots can be used simultaneously by declaring other variables in the application program. Two routines can be used to communicate between the ``new'' Fortran-90 header structures and the ``old'' Fortran-77 named commons:
SUBROUTINE GDF_HEADER_TO_COMMON (IMAG,COM)
SUBROUTINE GDF_COMMON_TO_HEADER (IMAG,COM)
where IMAGE is a header structure, and COM is
'X','Y' or 'Z'