How to read un-formatted data file saved via a VAX FORTRAN code with "map" and "union"

Question

guys. I am trying to read a scientific datum file stored by a VAX FORTRAN code. The data were stored in structure, of which the file and code descriptions are as follows. I googled that FORTRAN 77 might read the file, but my frequently used language is not FORTRAN. So can some one tell me how to read the data into a FORTRAN or C/IDL/etc. variables? For example, N units of the structure are stored in file "pxm.mos", how can I read the data into my variables? Thanks a lot! Here are the descriptions.

c     FILE name is "pxm.mos"
c     FILE AND RECORD STRUCTURE 
c       The files were created with form='unformatted', organization='sequential', 
c       access='sequential', recordtype='fixed', recordsize=512. 
c       The following VAX FORTRAN code specifies the record structure: 

            structure /PXMstruc/ 
              union 
                map 
                  integer*4 buffer(512) 
                end map 
                map 
                  integer*4 mod16           
                  integer*4 mod60          
                  integer*4 line            
                  integer*4 sct
                  integer*4 mfdsc          
                  integer*4 spare(3) 
                  real*4    datamin        
                  real*4    datamax        
                  real*4    data(0:427)   
                end map 
              end union 
            end structure 

            record /PXMstruc/ in 

Answer 1

This isn't hard. You can think of structure like a C struct, with unions. Each record is 2048 bytes (512 "longwords" in VAX terms) and consists of five 32-bit ints, an array of 3 ints for padding, two 32-bit floats and then an array of 428 floats. Given that the file is fixed length, there's no metadata to worry about. The union with "buffer" can be ignored.

I would be more concerned about how the file made its way to your computer, assuming it originated on a VMS system. You'll want to verify that the file size is an exact multiple of 2048 bytes. Most likely it transferred just fine, so declare a struct with the right layout and read it in, record by record.