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Stream I/O problem with write (lu, pos=address) buffer

 
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2351
Location: Sydney

PostPosted: Thu Nov 25, 2021 4:50 am    Post subject: Stream I/O problem with write (lu, pos=address) buffer Reply with quote

Paul,

(new post of old problem for clean thread)

I am trying to understand what is wrong, so I created a "simpler" example.
This uses records of 256 bytes (no header/footer) and reads a single byte from a random address.

If I:
1) write the records sequentially, without an address, then read randomly using pos=calculated_address appears to work.
2) write the records sequentially, with a calculated address, then read randomly using pos= with the same calculated address approach, it FAILS.
3) write the records randomly, with a calculated address, then read pos= with the same calculated address, it FAILS.
With this random write test 3, the file appears to be truncated, possible after the last write, as I got an end of file error for a valid address in the read test with 2**12 records of 2**8 length. (1MB file)

This program example is possibly easier to follow and identify the problem(s).

Test 1 : gives hope, as the read (pos=) test appears to work, but
Test 2 : write ( pos=) looks to be incorrect.
Test 3 : random write could be worrying for rewriting data ?
All tests appear to work with gFortran, hopefully showing my approach to stream I/O is as expected.

The calculated address:
for record:byte in read > address = (record-1)*256 + byte
for start of record in write > address = (record-1)*256+1
for byte read > byte value = mod (address,record_length)

https://www.dropbox.com/s/jpdu6f8ub0z9d63/stream_read.f90?dl=0

For stream I/O to be a useful approach for large files (> 4GB), pos= needs to support an 8-byte addresses.

This example above does not test Inquire, which also gives an incorrect address. The code below is taken from an earlier post.

Code:
   subroutine get_stream_pos (stream_unit, stream_address)
      integer*4 :: stream_unit
      integer*8 :: stream_address
      integer iostat

      inquire ( unit=stream_unit, pos=stream_address, iostat=iostat )
      write (*,*) 'stream is at', stream_address, ' iostat =',iostat
   end subroutine get_stream_pos
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2351
Location: Sydney

PostPosted: Thu Nov 25, 2021 5:00 am    Post subject: Reply with quote

Paul,

I have not yet tested the latest .dll's you have supplied.
I am using ftn95 Ver 8.80.0. Should I update to the latest version first ?

John
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PaulLaidler
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Joined: 21 Feb 2005
Posts: 7142
Location: Salford, UK

PostPosted: Thu Nov 25, 2021 7:59 am    Post subject: Reply with quote

John

Yes. Please test with the DLLs that I sent to you and let me know if it fixes these issues.
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