How can I debug a Fortran READ/WRITE statement with an implicit DO loop?

Question

The Fortran program I am working is encountering a runtime error when processing an input file.

At line 182 of file ../SOURCE_FILE.f90 (unit = 1, file = 'INPUT_FILE.1')
Fortran runtime error: Bad value during integer read

Looking to line 182 I see a READ statement with an implicit/implied DO loop:

182:    READ(IT4, 310 )((IPPRM2(IP,I),IP=1,NP),I=1,16)    ! read 6 integers
183:    READ(IT4, 320 )((PPARM2(IP,I),IP=1,NP),I=1,14)    ! read 5 reals

Format statement:

310 FORMAT(1X,6I12)

When I reach this code in the debugger NP has a value of 2. I has a value of 6, and IP has a value of 67. I think I and IP should be reinitialized in the loop.

My problem is that when I try to step through in the debugger once I get to the READ statement it seems to execute and then throw the error. I'm not sure how to follow it as it reads. I tried stepping into the function, but it seems like that may be a difficult route to take since I am unfamiliar with the gfortran library. The input file looks OK, I think it should be read just fine. This makes me think this READ statement isn't looping as intended.

I am completely new to Fortran and implicit DO loops like this, but from what I can gather line 182 should read in 6 integers according to the format string #310. However, when I arrive NP has a value of 2 which makes me think it will only try to read 2 integers 16 times.

How can I debug this read statement to examine the values read into IPPARM as they are read from the file? Will I have to step through the Fortran library?

Any tips that can clear up my confusion regarding these implicit loops would be appreciated!

Thanks!

NOTE: I'm using gfortran/gcc and gdb on Linux.

I think it's reading 16*NP integers, probably 6 per line although I've never tried implied do loops with a formatted file... — mgilson
– mgilson, Commented Jul 17, 2012 at 14:09

Kevin Lee · Accepted Answer · 2012-07-17 20:06:27Z

1

Is there any reason you need specific formatting on the read? I would use READ(IT4, *) where feasible...

Later versions of gfortran support unlimited format reads (see link http://fortranwiki.org/fortran/show/Fortran+2008+status)

Then it may be helpful to specify

310 FORMAT("*(1X,6I12)")

Or for older compilers

310 FORMAT(1000(1X,6I12))

answered Jul 17, 2012 at 20:06

Kevin Lee

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M. S. B. Over a year ago

These formats will read all the numbers from the same line, but need an extra space every six. (*I12) or (1000I12) would probably be more convenient. The suggestion to use list-directed I/O, READ (IT4, *), is by the easiest.

M. S. B. · Accepted Answer · 2012-07-18 18:19:36Z

1

The variables IP and I are loop indices and so they are reinitialized by the loop. With NP=2 the first statement is going to read a total of 32 integers -- it is contributing to the determination the list of items to read. The format determines how they are read. With "1X,6I12" they will be read as 6 integers per line of the input file. When the first 6 of the requested 32 integers is read fron a line/record, Fortran will consider that line/record completed and advance to the next record.

With a format of "1X,6I12" the integers must be precisely arranged in the file. There should be a single blank, then the integers should each be right-justified in fields of 12 columns. If they get out of alignment you could get the wrong value read or a runtime error.

edited Jul 18, 2012 at 18:19

answered Jul 17, 2012 at 14:47

M. S. B.

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