Question

I am editing a fortran 90 to read a file. A particular file happens to be ‘contaminated’ with some extra information, so I wanted to attempt a read and then rewind; reread if in error:

open(filenum,file=filename,form="unformatted",iostat=ierr) //'direct' access
...lots of stuff...
here = ftell(filenum)
read(filenum,iostat=ierr) var1, var2             //try reading as var1, var2
if(iswrong(var1, var2)) then                     //check if correct
  call fseek(filenum,here-ftell(filenum),1)      //rewind
  read(filenum,iostat=ierr) vara, varb, varc     //read as different type 
endif

However, when I compile this program, I get

Undefined symbols:
  "_fseek_", referenced from:
      ___myreader__subroutine_name in myreader.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

(I am trying to compile on gfortran (i686-apple-darwin8-gfortran-4.2)). I understand that fseek is not a standard fortran routine.

I wonder if there is an alternative. I understand I can do something like read(filenum,rec=somevalue) but how can I use this in a similar fashion? I also thought to attempt reading with read(,advance='no') for testing and then reading again with advance='yes' if it is in the correct format, but this requires a specific format expression, which is not specified. Thank you.

Answer 1

You have to use backspace to go back to the previous record. Traditional Fortran IO is record based, non stream based (like in C)