FORTRAN rewind on NFS filesystems

[Toon Moene]

Mathias Puetz wrote:

> we are running a Linux cluster with diskless slave nodes
> at our site and have a strange problem related to NFS.
> The problem is more likely to be with Fortran g77 than
> a real NFS problem. However, I'm not sure, the bug might
> as well be some 'feature' of the Linux NFS2 implementation.

>       PROGRAM rew
> 
>       CHARACTER(80) iwrote
> 
>       OPEN(FILE='testfile',UNIT=10,STATUS='new')
> 
>       WRITE(10,*) 'Totally Wrong!'
> 
>       REWIND(10)
> 
>       WRITE(10,*) 'Right!'
> c
> c     comment the above line, and uncomment below and
> c     it works on NFS
> c
> c     WRITE(10,*) 'Absolutely Right!'
> 
>       CLOSE(10)
> 
>       END
> 
> -------------------------------------------

> no compiler options in both cases.

> the contents of 'testfile' of the F77 version is
> 
> Right!
> 
> if run on an ext2 filesystem
> 
> and I get the error message
> 
> endfile: truncation failed in endfile
> apparent state: unit 10 named testfile
> last format: list io
> lately writing direct formatted external IO
> Aborted (core dumped)
> 
> if run on a NFS2 filesystem.

To see if it is a problem with your (mounting of) the NFS2 file system,
try this C equivalent to the Fortran code:

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

main()
{
   int f = open("testfile",O_RDWR | O_CREAT);

   if (f > 0) {

      write(f,"Totally wrong !\n", 16);

      lseek(f,0,SEEK_SET);

      ftruncate(f,0);
      write(f,"Right!\n", 8);

      close(f);
   }
   else {

      write(0,"Couldn't open file\n",19);
      return 1;
   }
}

The reason for the ftruncate is the requirement of Fortran that a write
statement on a sequential file always writes the record that's going to
be the last in the file.

So the sequence WRITE/REWIND/WRITE means: write a record, place the
"current record pointer" at the beginning of the file, clear the rest
and write a record (thus establishing a new "end-of-file").

Hope this helps,

-- 
Toon Moene (toon@moene.indiv.nluug.nl)
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Phone: +31 346 214290; Fax: +31 346 214286
GNU Fortran: http://gcc.gnu.org/onlinedocs/g77_news.html