How to get filename a symbolic link points to from within Fortran

[Mike Miller_8]

I need to be able to get the file name that a symbolic link points to from within a Fortran code. What is the easiest way to do this ?

[Steven Schweda · Accepted solution]

One method is to make a Fortran-callable C
function do the work, using the appropriate
C run-time function.

I haven't done any serious programming in
Fortran since it was FORTRAN, so you'll
probably want to edit this pretty heavily,
add better exception handling, and so on,
but, ...

dy # cat freadlink.c
#include /* For printf(). */
#include

int freadlink_( char *lname, char *ltext, int lname_len, int ltext_len)
{
int sts;

printf( " C-diag: lname_len = %d, ltext_len = %d.\n", lname_len, ltext_len);

/* NUL-terminate the Fortran string.
* Note that it's the caller's responsibility to provide the
* extra byte.
*/
lname[ lname_len] = '\0';

/* Get the link text. */
sts = readlink( lname, ltext, ltext_len);

/* Return the link text length. (-1 on error.) */
return sts;
}

dy # cat fln.f
program fln

implicit none

character* 1024 lname
character* 1024 ltext

integer* 4 lname_len
integer* 4 ltext_len

integer* 4 freadlink

lname = "slink"
lname_len = len( "slink")

write (*, *) " lname: >", lname(1: lname_len), "<"

ltext_len = freadlink( lname( 1: lname_len) , ltext)

write (*, *) " ltext_len = ", ltext_len, "."

if (ltext_len .ge. 0) then
write (*, *) " ltext: >", ltext( 1:ltext_len), "<"
endif

end

dy # ls -l
total 32
-rw-r--r-- 1 root sys 522 Mar 31 17:01 fln.f
-rw-r--r-- 1 root sys 519 Mar 31 17:00 freadlink.c
lrwxr-xr-x 1 root sys 9 Mar 31 16:46 slink -> link_text

dy # gcc -c freadlink.c
dy # gfortran fln.f -o fln freadlink.o

dy # ./fln
lname: >slink<
C-diag: lname_len = 5, ltext_len = 1024.
ltext_len = 9 .
ltext: >link_text<

Note that the details of how Fortran passes
a character string argument depend on the OS.
On VMS, it uses a descriptor. On UNIX, it
uses an extra (int) argument after the end
of the normal argument list. And on VMS, you
wouldn't need the "_" suffix on the C
function name.

Of course, if the Fortran run-time library
already offers a function like this, then
we've both wasted some time. (But I enjoy a
nice stroll down memory lane, and I've never
used "gfortran" in anger before, so it was
educational.)

[Mike Miller_8]

With a few mods and an alias command I was able to get this to work but then I found out that my check using stat was checking for hard links instead.

What is the easiest way to find out if a file is a symbolic link or not ?

[Steven Schweda]

> With a few mods and an alias command

Pray, tell.

> I was able to get this to work but then I
> found out that my check using stat

Can't see it.

> was checking for hard links instead.

Hard links are often called files.

> What is the easiest way to find out if a
> file is a symbolic link or not ?

I don't know. readlink() seems to offer one
way.

[...]
ERRORS

The readlink() function will fail if:
[...]
[EINVAL]

The path argument names a file that is not a symbolic link.
[...]

[Steven Schweda]

Compare inode values between stat() and
lstat() results?

There's probably a good way, too.

[Mike Miller_8]

I was able to get the following to work :

#include
#include
#include

int freadlink_( char *lname, char *buf, int lname_len, int buf_len)
{
int rc;
int i;

/* NUL-terminate the Fortran string. */
lname[ lname_len] = '\0';
for (i=0;i
// printf( " freadlink : lname = %s lname_len = %d, buf_len = %d.\n", lname, lname_len, buf_len);

/* Get the link text. */
rc = readlink( lname, buf, buf_len);
// printf(" freadlink : rc = %d\n",rc);

// if the readlink fails, return the filename as the output
if (errno!=0) strcpy(buf,lname);

// printf("freadlink : %s\n",buf);
return rc;
}


!FORTRAN CODE NEEDED AS A FUNCTION
!$HP$ ALIAS FREADLINK = 'freadlink_' (%REF, %REF, %VAL, %VAL)
Character* 255 Function linkFileName(filename)
Character* (*) filename

integer * 4 rc
character * 255 buf
integer * 4 bufsiz
integer * 4 ilen

bufsiz=255
ilen=len(trim(filename))

rc=freadlink(filename,buf,ilen,bufsiz)
linkFileName=buf

End

CALLING FORTRAN PROGRAM
character * 80 filename
character * 255 linkfilename

filename='test.fil'
write(6,100) trim(linkfilename(filename))
100 format(' test.fil -> ',A)

end

If test.fil is a link file then the filename pointed so is printed, if test.fil is not a link file then test.fil is printed.

Thanks for the help !

[Mike Miller_8]

Created Fortran callable C routine to call readlink to get the file pointed to by a symbolic link. If the filename passed in is not a symbolic link file then the filename is passed back out as the output.

[Mike Miller_8]

As it turns out you must also remove the nulls from the returned parm or if you pass it through asa the output will not print out right.

!$HP$ ALIAS FREADLINK = 'freadlink_' (%REF, %REF, %VAL, %VAL)
Character* 255 Function linkFileName(filename)
Character* (*) filename

integer * 4 rc
character * 255 buf
integer * 4 bufsiz
integer * 4 ilen

bufsiz=255
ilen=len(trim(filename))

rc=freadlink(filename,buf,ilen,bufsiz)
do i=1,255
if (buf(i:i).eq.char(0)) buf(i:i)=' '
end do
linkFileName=buf

End

[Mike Miller_8]

Created Fortran callable C routine to call readlink to get the filename a symbolic link points to.

[Steven Schweda]

> [...] you must also remove the nulls from
> the returned parm or if you pass it through
> asa [...]

Who's "asa"?

Or throw in another integer (C: int *)
argument, and pass the valid length back to
the caller (who then must avoid the invalid
residue). As usual, many things are
possible.

Irregardful, thanks for the report. I hadn't
seen the HP-Fortran-specific doo-dah before.
(I don't think that I ever needed to use a
"cDEC$", either, but I'd heard of that one.)
All very educational.