From:    Mike Ratledge 
To:      All                                      Msg #245, 03-Aug-88 12:45.00
Subject: File sharing enabled test

Someone asked the other day about an easy way to determine if file-sharing
is enabled at program run-time.  I use the following code in all of my
Turbo C program to do just that:

#define TRUE 1
#define FALSE 0

int sharing;


main (int argc, char *argv[])

{
  sharing = is_sharing(argv[0]);
  .
  .
  .
  if (sharing)
  {
    /* open file in shared mode */
    ...
  }
  else
  {
    /* use "normal" open */
    ...
  }
}


int is_sharing(char *arg)

{
 FILE *exe;

  if (_osmajor < 3)
    return(FALSE);
  exe = fopen(arg, "rb");
  ii = lock(fileno(exe), 0l, 500l);
  if (ii != -1)
  {
    ii = unlock(fileno(exe), 0l, 500l);
    fclose(exe);
    return(TRUE);
  }
  fclose(exe);
  return(FALSE);
}

What does this code do?  First - it checks to make sure it's running under
DOS 3.0+ - if not - no sharing.  Next - it opens the program itself (the
.EXE file) by using "argv[0]", which points to the actual program name
complete with the path under DOS 3.0 or later.  It then attempts to lock
the first 500 bytes of the program on disk, and if successful (i.e. return
!= -1) it unlocks the same bytes and closes the file (actually - the unlock
is superfluous, since closing the file releases all locks) and returns the
"TRUE" result.  If it fails - it closes the .EXE file and returns FALSE.
Note that this does not depend on opening a file in shared mode to test it.
 
Note that this code must be modified slightly to be useful for MicroSoft
C, since they use the "locking" procedure for both lock & unlock.  You
also have to "rewind" before the unlock, since M/S C works from the
current file-pointer forward.  I could post both - but I'm sure all you
C-jockeys out there know what I'm talking about if it concerns you (i.e.
you're using M/S C instead of Turbo).  I also have this coded in Turbo
Pascal if anyone needs it...