Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!bloom-beacon!mcgill-vision!mouse
From: mouse@mcgill-vision.UUCP (der Mouse)
Newsgroups: comp.unix.wizards
Subject: ftruncate broken? - Sun-based NFS
Message-ID: <1579@mcgill-vision.UUCP>
Date: 9 Jul 89 05:13:16 GMT
Organization: McGill University, Montreal
Lines: 62

The ftruncate() call appears to be broken on at least some systems with
NFS implementations based on Sun's.  I've tried this on a Sun-3 with
release 3.5 and on a VAX running mtXinu 4.3+NFS.  I also tried it on a
MicroVAX running real 4.3, and it did not exhibit the broken behavior.
But it's not directly an NFS problem, because it happens even when the
file is on a ufs filesystem.

The problem is that ftruncate() fails if the file modes prohibit
writing, even if the file descriptor used does permit writing.  For
example, try the following program on a handy Sun.  Notice that (unless
you try it as super-user), the ftruncate call fails.  Try it on a 4.3
machine, though, and everything's fine.

(I checked the Sun manpage, and there's not even a note in the BUGS
section warning about this, so presumably someone thinks it should
work the way it does on 4.3.)

Anybody have a simple fix?  (Patch a couple of bytes to noops somewhere
in the OBJ/ files perhaps?)  Will it be fixed in newer releases (4.x)?
I'm about ready to try to work out a fix on the mtXinu system, to which
we have source, but that's not much help on the Suns.

	#include <sys/file.h>
	
	int fd;
	char junk[8192];
	
	main()
	{
	 unlink("test.file");
	 fd = open("test.file",O_RDWR|O_CREAT|O_TRUNC,0666);
	 if (fd < 0)
	  { perror("open/create test.file");
	    exit(1);
	  }
	 if (write(fd,&junk[0],8192) != 8192)
	  { perror("write #1");
	    exit(1);
	  }
	 if (fchmod(fd,0444) < 0)
	  { perror("fchmod");
	    exit(1);
	  }
	 if (write(fd,&junk[0],8192) != 8192)
	  { perror("write #2");
	    exit(1);
	  }
	 if (ftruncate(fd,(unsigned long int)16000) < 0)
	  { perror("ftruncate");
	    exit(1);
	  }
	 if (close(fd) < 0)
	  { perror("close");
	    exit(1);
	  }
	 exit(0);
	}

					der Mouse

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