r+ on fopen.

[Lepreau%utah-20 at sri-unix.UUCP]

From:  Jay Lepreau <Lepreau at utah-20>

This is indeed a long-standing bug, to my knowledge first fixed
by John Demco (ubc-vision!demco), and later this year by Guy Harris.
Both messages are below, as each has info the other doesn't.

>From harpo!decvax!microsof!uw-beave!ubc-visi!demco Thu Feb 17 17:54:32 1983
Subject: Re: read/write same file with one open
Newsgroups: net.lang.c

I have found a bug in the fseek() function of the stdio package as
distributed by Berkeley (3/9/81). It can result in improper flushing with a
stream file which has been opened for reading and writing (by specifying
"r+", "w+", or "a+"). I don't know if the problem exists on System III or V.

First, here is some background information about the stdio package. If a
stream is opened for read/write, a flag bit called _IORW is set in the _iob
structure for the file. Two other flag bits, _IOWRT and _IOREAD, indicate
whether the file is currently being written or read. Changing from reading
to writing or vice versa can only be done after reading an end of file, or
calling fseek() or rewind(). In these cases the package tries to make sure
that neither of the _IOWRT or _IOREAD bits are set, so you can subsequently
do either a getc() or putc() and the package will enter reading or writing
mode correctly.

The problem is that fseek() called while in read mode can return with the
file still in read mode. Subsequent putc()'s will not be written. A quick
fix is to change

	resync = offset&01;

to

	if (!(iop -> _flag & _IORW))
		resync = offset & 01;
	else
		resync = 0;

Can someone tell me what this "resync" business is all about? It looks like
an attempt to keep the file's buffer aligned on an even byte offset into the
file, but it won't do that in all cases.



>From harpo!seismo!rlgvax!guy Sun Aug 14 22:50:11 1983
Subject: Standard I/O streams open for reading and writing
Newsgroups: net.unix-wizards,net.bugs.4bsd

The Standard I/O library for releases of UNIX from V7 on includes a feature
to permit you to open a stream for reading and writing.  This feature is
documented in USG UNIX (System III, System V).  There is one minor bug in
the V7 implementation, and an additional major bug in the 4.?BSD implementation.

The first bug is that if your "umask" takes away your own read or write
permission, you may not use the "w+" or "a+" modes.  The problem is that
these modes require "stdio" to do a "creat" to create the file, and then to
close and reopen the file in order to have it open for reading and writing
(USG doesn't have to do this, but interestingly enough the S3 "stdio" does
it anyway), so if your umask takes away read or write permissions the reopen
fails.  The fix is to change the lines

	f = creat(file, 0666);
	if (rw && f>=0) {
		close(f);
		f = open(file, 2);
	}

to

	if (rw) {
		int m = umask(0);
		f = creat(file, 0666);
		if (f>=0) {
			close(f);
			f = open(file, 2);
			chmod(file, 0666 & ~m);
		}
		umask(m);
	} else
		f = creat(file, 0666);

in "endopen.c" in V7, and in "fopen.c" and "freopen.c" in 4.?BSD.

The more serious bug is due to the way the 4.?BSD "stdio" handles "fseek".
If you have a stream open for reading and writing, you must do an "fseek"
if you want to switch from reading to writing or vice versa (this is documented
in the USG manuals, so it's not a buried feature).  However, if you do an
"fseek" to an odd-byte boundary on 4.?BSD on a stream where the last operation
was a read, it seeks to the previous byte and does a "getc" to move to the next
byte.  This is not a problem if the stream is open only for reading or only for
writing (I presume it was put in there for efficiency; PDP-11 UNIX moves data
from kernel to user or vice versa much more efficiently if it is on a word
boundary), but it makes it impossible to put an "fseek" after a read and
immediately before a write (because the "fseek" may do a read before you get
a chance to do your write).  The code in "fseek.c" to handle seeks on a
stream on which the last operation was a read has a section which reads:

		if (iop->_flag & _IORW) {
			iop->_ptr = iop->_base;
			iop->_flag &= ~_IOREAD;
		}

A line "resync = 0;" should be added after the "iop->_flag &= ~_IOREAD;".  This
way, "fseek" will behave as it did if the stream is only open for reading, but
will not do the "resync" if it is open for reading and writing.

	Guy Harris
	{seismo,mcnc,we13,brl-bmd,allegra}!rlgvax!guy

-------