read/write same file with one open

utzoo!decvax!microsof!uw-beave!ubc-visi!demco utzoo!decvax!microsof!uw-beave!ubc-visi!demco
Thu Feb 17 17:54:32 AEST 1983

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;


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

		John Demco

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