From: rishab AT dxm DOT ernet DOT in
Date: Sat, 12 Feb 94 00:14:23 IST
Subject: GO32 crashing with fstream append
To: djgpp AT sun DOT soe DOT clarkson DOT edu


Hello world!

After my last mail on go32 crashes wihth IOStream, I've created some simple
programs that crash. I've gone through them with go32's debugger. The problem
seems to be caused by a scrambled pointer-to-virtual-table. This reminds me of
a previous post on libg++ 2.5.3, which fixed a gcc code-gen bug with a hack in
streambuf.h; apparently ifstream v-tables were getting mangled incorrectly.

Sources for DJGPP libs seem to be derived from lg++2.5.0, iac definitely NOT
2.5.3, going by the differing changelog files in the libio directory.

Is there a version of the sources derived from libg++ 2.5.3? Does, or would 
that fix the problems? I'm trying to apply dj diffs to the gnu libg++ 2.5.3.

Here are the sources. ouchout and ouchin crash in 
[io]fstream::open(const char...), line close to end, while trying to call
virtual function seekoff() (due to ios::app or ios::ate). Unfortunately, the 
pointer to vtable points to junk.

Seekout crashes in sputn(...) ?i think? called by ostream::write(...), when
sputn tries (so it appears) to find an address from a vtable, tho in this case
the scrambled pointer to vtable points to memory out of bounds, crashing.

Note that the first call to write() does not crash. If you use << instead of 
write, seekout does not crash at all. You may want to use write for tables...

--------------------
// ouchout.cc

#include <fstream.h>
#include <stdio.h> // to use puts(). cout buffers don't clear before the crash.

void main()
{
	char c='!';
	puts("\n opening... \n\n\n");   // will print
	ofstream ofile("qq.DAT", ios::app); // appending, try with an existing file
	puts("\n  writing... \n\n\n"); // we'll never get here
	ofile << c;
}

---------------
// ouchin.cc

#include <fstream.h>
#include <stdio.h>

void main()
{
	char c;
	puts("\n opening... \n\n\n"); // ok
	ifstream ifile("qq.DAT", ios::ate);
	puts("\n  reading... \n\n\n"); // we've died already
	ifile >> c;
}

--------------
// seekout.cc

#include <fstream.h>
#include <stdio.h>

void main()
{
	char c='!';
	puts("\n opening... \n\n\n");
	ofstream ofile("qq.DAT");
	puts("\n  seeking 0... \n\n\n");
	ofile.seekp(0, ios::cur);
	puts("\n  writing at 0... \n\n\n");
	ofile << c;
	puts("\n  seeking to end... \n\n\n");
	ofile.seekp(0, ios::end);               // ok so far, we can call 
	                                        // virtual fn seekoff thru seekp
	puts("\n  writing from end... \n\n\n"); // this prints
	ofile.write(&c,1);
	puts("\n  done! \n\n\n"); // we never get here
}

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Rishab Aiyer Ghosh                            "What is civilisation
rishab AT doe DOT ernet DOT in, rishab AT dxm DOT ernet DOT in        but a ribonucleic
Voicemail +91 11 3760335; Vox/Fax/Data 6853410      hangover?"
H-34C Saket New Delhi 110017 INDIA
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