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From: "Tom Scott" <tscott AT StorageMatrix DOT com>
To: <cygwin AT cygwin DOT com>
Subject: Code generation bug for operator new[] when -fcheck-new in GCC 3.3.1
Date: Wed, 26 Nov 2003 16:16:37 -0500
Organization: Storage Matrix, Inc.
Message-ID: <000c01c3b462$94729dc0$6400a8c0@DELL2K>
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I've found that the following sample, which uses nothrow memory allocation
semantics, generates a segmentation violation:

// g++ -g -fcheck-new -fno-exceptions -fno-rtti sample.cpp
#include <stdlib.h>
 
class foo {
public:
    int v;
    foo(){ v = 0; }
    ~foo() {}
    void* operator new[](size_t size) {
        return 0;   // simulated memory failure
    }
    void operator delete[](void* p, size_t size) { }
};
 
main()
{
    foo *p = new foo[2];
    // p==4 here
    if (p) delete [] p;
    return 0;
}

The segmentation violation results from a bug in the code that is generated
to call operator new[].  The return of operator new[] is correctly checked
for non-zero before calling the ctor ("-fcheck-new" semantics), but this
return value is subsequently incremented by 4.  As a result, p is set to 4
(not 0) when memory runs out.
 
A work around is to modify applications so that the return value of "new
class[]" is checked and to treat a return of 4 the same as 0.
 
Tom


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