X-Authentication-Warning: delorie.com: mail set sender to djgpp-bounces using -f
X-Recipient: djgpp AT delorie DOT com
X-Original-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20161025;
        h=subject:to:references:from:openpgp:autocrypt:message-id:date
         :user-agent:mime-version:in-reply-to:content-language
         :content-transfer-encoding;
        bh=ei2+Ev9f7TwDDdZlBd+nlDy93Ac/GJXfL66Q9hobClM=;
        b=olwStKkUgYk+FpvnX0J81XLhJdSOi+shscEky5WgjQxn4ZuOHPvbUREPy54rplFfs5
         dKq7D4jRe01m0F0eQUp/M77BQivgyilKdLNuIoFhErGphp7zBzLDEbNl4XJTauL5BcR7
         5sM8qEobJGYV0XXYdT+DFZ01uLsw+/yc/zCAgs6ZJcPBdCMSChMK3zp1M0p61NnhRMAD
         HQOJSoJmWg0Pkyh3n7tRfdk6SICPLJBuqkVDEF4C0p88t8KMeZQhQ8R5mvtQz/YHHXzq
         NMRR7ZLlF0tU6RZc/Nd5vT1UArpL5h64pzs2uNcA0IxpZsTIl31PKaKDPtGuUUUXBVpf
         RZdg==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=1e100.net; s=20161025;
        h=x-gm-message-state:subject:to:references:from:openpgp:autocrypt
         :message-id:date:user-agent:mime-version:in-reply-to
         :content-language:content-transfer-encoding;
        bh=ei2+Ev9f7TwDDdZlBd+nlDy93Ac/GJXfL66Q9hobClM=;
        b=foGWIfQOhM6RnGRQUx9BSIYDCyUWrfvTwAUr1PMNiHdPM0Jz6djH1iyse8sgWYqiJJ
         riqSfzM+h9FA2CcKi53X10jvi7Gh7ulObEJbr9BGEfzPoywfFEvONJkPL6AY6ppUNLL/
         kqlJyeo35na4NrxU7dRtbpn0TI9A5QgixWgrietPUt0OpTynnvY5rpaDfKR7A/xaNjwK
         WIQgvBFAPUxiuVAwMVDN9pCpCOriAO+EtwKgovSI0+mvxvoJNHJWwkHCleUz/kmPv7WP
         4LGbWgkN/3AFzDmCneGgQV/iukcDs2+iL9g9n9bKbLqSgRTZFHiulnCfLUgfOIrn7tM0
         pwlg==
X-Gm-Message-State: APjAAAVcjdGfe5kxPpbvvaqtHDh/GDbmApuwKoqvvzmKpBwgaoAGoP/d
	i41kXsjTFtVxn+TBCev5SDzvmqCq
X-Google-Smtp-Source: APXvYqxfbOhkNmz7RkS5nvplWRzHQUqw1xzDbn2UpmiVSe1DDw8q+mMDiM9UHOm4ylSYzs43VIYZEQ==
X-Received: by 2002:a17:907:384:: with SMTP id ss4mr78695482ejb.166.1560788808772;
        Mon, 17 Jun 2019 09:26:48 -0700 (PDT)
Subject: Re: malloc() returns pointer to already allocated memory
To: djgpp AT delorie DOT com
References: <158e5d20-0a90-4beb-de48-da328379d8fb AT gmail DOT com>
 <qe76u1$1kj8$1 AT gioia DOT aioe DOT org>
From: "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Openpgp: id=D1694EA4DA1338AF4905293BA5102F469FA45960
Autocrypt: addr=jwjagersma AT gmail DOT com; prefer-encrypt=mutual; keydata=
 mJMEWlMYlBMJKyQDAwIIAQENBAMEORD9eiW30pI0XzJbuE7/4WF3ZyEOkpRgrmZnpTRmzXMW
 z22N+2YrczEM+q+NzM/wn8XOVH4hZ7eSaUsLT86YLS+gF/F44IbRZVCHxdrL+qSQQ0SafwDM
 Aaddx2azX+6MtlFO2B6oDbCHT7Nn5oQi3IgR+pgBzaCkfxyOnU1wjvO0JEouVy4gSmFnZXJz
 bWEgPGp3amFnZXJzbWFAZ21haWwuY29tPojQBBMTCgA4FiEE0WlOpNoTOK9JBSk7pRAvRp+k
 WWAFAlpTGJQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQpRAvRp+kWWA3xAH/WIBG
 sLngyIrTsn5uKz4JygtlCfjMRRSwl3/UIaW96VvIM/wUyzHl+vlyLmuclOjAiTVia/Js03Ar
 zB9iCHZQ0QH/W8cRwHjRNZRukU2IR8eqoGsl6hTUrrAAOKOVO04wrjO4jGDHg3Sw2GxBx7Md
 mD0QRId/gFwR/1UWS0jBdSUWC7iXBFpTGJQSCSskAwMCCAEBDQQDBFipNF/RUEAer/hhT6yJ
 5no1ZLAZgpypvWZ3xJSrBKmcDTaOmNKNRrw1dNtSZ6M6wYcBmtCq7uvR5iiGyK3nD+olwe7a
 4WDkhEEvbmdAYl2LnBsYeyruoW+N+KiMalvf9SphDmBi2NwdSZ1IhnBpxlUns8jqlgZGvG1G
 xE1sleq7AwEKCYi4BBgTCgAgFiEE0WlOpNoTOK9JBSk7pRAvRp+kWWAFAlpTGJQCGwwACgkQ
 pRAvRp+kWWBnyAH+JLrW/cK2aVDvq2nxn471opBgHyExzMi0hvbuy8BjyEt+AfL0RNCFcdds
 CQakIGDv5k69TAgOftECgASwv6CG5wH8D3jnksh6YvYMf0hCBlSfwJ5ehIeqQTj0m2410RgC
 KWywqmfU1X5YVXgmuXatRNgV/ggXS2Q7GnSP/StQzlh3sQ==
Message-ID: <c667a5a6-744f-3f50-e8cd-dbd4c3ec4db9@gmail.com>
Date: Mon, 17 Jun 2019 18:25:42 +0200
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.1
MIME-Version: 1.0
In-Reply-To: <qe76u1$1kj8$1@gioia.aioe.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Reply-To: djgpp AT delorie DOT com
Errors-To: nobody AT delorie DOT com
X-Mailing-List: djgpp AT delorie DOT com
X-Unsubscribes-To: listserv AT delorie DOT com
Precedence: bulk

On 2019-06-17 07:05, Rod Pemberton wrote:
> Second problem is you don't call memset() prior to using memory, nor
> free() after you're done using it.  Of course, calling memset() would
> prevent your method of "clobber" detection from working.  But, not
> calling memset() means you don't know if the magic clobber value is:
> a) from you setting it within your program, or
> b) from some random garbage values in memory.

The probability of one 32-bit memory address having a particular value 
is 1/(2^32), so about 1 in 4 billion. for a machine with, say, 256MB 
memory, you'd expect to find this value roughly once every 64 runs (IF 
all memory is initialized to perfectly random values, which it isn't. 
You're far more likely to find all ones or all zeroes on a clean boot). 
Clearly that isn't the case here, the clobber is detected every single time.
Clearing allocated memory by memset(), you wouldn't be able to find 
clobbered memory unless you kept track of all allocated pointers. Which 
requires a large table... allocated with malloc(). If malloc is bugged, 
you'd end up clobbering this table too.
Initially I did use a fixed magic value, but that causes false-positives 
on subsequent runs since memory isn't reset to zero. Using 
randomly-sized allocations turned out to be unnecessary since it happens 
with fixed size allocations too.

> Finally, you didn't report which version of DJGPP, or CWSDPMI, and
> whether or not your code is operating in a Windows 98/SE/ME/XP etc
> console.  When operating in a Windows console, CWSDPMI is not being
> used.  The Windows DPMI host is being used.

I'm testing on a Pentium 3 machine running FreeDOS, with cwsdpmi r7 and 
hdpmi32 v3.17. I'm cross-compiling from mingw64 with gcc 9.1, using the 
precompiled libraries from djcrx205.zip.