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From: "JC" <jessechao AT comcast DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Free() crash my program!
Date: Thu, 7 Oct 2004 22:57:08 -0700
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Hi,

I used djgpp to develop a simple program, using dosmemget(),
to dump some DOS memory. The code is like following:

    p = (UINT8 *)malloc(len+1);
    if (p == NULL)
    {
        ...
    }
    dosmemget(addr, len, p);
    /* Code to printf the contents in *p */
 ...
 free(p);

The code compiled & ran fine. But when the code execute
the free(). The code crashed and got SIGSEG error:

    Exiting due to signal SIGSEGV
    General Protection Fault at eip=00006ac4
    ...

According to the FAQ, this might happen if the code put
data into the buffer that is longer than malloc. But my
code just read the data from buffer and free() them. How
come I had this crash? I also noticed: if the contents
of the buffer are all zeros (unused area?), it worked fine.
But if the buffer had data in it other than 0 (code? data?),
most likely it will crash. Anyone can help me on this?

Thanks in advance.