Subject: Can malloc return invalid memory?
To: None <>
From: D'Arcy J.M. Cain <>
List: current-users
Date: 04/02/2000 06:16:48
I know it sounds like an odd question but I have a very weird problem
with Python running on -current (1.4X) as of a few days ago and, in
fact, I have been struggling with this for a few weeks.  Here is the
code from stringobject.c in Python.  For clarity I removed some code
that was ifdef'd out and even if it wasn't I'm pretty sure it never
got hit since I know it reached the code below.

PyObject *
PyString_FromStringAndSize(str, size)
    const char *str;
    int size;
    register PyStringObject *op;
    op = (PyStringObject *)
        malloc(sizeof(PyStringObject) + size * sizeof(char));
    if (op == NULL)
        return PyErr_NoMemory();
    op->ob_type = &PyString_Type;

GDB tells me the following.

#0  PyString_FromStringAndSize (
    str=0x9164f47 "2000:04:01:13:57:10:EST 200 931 /images/refnews.gif",
    size=23) at stringobject.c:99

So I know that there is a string at least 23 characters and the size request
is 23.  At this point the string is irrelevant anyway.

The program crashes on that last line with:

Program terminated with signal 11, Segmentation fault.

The value of op is 0x9233000.  When I try to print op->ob_type I get
the following.

(gdb) p op->ob_type
Cannot access memory at address 0x9233004.

So malloc has not returned NULL but the space it does return seems to be
innaccessible to the program.  The only thing that is perhaps different
about my script is that it has huge memory requirements but malloc didn't
return an error so it isn't a matter of running out of memory.

Any ideas?

D'Arcy J.M. Cain <darcy@{druid|vex}.net>   |  Democracy is three wolves                |  and a sheep voting on
+1 416 425 1212     (DoD#0082)    (eNTP)   |  what's for dinner.