[9fans] zero length arrays in gcc

[9fans] zero length arrays in gcc

[Fernan Bolando]

Hi all

nhc98 uses a few of

static unsigned startLabel[]={};

which is a zero length array. It appears that it uses this as
reference to calculate the correct pointer for a bytecode.

pcc does not allow this since zero lenth array is another gcc
extension. I tried declaring it as

static unsigned startLabel[];

The resulting bytecode can then be compiled however it will only
crash. I traced it a pointer that tries to read an unallocated section
in memory.

Is it possible emulate zero pointer in pcc??

--
http://www.fernski.com

Re: [9fans] zero length arrays in gcc

[Bakul Shah]

On Wed, 23 Sep 2009 02:49:44 +0800 Fernan Bolando <fernanbolando@mailc.net>  wrote:
> Hi all
>
> nhc98 uses a few of
>
> static unsigned startLabel[]={};
>
> which is a zero length array. It appears that it uses this as
> reference to calculate the correct pointer for a bytecode.
>
> pcc does not allow this since zero lenth array is another gcc
> extension. I tried declaring it as
>
> static unsigned startLabel[];
>
> The resulting bytecode can then be compiled however it will only
> crash. I traced it a pointer that tries to read an unallocated section
> in memory.
>
> Is it possible emulate zero pointer in pcc??

Would something like this work?

unsigned foo[1];
#define startLabel (&foo[1])

Re: [9fans] zero length arrays in gcc

[Fernan Bolando]

Hi all

both option did not solve my lost pointer problem. I am still
verifying if it is due to something unrelated to zero length arrays

option1
 unsigned foo[1];
 #define startLabel (&foo[1])

option 2
static unsigned startLabel0;
static unsigned startLabel[] = &startLabel0

thanks
fernan

--
http://www.fernski.com

Re: [9fans] zero length arrays in gcc

[Russ Cox]

Assuming there are not many of these, the
easiest thing to do is fix the nhc98 code.
Figure out what the field after the label is
and take that address instead.

Alternately, you can use a Plan 9 extension
to defeat the gcc extension.  Wherever it says

struct Foo{
    ...
    uchar startLabel[];
    int field1;
    char field2;
    double field 3;
    uchar endLabel[];
    ...
}

you can replace it with

typedef struct insideFoo insideFoo;
struct insideFoo {
    int field1
    char field2;
    double field3;
}

struct Foo {
    ...
    insideFoo;
    ...
}

and then when it says something like

    p = x.startLabel;
    q = x.endLabel;
    memmove(&data, &x.startLabel, x.endLabel - x.startLabel);

you can write

    p = (char*)&x.insideFoo;
    q = (char*)(&x.insideFoo+1);
    memmove(&data, &x.insideFoo, sizeof x.insideFoo);

Russ

Re: [9fans] zero length arrays in gcc

[erik quanstrom]

> pcc does not allow this since zero lenth array is another gcc
> extension. I tried declaring it as
>
> static unsigned startLabel[];
>
> The resulting bytecode can then be compiled however it will only
> crash. I traced it a pointer that tries to read an unallocated section
> in memory.

the only thing that makes sense is

static unsigned startLabel0;
static unsigned startLabel[] = &startLabel0

this might not work.  one can't rule out deep and dark
magic with gcc.

- erik