how to put array size inside array initializer itself in C?

Question

I have an array that's used in a communication protocol (a USB device descriptor). This protocol calls for the array size in the array header. So I would like to do that (which is forbidden):

static uint8_t array[]= {
    TYPE,
    sizeof(array),
    other data...
};

The array being in a read only part of the memory, I can't override the relevant cell after the fact, and I'm not really willing to copy it to override the cell (it's in a minimal embedded system). I need it to look like "on the wire" because it will go through DMA.

Is there some kind of magic that could work around this limitation? I'm willing to use C99 or GNU extensions. I won't switch just for that, but I'm curious about a C++ solution too.

And specifying the size of the array in the declaration, e.g. as static uint8_t array[18] = ... is presumably no more helpful a solution than 'why don't you just type the size of the array by hand and not use sizeof'? — Tommy
– Tommy, Commented Apr 29, 2013 at 19:34
"The array being in a read only part of the memory" It can't be unless you omitted a const. — Daniel Fischer
– Daniel Fischer, Commented Apr 29, 2013 at 19:39
@danielFischer good catch, thanks. But with or without the 'const' this stuff ends up in a locked part of the memory map. — nraynaud
– nraynaud, Commented Apr 29, 2013 at 19:58

Bryan Olivier · Accepted Answer · 2013-04-29 19:47:59Z

3

If you would not use an array, but a struct, which it looks like you really want, then it would be possible:

typedef struct
{       int     type, size, other;
} pack_t;
static pack_t a =
{       10, sizeof(a), 11
};

answered Apr 29, 2013 at 19:47

Bryan Olivier

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3 Comments

nraynaud Over a year ago

here is the result: gist.github.com/nraynaud/5484759 I tink the line count is high, do you see a more compact way to do it? It's too bad I can't use the '=' directly in the struct.

Bryan Olivier Over a year ago

You could of course lose the field names, but that doesn't aid to the readability. If you have to make many of those descriptors, I would make a macro filling in most of the standard values and taking the varying ones from the macro parameters.

Tommy Over a year ago

With GCC you'll likely want to add __attribute__((packed)) to the definition of your struct, just to make absolutely sure there's no padding.

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