Re: Determining size of embedded blobs

[Ian Lance Taylor <iant at google dot com>]

Jean Hoderd <jhoderd@yahoo.com> writes:

> I want to embed an image into an executable.  I managed
> to do it by running "ld -r -b binary -o blob.o blob.png"
> on the blob.png file, thus creating a blob.o that I can
> link into my main program.  I have access to the binary
> blob by declaring an extern variable:
>
> extern char _binary_blob_png_start [];
>
> Normally I know the size of the blob I just linked in.
> But suppose I didn't.  Is there a way I can determine
> it from within the C program?  I know that by running
> "objdump -x blob.o" I can see the symbol table on the
> obj file, and that the symbols _binary_blob_png_end and
> _binary_blob_png_size are also defined:
>
> SYMBOL TABLE:
> 0000000000000000 l    d  .data  0000000000000000 .data
> 0000000000000400 g       .data  0000000000000000 _binary_blob_png_end
> 0000000000000000 g       .data  0000000000000000 _binary_blob_png_start
> 0000000000000400 g       *ABS*  0000000000000000 _binary_blob_png_size
>
> I assumed that _binary_blob_png_end was a pointer to the
> end of the blob, but it doesn't seem so.  Also, I always
> get a segfault if I assume that _binary_blob_png_size is
> an integer value.

As you've observed, _binary_blob_png_size is the size.  To access it
from C, you have to write something like:
    extern char _binary_blob_png_size[];
    size_t size = (size_t) &_binary_blob_png_size[0];
It's an awkward translation from a linker symbol to a C value.

Ian