Re: C23 thoughts and opinions

On 01/06/2024 12:24, bart wrote:

On 01/06/2024 02:25, Scott Lurndal wrote:

bart <bc@freeuk.com> writes:

 

Little of this seems to work, sorry. You guys keep saying, do this, do
that, no do it that way, go RTFM, but nobody has shown a complete
program that correctly shows the -size symbol to be giving anything
meaningful.

>
But something is not right. I've also asked what is the point of the
-size symbol if you can just do -end - -start, but nobody has explained.

>
$ cat /tmp/m.c
#include <stdio.h>
#include <stdint.h>
>
extern uint64_t _binary_main_cpp_size;
extern uint8_t *_binary_main_cpp_start;
extern uint8_t *_binary_main_cpp_end;
>
int main()
{
     printf("%p\n", &_binary_main_cpp_size);
     printf("%p\n", &_binary_main_cpp_start);
     printf("%p\n", &_binary_main_cpp_end);
     return 0;
}
$ objcopy -I binary -B i386 -O elf64-x86-64 main.cpp /tmp/test.o
$ cc -o /tmp/m /tmp/m.c /tmp/test.o
$ /tmp/m
0x30e2
0x601034
0x604116
$ nm /tmp/m | grep _binary_main
0000000000604116 D _binary_main_cpp_end
00000000000030e2 A _binary_main_cpp_size
0000000000601034 D _binary_main_cpp_start

When I tried it on my Linux system, I get an error "relocation R_X86_64_PC32 against absolute symbol `_binary_main_cpp_size' in section `.text' is disallowed".  This is, I think, the correct response - from C you do not have direct access to absolute linker symbols.  There is no space allocated for it in the executable, and it only exists as a constant in the link stage.  Without an allocated address, declaring it as "extern" makes no sense.  It makes even less sense to try to look at the /address/ of the symbol and think that it holds useful information. It's not often I say this, but I think Scott has got things muddled here.
The only workable use of the symbol "_binary_main_cpp_size" would be in a linker script.  If you want the size of the binary blob in your code, use the obvious solution :
size_t size = &_binary_main_cpp_end - &_binary_main_cpp_start;
Sometimes it would be useful to have the size of the blob as a constant at compile time - such as for declaring another array of the same size, or using static assertions to check the size.  You can't do that with the objcopy blob inclusion.  But you /can/ do it using "xxd -i" (or similar scripts), or with #embed.
I am at a loss to see any advantages of the objcopy method in practical use for blob embedding.