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From: jak <nospam@please.ty>
Newsgroups: comp.lang.c
Subject: Re: C23 thoughts and opinions
Date: Sat, 1 Jun 2024 13:59:32 +0200
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bart ha scritto:
> On 01/06/2024 02:37, jak wrote:
>> bart ha scritto:
>>> I can see that the first two can be subtracted to give the sizes of 
>>> the data, which is 70 or 0x46. 0x46 is the last byte of the address 
>>> of _size, so what's happening there? What's with the crap in bits 16-47?
>>>
>>> I can extract the size using:
>>>
>>>     printf("%d\n", (unsigned short)&_binary_hello_c_size);
>>>
>>> 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.
>>
>>      typedef unsigned char uchar;
>>      extern uchar _binary_hello_c_size[];
>>      long hello_c_size = _binary_hello_c_size - (uchar *)0;
> 
> What result for the size did you get when you ran this?
> 
> It seems people are just guessing what might be the right code and 
> posting random fragments!
> 

I wrote it that way precisely because I believed it was the clearest
way. With the extern you can retrive the relative values ​​that in the
case of _start and _end correspond to the initial and final address of
the object, in fact you can get the length of the object by subtracting
the starting address from the final one:

extern char _binary_hello_c_start[];
extern char _binary_hello_c_end[];

long len = _binary_hello_c_end - _binary_hello_c_start;

Unfortunately, _size is provided in the same way as _start and _end 
addresses, then, since it is not an address but a length and in C:
Address +/- Value = Address
Address +/- Address = Value
so, to retrive this length that in the program it is seen as an address 
it is sufficient to subtract the starting address which in the case of a 
length is zero.

extern char _binary_hello_c_size[];

long len = _binary_hello_c_size - (char *)0;

surely you can also recover the value with a cast:

long len = (long)_binary_hello_c_size;

but the example I sent you had seemed more explanatory while the cast
seems to me a blow of hoe.
Here nobody invents anything. I'm sorry you think this.
/*
  *  example:
  *  file to embed:
  *  --- start file.txt ---
  *  line number  1
  *  line number  2
  *  line number  3
  *  line number  4
  *  line number  5
  *  line number  6
  *  line number  7
  *  line number  8
  *  line number  9
  *  line number 10
  *  line number 11
  *  line number 12
  *  line number 13
  *  line number 14
  *  line number 15
  *  line number 16
  *  line number 17
  *  line number 18
  *  line number 19
  *  line number 20
  *  --- end file.txt ---
  *  objcopy --input-target binary --output-target pe-x86-64 
--binary-architecture i386 file.txt file.txt.o
  *  gcc embed.c file.txt.o -o embed
  */

#include <stdio.h>

int main()
{
     typedef unsigned char uchar;
     extern uchar _binary_file_txt_start[];
     extern uchar _binary_file_txt_size[];
     long file_txt_size = _binary_file_txt_size - (uchar *)0;

     for(long i = 0; i < file_txt_size; i++)
         putchar(_binary_file_txt_start[i]);

     return 0;
}
output: show file.txt content