Re: Variables assigned to ram are increasing elf file size (=> occupying flash)

[Dan Kegel <dank@kegel.com>]

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If you're doing embedded development, you may need to go under the hood a
bit.  It's hard to say without knowing your tool chain and without a
minimal test case.

https://interrupt.memfault.com/blog/how-to-write-linker-scripts-for-firmware
may be illuminating...

Good luck!
- Dan

Rachel Sapir <rachel@akribis-sys.com> schrieb am Mi., 2. Sept. 2020, 02:44:

> Hello Dan!
>
>
>
> Thank you very much for your reply.
>
>
>
> However, I am still confused.
>
>
>
> When I look at the size of my elf file it looks like this:
>
>
>
>    text       data        bss        dec        hex    filename
>
> 638774       6184    4249936    4894894    4ab0ae    MAS02.elf
>
>
>
> So, indeed the bss section is the largest. If I look at the size of the
> elf file it is 7,789,044 bytes, which I guess includes not only the
> sections, but also some other parts of the format. Still, more than 10
> times larger than text + data.
>
>
>
> Now I take one array, that is just a global long variable and change its
> size from 1001 to 100001. This means adding 99000 * 4 = 396000 bytes to bss.
>
>
>
> Now the size looks like this:
>
>
>
>    text       data        bss        dec        hex    filename
>
> 638774       6184    5298512    5943470    5ab0ae    MAS02.elf
>
>
>
> The bss section is 1048576 larger. This is the first surprise to me, since
> it is 2.5 times what I would expect. Not even a round factor that I could
> try to explain.
>
>
>
> The size of the file on the disk is now 7,789,044 bytes. So the size of
> the elf looks like it was not affected by this change.
>
>
>
> When I change the size of several very large arrays, the size file looks
> like this:
>
>
>
>    text       data        bss        dec        hex    filename
>
> 638710       6184    8444240    9089134    8ab06e    MAS02.elf
>
>
>
> Again, only the bss is larger.
>
>
>
> But the size of the elf file is: 11,983,348 bytes. It is significantly
> larger.
>
>
>
> Do you have any idea why?
>
>
>
> Now I tried another thing. I thought this may be related to the fact that
> the large array I am increasing is a two dimensional array (I know, I’m
> grasping at straws and theorizing in crazy ways). I increased another two
> dimensional array, by another random size change, and my file size looked
> like this:
>
>
>
>    text       data        bss        dec        hex    filename
>
> 638710       6184    5298512    5943406    5ab06e    MAS02.elf
>
>
>
> The bss is just the same, to the byte, like the time I changed the one
> dimension array. But now the size of the elf file is 11,983,348 bytes,
> Exactly the same as when changing several other two dimensional arrays.
> This is very very weird. And I swear I am refreshing files and folders and
> looking at the time they were modified.
>
>
>
> Even if you can't help, at least writing it down may help. (but of course
> help will be better 😊)
>
>
>
> Thanks again,
>
> Rachel
>
>
>
>
>
>
>
>
>
>
>
> *From:* Dan Kegel <dank@kegel.com>
> *Sent:* Tuesday, September 1, 2020 6:42 PM
> *To:* Rachel Sapir‏ <rachel@agito.co.il>
> *Cc:* gcc-help‏ <gcc-help@gcc.gnu.org>; Rachel Sapir‏ <
> rachel@akribis-sys.com>
> *Subject:* Re: Variables assigned to ram are increasing elf file size (=>
> occupying flash)
>
>
>
> Unless they're initialized, the arrays should be in BSS, which shouldn't
> be in the elf nor stored to flash (since it's all zero, and should be
> cleared on startup).
>
>
>
> Since you didn't include a test case, here's a tiny example:
>
>
>
> dank@thinky:~$ cat foo.c
> #include <stdio.h>
> #include <stdlib.h>
> int blarg[500000];
>
> int main(int argc, char **argv)
> {
>    int i = atoi(argv[1]);
>    int j = atoi(argv[2]);
>    blarg[i] = 7;
>    printf("blarg[%d] == %d\n", j, j);
>    return 0;
> }
> dank@thinky:~$ gcc -O2 foo.c
> dank@thinky:~$ size a.out
>    text   data    bss    dec    hex filename
>    1783    608 2000032 2002423 1e8df7 a.out
> dank@thinky:~$ ls -l a.out
> -rwxrwxr-x 1 dank dank 16768 Sep  1 08:39 a.out
>
>
>
> Note the small size of a.out.
>
> - Dan
>
>
>
> On Tue, Sep 1, 2020 at 3:21 AM Rachel Sapir <rachel@agito.co.il> wrote:
>
> Hello,
>
> I noticed that when I increase array sizes (I have some very big arrays)
> the .elf file is increased by the same size. Since the elf file is saved to
> the flash, it occupies flash area unnecessarily.
>
> Is there a way to prevent this from happening?
>
> Best regards,
> Rachel
>
>

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