Hi,
Is there any way to use --gc-sections and still keep a specific section?
I have an "application info" section located at a specific address in flash, which is modified with serial#, etc, before uploading the fw to the device. The fw references the array the section consists of, but still the linker throws it away when using --gc-sections. any ideas?
JohanEkdahl 
I think (but am not sure) you need to tinker a bit with the linker script:
thanks, johan... i'll try that... i still thinks it's weird that it gets thrown out when the code references data in there, but maybe that's because it has a special name in order to be able to relocate it.
JohanEkdahl I pointed to KEEP as I seem to recall this having been mentioned here before. I'm not sure, though. Linker scripts is something I still haven't got firmly under my belt...
Let us know how it goes!
I have an "application info" section located at a specific address in flash, which is modified with serial#, etc, before uploading the fw to the device. The fw references the array the section consists of, but still the linker throws it away when using --gc-sections. any ideas?
"fw" is the firmware in question (the "hex" file so to speak)..
when --gc-sections is used, the mentioned section is not in the hex file anymore (or possibly not at the address it's supposed to, but that's unlikely).
there is a function in the code that references the array stored in the section.. i guess the function just reads whatever is present in flash at the address where the section is supposed to be located..
clawson 
BUT gc-sections is a TWO part operation. You build the code files with -function-sections which puts each function into a separately named section (so not .text) then when linked gc-sections is used to "garbage collect unused sections" - in other words don't link in any function that is never referenced. If you want to over-ride this then build the .c file that you don't want discarded (could be a singe function) without function-sections then gc-sections won't touch it as it's already in .text
The .map fie should confirm which sections were discarded and which were linked - trawling about in the .hex is NOT the way to check. Also check the .sym file which is basically avr-nm output of the .elf - look for " T " entries.
Cliff
right.. i know it's a 2-part operation, i just wanted to know why a (customly named by me) section containing referenced data is thrown out, but sections customly named by the compiler aren't (the "function" sections). But I guess the sections named by the compiler has some part of their names matched by a KEEP wildcard as mentioned by Johan above.
I'm not trawling the hex file for sections.. We are using a custom piece of software to update the flash over an rs485 interface using a bootloader in the avr. this piece of software manipulates part of the section in question directly in the hex data (to set serial#, etc) before upload.
cheers..
"fw" is the firmware in question (the "hex" file so to speak)..
when --gc-sections is used, the mentioned section is not in the hex file anymore (or possibly not at the address it's supposed to, but that's unlikely).
there is a function in the code that references the array stored in the section.. i guess the function just reads whatever is present in flash at the address where the section is supposed to be located..
IIRC --gc-section keeps .text,
KEEPed sections and any sections they reference,
directly or indirectly.
Does your function qualify?
I figured out what's going on..
Some of the code was copied from an older project (without --gc-sections) from another dev, and he specified the relocation address of the section as a constant, hence the mentioned array was in fact not referenced anywhere..
sorry for the rant...
