I am facing a trivial prblem.
I am doing an ldr r0, _buff in arm assembly, where _buff is defined in a c file. _buff is not static.
How should I define external linkage in assembly file (similar to extern in C) is it required or is there anything which I am missing.
You don't need to. If the symbol can't be found in the source file, it'll be assumed to be defined elsewhere.
It's not necessary to do that. If *_buff* has been defined as global in the C file, you can compile and build the files together:
arm-none-gnuaebi-gcc -o output assembly.s cfile.c
You can also compile them separately, it's also going to work. But _buff does not exist, you will get a link error.
