How to use an external C function in assembly?

Question

I am creating code that calls a C function that gets the square root of an integer. The C function is external and in my folder. When calling it, i do not know how to give it a parameter so it knows what to calculate

I am using NASM Intel x86_64, Assembler is gcc, i do not know the current edition. I am using Linux

Currently my code is:

xor rsi, rsi       ;just clears rsi
mov rsi, r15       ;r15 held the value that is supposed to be the parameter
call square

It currently just prints out a 0 as it is not taking a correct value.

How do i get it to take in the parameter?

Thank you.

Answer 1

According to the System V AMD64 ABI calling convention the first argument should be in the rdi register.

You need to copy the value to rdi instead of rsi .