implementing Calling External C Functions from x86 Assembly Using NASM and GCC
I'm working on a project and hit a roadblock. I’m trying to call a C function from my x86 assembly code using NASM and GCC, but I'm running into issues with the parameters not being passed correctly. My assembly code is supposed to call a simple C function that takes an integer and returns its square. Here’s the relevant assembly snippet: ```nasm section .data num db 5 section .text extern square global _start _start: ; Load the number into eax movzx rdi, byte [num] ; Zero-extend the byte to 64 bits as C expects an int call square ; Exit the program mov rax, 60 ; syscall: exit xor rdi, rdi ; status: 0 syscall ``` And my C function, defined in a separate file, looks like this: ```c #include <stdio.h> int square(int x) { return x * x; } ``` I compile the C code with `gcc -c square.c -o square.o` and link it with my assembly code using `nasm -f elf64 -o main.o main.asm && gcc -o program main.o square.o`. However, when I run the program, it doesn’t seem to return the expected value. Instead, using `gdb`, I noticed that the return value is not being stored correctly, and I end up with garbage values in `rax`. I’ve tried adjusting the way I pass parameters by using different registers, but I keep running into issues. Are there specific calling conventions I need to adhere to when interfacing between assembly and C? How can I ensure that the parameters are passed correctly and that I get the right return value from the C function? Any ideas what could be causing this? Cheers for any assistance!