Calling a c function from assembly example.

hello.c

int hello() {
    return 123;
}

main.asm

extern hello
extern printf
extern exit

global _start

section .data
fmt: db "%d", 10, 0
section .text

_start:
    call  hello     ; hello()

    mov   rsi,rax
    mov   rdi,fmt
    call  printf  ; printf(format, rsi)

    push  0
    call exit     ; exit(0)

Makefile

all:
    gcc -Wall -c hello.c
    nasm -f elf64 main.asm
    ld main.o hello.o -lc -I /lib64/ld-linux-x86-64.so.2

clean:
    rm a.out hello.o main.o