Linking C with NASM

I have a NASM file and a C file. How do I call a function in the C file from the NASM file? How do I call a NASM function from the C file?

Many Thanks
DD

Calling assembly function from C:

C file:

#include <stdio.h>

int add(int a, int b);

int main(int argc, char **argv)
{
printf("%d\n", add(2, 6));
return 0;
}

assembly file:

global add

section .data

section .text

add:
mov eax, [esp+4] ; argument 1
add eax, [esp+8] ; argument 2
ret

compiling:

$ nasm -f elf add.asm
$ gcc -Wall main.c add.o
$ ./a.out
8
$

Calling C function from assembly:

C file:

int add(int a, int b)
{
return a + b;
}

assembly file:

extern add
extern printf
extern exit

global _start

section .data
format db "%d", 10, 0
section .text

_start:
push 6
push 2
call add ; add(2, 6)

push eax
push format
call printf ; printf(format, eax)

push 0
call exit ; exit(0)

compiling:

$ gcc -Wall -c add.c
$ nasm -f elf main.asm
$ ld main.o add.o -lc -I /lib/ld-linux.so.2
$ ./a.out
8
$