c library tutorial added

x86_64/6_clib.asm

@@ -0,0 +1,72 @@
+; We need to tell it about main when using c lib functions
+; else it will complain about it during linking
+global main
+
+; c library symbols for the compiler (linker doesn't care)
+extern printf
+extern atoi
+
+section .data
+	; Strings to printf'
+	fmt: db '%d + %d = %d',0xA,0x0 ; append with newline and null terminator
+	usageMsg: db 'usage: %s first_number second_number',0xA,0x0
+
+section .text
+; Note that this will be called by some external function (actually,
+; the compiler will generate its own _start that eventually calls this)
+main:
+	push rbp
+	mov rbp, rsp
+	push rbx
+	push r12
+	push r13
+	push r14
+	push r15
+
+	; main takes an int and a char pointer as arguments
+	mov r12, rdi
+	mov r13, rsi
+
+	; This segfaults when moved below the line "jne .fail"
+	; Why?
+	sub rsp, 8 ; allocate 8 bytes = 2 integers
+
+	cmp r12, 3 ; argc == 3
+	jne .fail
+
+	mov rdi, qword [r13 + 8] ; argv[1]
+	call atoi
+	mov dword [rsp], eax ; store integer 1
+
+	mov rdi, qword [r13 + 16] ; argv[2]
+	call atoi
+	mov dword [rsp + 4], eax ; store integer 2
+
+	; add the two numbers
+	mov ecx, dword [rsp]
+	add ecx, dword [rsp + 4]
+
+	; print the result
+	mov rdi, fmt
+	mov esi, [rsp]
+	mov edx, [rsp + 4]
+	call printf
+
+	; main returns an integer
+	xor rax, rax
+	jmp .end
+.fail:
+	mov rdi, usageMsg
+	mov rsi, [r13] ; argv[0] = the executable name
+	call printf
+	mov rax, 1
+.end:
+	pop r15
+	pop r14
+	pop r13
+	pop r12
+	pop rbx
+	mov rsp, rbp
+	pop rbp
+
+	ret