diff --git a/src/ft_calloc.s b/src/ft_calloc.s
index 958f917..e4dab1a 100644
--- a/src/ft_calloc.s
+++ b/src/ft_calloc.s
@@ -12,10 +12,14 @@ section .text
 		mov rdi, rbx
 		call malloc wrt ..plt 
 
+		cmp rax, 0
+		je end
+
 		mov rdi, rax
 		mov rsi, rbx
 		call bzero wrt ..plt
 
-		pop rbx
-		ret
+		end:
+			pop rbx
+			ret
 
diff --git a/src/ft_strcmp.s b/src/ft_strcmp.s
index 94202c9..308ed2d 100644
--- a/src/ft_strcmp.s
+++ b/src/ft_strcmp.s
@@ -6,8 +6,8 @@ section .text
 	global ft_strcmp
 	; rdi = s1, rsi = s2
 	ft_strcmp:
+		mov rax, -1 ; counter
 
-		mov rax, -1
 		loop:
 			inc rax
 			mov cl, [rdi + rax] ; cl = s1[rax]
diff --git a/src/ft_strcpy.s b/src/ft_strcpy.s
index 4740275..ae99d85 100644
--- a/src/ft_strcpy.s
+++ b/src/ft_strcpy.s
@@ -7,11 +7,10 @@ section .text
 	; rdi = dst, rsi = src
 	ft_strcpy:
 		mov rax, 0 ; counter
-		mov rcx, 0
 		mov rdx, 0
 
 		loop:
-			mov dl, [rsi + rax]
+			mov dl, [rsi + rax] ; get char from src
 			mov [rdi + rax], dl ; replace char by char
 
 			inc rax
diff --git a/src/ft_strdup.s b/src/ft_strdup.s
index 6735132..be42e60 100644
--- a/src/ft_strdup.s
+++ b/src/ft_strdup.s
@@ -14,9 +14,13 @@ section .text
 		inc rdi
 		call malloc wrt ..plt
 
+		cmp rax, 0
+		je end
+
 		mov rdi, rax ; put malloc result in dest
 		mov rsi, rbx ; put s in src
 		call ft_strcpy
 
-		pop rbx
-		ret
+		end:
+			pop rbx
+			ret
diff --git a/src/ft_strlen.s b/src/ft_strlen.s
index fa8cf39..ba5b230 100644
--- a/src/ft_strlen.s
+++ b/src/ft_strlen.s
@@ -2,18 +2,18 @@ bits 64
 
 section .text
 	global ft_strlen
+	; rdi = str
 	ft_strlen:
 		; code
-		mov rbx, rdi
-		mov rsi, -1
+		mov rcx, -1 ; counter
 
 		loop:
-			inc rsi
-			mov al, [rbx + rsi]
+			inc rcx
+			mov al, [rdi + rcx]
 
 			cmp al, 0
 			jne loop
 
 		; return
-		mov rax, rsi
+		mov rax, rcx
 		ret