echo server, multiple clients, lines <256 bytes

readline.s

@@ -1,34 +1,41 @@
-; Returns length in rax
+; Returns:
+; rax - read(2) length
+; rdi - if line is complete, line length;
+;       if line was incomplete, 0;
+;       if EOF or read(2) returned an error, undefined
 ; Errors (in rax):
 ; -1024 - line too long
 ; other negative - error from read(2)
+; 0 - EOF from read(2)
 ; Arguments:
 ; rdi - fd
 ; rsi - buffer
 ; rdx - max length
 readline:
-    push rdi
     push rsi
     push rdx
     mov rax, SYS_READ
     syscall
     pop rdx
     pop rsi
-    pop rdi
     cmp rax, 0
     jl return
     mov r10, 0
 
 readline__scan:
-    cmp r10, rax
+    cmp r10, rdx
     jge readline__overflow
+    cmp r10, rax
+    jge readline__incomplete_line
     mov r11b, [rsi + r10]
     add r10, 1
     cmp r11b, 0x0a  ; '\n'
     jne readline__scan
+    mov rdi, r10
+    ret
 
-readline__return:
-    mov rax, r10
+readline__incomplete_line:
+    mov rdi, 0
     ret
 
 readline__overflow: