; Returns length in rax
; Errors (in rax):
; -1024 - line too long
; other negative - error 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
    jge readline__overflow
    mov r11b, [rsi + r10]
    add r10, 1
    cmp r11b, 0x0a  ; '\n'
    jne readline__scan

readline__return:
    mov rax, r10
    ret

readline__overflow:
    mov rax, -1024
    ret