section .data
    file_error_msg db "Could not open file: "
    file_error_msg_len equ $ - file_error_msg


section .text
global fopen_read
    
extern strlen

extern eprint_str
extern panic
extern error_to_str
extern eprint_error

;; Opens file for reading:
;; rdi: pointer to filename (null-terminated)
fopen_read:
    mov rax, 2           ; syscall: open
    mov rsi, 0           ; flags: O_RDONLY
    mov rdx, 0           ; mode
    syscall
    cmp rax, 0
    jl .file_error
    ret                         ;fd in rax

.file_error:
    push rdi
    mov rdi, rax
    call eprint_error

    lea rdi, [rel file_error_msg]
    mov rsi, file_error_msg_len
    call eprint_str

    pop rdi
    call strlen                 ; get length of filename
    mov rsi, rax                 ; r9 = filename length
    call eprint_str

    mov rdi, 10
    push rdi
    mov rdi, rsp
    mov rsi, 1
    call eprint_str
    pop rdi

    call panic