trying not to segment

lang/src/main.asm

@@ -7,6 +7,10 @@ section .data
     hello_msg_len equ $ - hello_msg
     die_msg db "panic occured!", 10
     die_msg_len equ $ - die_msg
+    file_error_msg db "Could not open file: "
+    file_error_msg_len equ $ - file_error_msg
+    error_msg db "Error: "
+    error_msg_len equ $ - error_msg
 
 section .text
 global _start
@@ -21,10 +25,15 @@ _start:
     mov rax, [rsp]        ; argc
     cmp rax, 1
     jle .no_filename      ; if argc <= 1, no filename provided
+    ; get argv[1]
+    mov rax, [rsp + 16]   ; argv[1]
+
+    ; open file for reading
+    mov rdx, rax          ; filename pointer
+    call fopen_read
     mov rax, 42
     ret
 
-
 .no_filename:
     call die
 
@@ -38,6 +47,13 @@ die:
     syscall
     ret                         ; should never reach here
 
+;; ==============================
+;; Helper functions
+;; ==============================
+
+;; Writes a string to stderr:
+;; rdx: pointer to string
+;; rcx: length of string
 eprint_str:                      ; print string-length pair to stderr
     ; str pointer in rdx
     ; length in rcx
@@ -48,4 +64,81 @@ eprint_str:                      ; print string-length pair to stderr
     syscall
     ret
 
+;; calculates length of null-terminated string
+;; rdx: pointer to string
+strlen:
+    xor rax, rax         ; length counter
+.strlen_loop:
+    cmp byte [rdx + rax], 0
+    je .strlen_done
+    inc rax
+    jmp .strlen_loop
+.strlen_done:
+    ret
+
+;; Copy bytes from one memory location to another
+;; rdx: destination pointer
+;; rcx: source pointer
+;; r8: number of bytes to copy
+memcpy:
+    xor r10, r10 
+.memcpy_loop_byte:
+    cmp r10, r8
+    jge .memcpy_done
+    mov al, [rcx + r10]
+    mov [rdx + r10], al
+    inc r10 
+    jmp .memcpy_loop_byte
+.memcpy_done:
+    ret
+
+;; Opens file for reading:
+    ;; rdx: pointer to filename (null-terminated)
+fopen_read:
+    mov rax, 2           ; syscall: open
+    mov rdi, rdx         ; filename
+    mov rsi, 0           ; flags: O_RDONLY
+    mov rdx, 0           ; mode
+    syscall
+    cmp rax, 0
+    jl .file_error
+    ret                         ;fd in rax
+.file_error:
+    ; allocate stack space for error message
+    mov rdx, rdi                ; filename is in rdi
+    call strlen                 ; get length of filename
+    mov r9, rax
+    mov rsi, r9
+    add rsi, file_error_msg_len ; add length of error message prefix
+    add rsi, 1                  ; for new line
+    ; align to 16 bytes
+    add rsi, 15
+    and rsi, -16                
+    sub rsp, rsi ; alloca
+    push rsi ; save length for dealloca
+    ; copy file_error_msg
+    lea rdx, [rsp + 8]
+    mov rcx, file_error_msg
+    mov r8, file_error_msg_len
+    call memcpy
+    ; copy filename
+    lea rdx, [rsp + 8 + file_error_msg_len]
+    mov rcx, rdi
+    mov r8, r9
+    call memcpy
+    ; trailing newline
+    lea rdx, [rsp + 8 + file_error_msg_len + r9]
+    mov byte [rdx], 10
+    ; print error message
+    lea rdx, [rsp + 8]
+    mov rcx, r9
+    add rcx, file_error_msg_len
+    add rcx, 1
+    call eprint_str
+    pop rsi
+    add rsp, rsi ; dealloca
+    ret
+    
+
+