Trying to learn NASM

show.nasm

global _main
extern _printf

section .data
    display: db "Enter a string: "
    display_length: equ $ - display
    out_str: db "Length: "
    out_str_length equ $ - out_str
    newline: db 10
    fmt: db "%d", 0

section .bss
    in_word: resw 40
    fword: resb 100
    w_len: resb 100


section .text


_main:
    %define SYS_WRITE 0x2000004
    %define SYS_READ 0x2000003
    %define SYS_EXIT 0x2000001


    mov rax, SYS_WRITE
    mov rdi, 1
    mov rsi, display
    mov rdx, display_length
    syscall

    mov rax, SYS_READ
    mov rdi, 0
    mov rsi, in_word
    mov rdx, 40
    syscall

    mov rax, SYS_WRITE
    mov rdi, 1
    mov rsi, out_str
    mov rdx, out_str_length
    syscall

    ; everything up until this point works

    mov rdi, in_word ; set rdi to first char in the input word
    call strlen


    mov rsi, rax ; copy rax into rsi (AKA second parameter)
    mov rdi, fmt ; load fmt into rdi (first parameter)
    xor rax, rax ; clear rax
    sub rsp, 8
    call _printf ; call printf
    add rsp, 8
    ; somewhere in printf the program segfaults, LLDB gives this info:


;     Process 44720 stopped
;     * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
;         frame #0: 0x00007ff819804a37 libsystem_c.dylib`__vfprintf + 53
;     libsystem_c.dylib`__vfprintf:
; ->  0x7ff819804a37 <+53>: movdqa %xmm0, -0x180(%rbp)
;     0x7ff819804a3f <+61>: movq   0x10(%rcx), %rcx
;     0x7ff819804a43 <+65>: movq   %rcx, -0x170(%rbp)
;     0x7ff819804a4a <+72>: leaq   -0x290(%rbp), %rcx



    ; mov rax, SYS_WRITE ; write newline
    ; mov rdi, 1
    ; mov rsi, newline
    ; mov rdx, 1
    ; syscall


    mov rax, SYS_EXIT ; exit
    xor rdi, rdi
    syscall


strlen:
    xor rax, rax ; set rax to 0
    mov rax, 0 ; idk if this is necessary, but I wanted to be sure *inc* worked

strlen_step:
    cmp byte [rdi], 0 ; check if byte is null terminator
    je strlen_end ; if yes, jump to end of loop
    inc rax ; else, increment the count
    inc rdi ; also increment the character of the string
    jmp strlen_step ; restart loop

strlen_end:
    xor rdi, rdi ; clear rdi register as it's no longer needed
    dec rax
    ret ; return the value of rax

Just found out you can set lldb to use intel flavored syntax so here's that:

Process 45111 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x00007ff819804a37 libsystem_c.dylib`__vfprintf + 53
libsystem_c.dylib`__vfprintf:
->  0x7ff819804a37 <+53>: movdqa xmmword ptr [rbp - 0x180], xmm0
    0x7ff819804a3f <+61>: mov    rcx, qword ptr [rcx + 0x10]
    0x7ff819804a43 <+65>: mov    qword ptr [rbp - 0x170], rcx
    0x7ff819804a4a <+72>: lea    rcx, [rbp - 0x290]
Target 0: (show) stopped.```