x86_64 nasm read syscall isn t null-terminated

Two changes had to be made: Firstly, I needed to fix the strlen function. (which I have renamed to _strlen) The issue was that I forgot to set rcx to zero before I began the counting, Lastly, directly after we do the syscall in stdin we then add a null-byte at the end of the buffer. (mov byte [rsi+rax-1], 0)

;##################################################################################################################################
;
; hello.asm
;
; 15/09/2023
;
; Logan Seeley
;
; Compiling:
;   Create object file:     $ nasm -f elf64 hello.asm
;   Link object file:       $ ld -m elf_x86_64 hello.o -o hello
;   Execute file:           $ ./hello
;   Print exit code:        $ printf "
[Process finished with exit code $?]
"
;
;   All in one command:
;   $ nasm -f elf64 hello.asm; ld -m elf_x86_64 hello.o -o hello; ./hello; printf "

[Process finished with exit code $?]
"
;
;   TODO:  Use call-preserved registers RBX, RBP, RSP, and R12-R15 for functions instead of using RDI, RSI, RDX, RCX, R8, R9
;           RBX, RBP and RSP should be used for args
;           R12-15 should be used for return values
;
;##################################################################################################################################

            global    _start

            section     .data
prompt:     db          "What is your name?", 10, 0     ; db: Define byte (8 bits)
                                                        ; Declare prompt to be "What is your name?"
                                                        ; 10 = "
" (newline character)
                                                        ; 0  = null-byte (for zero-termination)
hello:     db         "Hello, ", 0

bufsize     equ         2048
rl:         resb        bufsize

            section     .text
_start:
            mov         rsi,    prompt  ; Move string into rsi to be written to STDOUT
            call        stdout          ; Write to STDOUT
            call        stdin
            mov         rsi,    hello
            call        stdout
            mov         rsi,    rl      ; Move input into rsi
            call        stdout

            jmp         ExitSuccess     ; Return with exit code 0
_strlen:
            xor         rcx,    rcx     ; Move 0 into rcx so we count up from 0
            push        rsi             ; Save string to stack
strlen_next:
            cmp         [rsi], byte 0   ; Compare char to null byte
            jz          strlen_null     ; Jump if null byte

            inc         rcx             ; Char wasn t null, increment rcx (string length)
            inc         rsi             ; Next char
            jmp         strlen_next     ; Repeat, until we get a null byte
strlen_null:
            pop         rsi             ; Load string from the stack
            ret                         ; Return to call
stdin:
            mov         rax,    0
            mov         rdi,    0
            mov         rsi,    rl
            mov         rdx,    _strlen
            syscall
            mov         byte [rsi+rax-1], 0     ; Setting last character of buffer to null-byte
                                                ; Replacing last character instead of concatting,
                                                ; because last character is a newline from user pressing enter
            ret
stdout:
            mov         rax,    1       ; syscall write
            mov         rdi,    1       ; File descriptor STDOUT
            call        _strlen         ; String length of rsi
            mov         rdx,    rcx     ; Move strlen of rsi into rdx
            syscall

            ret
ExitSuccess:
            mov         rax,    60      ; syscall exit
            mov         rdi,    0       ; Move exit code into rdi
            syscall