asm-cat/cat.s

%define SYS_open 0x2
%define SYS_close 0x3
%define SYS_write 0x1
%define SYS_read 0x0
%define SYS_lseek 0x8
%define SYS_exit 0x3c

%define O_RDONLY 0x0
%define SEEK_SET 0x0
%define FD_stderr 2
%define FD_stdout 1
%define FD_stdin 0

%define BUFFER_SIZE 2048

section .text
global _start
_start:
    pop rax                     ; argc
    pop QWORD [exec_name]       ; argv[0]

check_arg_loop:
    mov rdi,[rsp]               ; argv[i]
    ; check for null
    cmp rdi,0
    je print_stdin_and_exit

    ; check for flags
    call check_flag
    cmp rax,0
    je print_files              ; no more flags

    pop rdi                     ; remove argv[i] (a flag)
    jmp check_arg_loop

print_files:
    mov rdi,[rsp]               ; argv[i]
    cmp rdi,0
    je print_files_end

    ; check if argv[i] is '-'
    mov rsi,dash_str
    call streql
    cmp rax,0
    jne print_files_stdin

    ; open file
    mov rdi,[rsp]               ; argv[i]
    call open_file

    ; print file
    mov rdi,rax
    pop rsi                     ; argv[i]
    call print_file

    jmp print_files

print_files_stdin:
    pop rdi                     ; argv[i]
    mov rdi,FD_stdin
    mov rsi,stdin_name
    call print_file

    ; rewind stdin
    mov rax,SYS_lseek
    mov rdi,FD_stdin
    mov rsi,0
    mov rdx,SEEK_SET
    syscall

    jmp print_files

print_files_end:
    xor rdi,rdi
    jmp exit

open_file:                      ; int open_file(const char *path)
    push rdi                    ; path

    ; open file
    mov rax,SYS_open
    ; rdi already set
    mov rsi,O_RDONLY
    syscall

    ; check return
    cmp rax,0
    jl open_error

    pop rdi                     ; path
    ret
open_error:
    call start_error

    ; message
    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,open_error_text
    mov rdx,open_error_text_len
    syscall

    ; detail
    mov rdi,[rsp]               ; path
    call strlen

    mov rdx,rax
    mov rax,SYS_write
    mov rdi,FD_stderr
    pop rsi                     ; path
    syscall

    call end_error

    mov rdi,1
    jmp exit

print_stdin_and_exit:
    mov rdi,FD_stdin
    mov rsi,stdin_name
    call print_file
    xor rdi,rdi
    jmp exit

print_file:                     ; void print_file(int fd, const char *name)
    push rdi                    ; fd
    push rsi                    ; name
print_file_loop:
    mov rax,SYS_read
    mov rdi,[rsp + 8]           ; fd
    mov rsi,read_buff
    mov rdx,BUFFER_SIZE
    syscall

    ; check error
    cmp rax,0
    jl read_file_error

    ; no more bytes
    je print_file_end

    ; print message
    mov rdx,rax
    mov rax,SYS_write
    mov rdi,FD_stdout
    mov rsi,read_buff
    syscall

    jmp print_file_loop

print_file_end:
    pop rsi                     ; name
    pop rdi                     ; fd
    ret

read_file_error:
    call start_error

    ; message
    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,read_error_text
    mov rdx,read_error_text_len
    syscall

    ; detail
    mov rdi,[rsp]               ; name
    call strlen

    mov rdx,rax
    mov rax,SYS_write
    mov rdi,FD_stderr
    pop rsi                     ; name
    syscall

    call end_error

    mov rdi,1
    jmp exit

;; check arg for -u, exit if unknown flag
;; return 1 if found, 0 otherwise
check_flag:                     ; bool check_arg(const char *arg)
    xor rax,rax
    ; check if arg[0] == '-'
    cmp BYTE [rdi],'-'
    jne check_flag_end

    inc rdi
    ; rax still set
check_flag_loop:
    ; check null byte
    mov cl,[rdi]
    cmp cl,0
    je check_flag_end

    ; check for u
    cmp cl,'u'
    jne check_flag_error

    mov rax,1
    inc rdi
    jmp check_flag_loop
check_flag_error:
    push rdi                    ; arg + i
    call start_error

    ; message
    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,arg_error_text
    mov rdx,arg_error_text_len
    syscall

    ; character
    mov rax,SYS_write
    mov rdi,FD_stderr
    pop rsi                     ; arg + i
    mov rdx,1
    syscall

    call end_error

    mov rdi,1
    jmp exit
check_flag_end:
    ret

;; print exec_name and a colon_space
start_error:                    ; void start_error(void)
    mov rdi,[exec_name]
    call strlen
    mov rdx,rax
    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,[exec_name]
    syscall

    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,colon_space
    mov rdx,colon_space_len
    syscall
    ret

;; print newline
end_error:                      ; void end_error(void)
    mov rax,SYS_write
    mov rdi,FD_stderr
    mov rsi,newline_ptr
    mov rdx,1
    syscall
    ret

streql:                         ; bool streql(const char *s1, const char *s2)
    ; check if both are zero
    mov cl,[rdi]                ; s1[i]
    or cl,[rsi]
    jz streql_true

    ; check if equal
    mov cl,[rdi]
    xor cl,[rsi]
    jnz streql_false

    inc rdi
    inc rsi
    jmp streql
streql_true:
    mov rax,1
    ret
streql_false:
    xor rax,rax
    ret

exit:                           ; NORETURN void exit(int status)
    mov rax,SYS_exit
    syscall

strlen:                         ; uint64 strlen(const char *str)
    mov rax,rdi
strlen_loop:
    cmp BYTE [rax],0
    je strlen_end
    inc rax
    jmp strlen_loop
strlen_end:
    sub rax,rdi
    ret

section .bss
    exec_name resq 1
    read_buff resb BUFFER_SIZE

section .data
    dash_str db "-",0x0
    colon_space db ": "
    colon_space_len equ $ - colon_space
    newline_ptr db 0xA
    stdin_name db "<stdin>",0x0
    arg_error_text db "unknown flag: "
    arg_error_text_len equ $ - arg_error_text
    read_error_text db "read error: "
    read_error_text_len equ $ - read_error_text
    open_error_text db "could not open file for reading: "
    open_error_text_len equ $ - open_error_text