asm-cat/cat.s

;; -*- mode: nasm -*-
;; Select the one that matches your os
%include "linux.s"
;%include "freebsd.s"

%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