why 1 byte of sizeof empty class

本文探讨了Linux内核学习对于个人成长的价值,解释了如何确保多线程环境中的实时AD线程,并深入解析了类在C++中的内存占用原理,包括为何即使类为空也会分配1字节内存,以及这与内存对齐的关系。

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2011-11-03

1. studying linux kernel any meaning to my life?

2. multithreading, how to assure the real-time AD thread?

3. why 1 byte of sizeof empty class.

Yes, the compiler will generate 1 byte of memory to mark the existence of the class.  This doesn't answer WHY though.  The reason is the language standard states that all classes must have a memory size of at least 1 byte so that the class doesn't occupy the same memory space with another class.  This is to prevent name mangling.  i.e., if I declare a class A {};, the compiler will still generate an entry in its table to something called "A".  If behind that I declare another class, say class B, if A takes 0 bytes of memory, and B's data gets written in the place where A was declared.  In this case, an instantiation of A would take on the properties of B.


one byte will relate to one unique address. is this problem relative to byte allignment?

import tkinter as tk from tkinter import scrolledtext, ttk, messagebox import os import logging from datetime import datetime class SimpleCLexer: def __init__(self): self.tokens = [] def tokenize(self, input_str): tokens = [] pos = 0 line = 1 column = 0 length = len(input_str) # 定义C语言的关键词和类型 keywords = { 'void', 'int', 'char', 'float', 'double', 'short', 'long', 'signed', 'unsigned', 'struct', 'union', 'enum', 'typedef', 'static', 'extern', 'auto', 'register', 'const', 'volatile', 'return', 'if', 'else', 'switch', 'case', 'default', 'for', 'while', 'do', 'break', 'continue', 'goto', 'sizeof' } types = {'U1', 'U2', 'U4', 'S1', 'S2', 'S4'} while pos < length: char = input_str[pos] # 跳过空白字符 if char in ' \t': pos += 1 column += 1 continue # 处理换行 if char == '\n': line += 1 column = 0 pos += 1 continue # 处理单行注释 if pos + 1 < length and input_str[pos:pos+2] == '//': end = input_str.find('\n', pos) if end == -1: end = length pos = end continue # 处理多行注释 if pos + 1 < length and input_str[pos:pos+2] == '/*': end = input_str.find('*/', pos + 2) if end == -1: end = length else: end += 2 pos = end continue # 处理标识符 if char.isalpha() or char == '_': start = pos pos += 1 while pos < length and (input_str[pos].isalnum() or input_str[pos] == '_'): pos += 1 token_text = input_str[start:pos] token_type = 'IDENTIFIER' # 检查是否为关键字或类型 if token_text in keywords: token_type = 'KEYWORD' elif token_text in types: token_type = 'TYPE' tokens.append({ 'type': token_type, 'text': token_text, 'line': line, 'column': column }) column += (pos - start) continue # 处理数字 if char.isdigit(): start = pos pos += 1 while pos < length and (input_str[pos].isdigit() or input_str[pos] in '.xXabcdefABCDEF'): pos += 1 tokens.append({ 'type': 'NUMBER', 'text': input_str[start:pos], 'line': line, 'column': column }) column += (pos - start) continue # 处理字符串 if char == '"': start = pos pos += 1 while pos < length and input_str[pos] != '"': if input_str[pos] == '\\' and pos + 1 < length: pos += 2 else: pos += 1 if pos < length and input_str[pos] == '"': pos += 1 tokens.append({ 'type': 'STRING', 'text': input_str[start:pos], 'line': line, 'column': column }) column += (pos - start) continue # 处理字符 if char == "'": start = pos pos += 1 while pos < length and input_str[pos] != "'": if input_str[pos] == '\\' and pos + 1 < length: pos += 2 else: pos += 1 if pos < length and input_str[pos] == "'": pos += 1 tokens.append({ 'type': 'CHAR', 'text': input_str[start:pos], 'line': line, 'column': column }) column += (pos - start) continue # 处理运算符和标点符号 operators = { '(', ')', '{', '}', '[', ']', ';', ',', '.', '->', '++', '--', '&', '*', '+', '-', '~', '!', '/', '%', '<<', '>>', '<', '>', '<=', '>=', '==', '!=', '^', '|', '&&', '||', '?', ':', '=', '+=', '-=', '*=', '/=', '%=', '<<=', '>>=', '&=', '^=', '|=', ',' } # 尝试匹配最长的运算符 matched = False for op_len in range(3, 0, -1): if pos + op_len <= length and input_str[pos:pos+op_len] in operators: tokens.append({ 'type': 'OPERATOR', 'text': input_str[pos:pos+op_len], 'line': line, 'column': column }) pos += op_len column += op_len matched = True break if matched: continue # 无法识别的字符 tokens.append({ 'type': 'UNKNOWN', 'text': char, 'line': line, 'column': column }) pos += 1 column += 1 return tokens class FunctionAnalyzer: def __init__(self): self.function_name = "" self.parameters = set() self.local_vars = [] self.global_vars = [] self.function_calls = [] self.current_function = None self.in_function = False self.in_function_body = False self.brace_depth = 0 self.variable_declarations = {} self.control_structures = {"if", "for", "while", "switch", "return", "else"} self.macro_definitions = set() self.recorded_globals = set() self.storage_classes = {"static", "extern", "auto", "register"} # 定义允许的类型(修复错误) self.basic_types = {'void', 'int', 'char', 'float', 'double', 'short', 'long', 'signed', 'unsigned'} self.type_aliases = {"U1", "U2", "U4", "S1", "S2", "S4"} self.allowed_types = self.basic_types | self.type_aliases # 添加函数前缀识别 self.function_prefixes = { "vd_": "void", "u1_": "U1", "u2_": "U2", "u4_": "U4", "s1_": "S1", "s2_": "S2", "s4_": "S4" } def analyze(self, tokens): self.tokens = tokens self.pos = 0 self.current_line = 0 # 第一步:识别宏定义(全大写标识符) self._identify_macros() # 第二步:分析函数体 while self.pos < len(self.tokens): token = self.tokens[self.pos] self.current_line = token['line'] # 检测函数定义 if token['text'] in self.storage_classes or token['text'] in self.allowed_types: if self._is_function_definition(): self._handle_function_definition() continue # 在函数体内检测变量声明 if self.in_function_body: if token['text'] in self.allowed_types: self._handle_variable_declaration() continue # 检测函数调用 if token['type'] == 'IDENTIFIER' and self.pos + 1 < len(self.tokens): next_token = self.tokens[self.pos + 1] if next_token['text'] == '(': self._handle_function_call() continue # 检测变量使用 if token['type'] == 'IDENTIFIER': self._handle_identifier_use(token) # 跟踪大括号深度 if token['text'] == '{': self.brace_depth += 1 if self.in_function and self.brace_depth == 1: self.in_function_body = True elif token['text'] == '}': self.brace_depth -= 1 if self.brace_depth == 0 and self.in_function: self.in_function = False self.in_function_body = False self.pos += 1 return self def _identify_macros(self): """识别宏定义(全大写标识符)""" for token in self.tokens: if token['type'] == 'IDENTIFIER' and token['text'].isupper(): self.macro_definitions.add(token['text']) def _is_function_definition(self): pos = self.pos storage_class = None # 检测存储类说明符 if self.tokens[pos]['text'] in self.storage_classes: storage_class = self.tokens[pos]['text'] pos += 1 # 检测返回类型 if pos >= len(self.tokens) or self.tokens[pos]['text'] not in self.allowed_types: return False return_type = self.tokens[pos]['text'] pos += 1 # 处理指针声明 if pos < len(self.tokens) and self.tokens[pos]['text'] == '*': return_type += '*' pos += 1 # 检测函数名 if pos < len(self.tokens) and self.tokens[pos]['type'] == 'IDENTIFIER': func_name = self.tokens[pos]['text'] pos += 1 else: return False # 检测参数列表开头的'(' if pos < len(self.tokens) and self.tokens[pos]['text'] == '(': pos += 1 else: return False # 参数列表必须包含至少一个参数或void if pos < len(self.tokens) and self.tokens[pos]['text'] == ')': # 空参数列表 pos += 1 else: # 非空参数列表 depth = 1 while pos < len(self.tokens) and depth > 0: if self.tokens[pos]['text'] == '(': depth += 1 elif self.tokens[pos]['text'] == ')': depth -= 1 pos += 1 # 检测函数体开头的'{' (允许最多5个token的间隔) found_brace = False for i in range(min(5, len(self.tokens) - pos)): if self.tokens[pos + i]['text'] == '{': found_brace = True break return found_brace def _handle_function_definition(self): start_pos = self.pos storage_class = None # 处理存储类说明符 if self.tokens[self.pos]['text'] in self.storage_classes: storage_class = self.tokens[self.pos]['text'] self.pos += 1 # 获取返回类型 return_type = self.tokens[self.pos]['text'] self.pos += 1 # 处理指针声明 if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == '*': return_type += '*' self.pos += 1 # 获取函数名 if self.pos < len(self.tokens) and self.tokens[self.pos]['type'] == 'IDENTIFIER': func_name = self.tokens[self.pos]['text'] self.pos += 1 else: self.pos = start_pos return # 记录函数名 self.function_name = func_name self.current_function = func_name self.variable_declarations[func_name] = True # 跳过 '(' if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == '(': self.pos += 1 # 提取参数 params = [] current_param = [] depth = 1 param_line = self.current_line while self.pos < len(self.tokens) and depth > 0: token = self.tokens[self.pos] if token['text'] == '(': depth += 1 elif token['text'] == ')': depth -= 1 if depth == 0: break elif token['text'] == ',' and depth == 1: # 提取参数类型和名称 param_type, param_name = self._extract_param_info(current_param) if param_type and param_name: params.append({ 'type': param_type, 'name': param_name, 'line': param_line }) self.variable_declarations[param_name] = True current_param = [] param_line = token['line'] self.pos += 1 continue current_param.append(token) self.pos += 1 # 处理最后一个参数 if current_param: param_type, param_name = self._extract_param_info(current_param) if param_type and param_name: params.append({ 'type': param_type, 'name': param_name, 'line': param_line }) self.variable_declarations[param_name] = True # 记录参数 self.parameters = params param_names = [p['name'] for p in params] if params else [] # 查找函数体开头的'{' while self.pos < len(self.tokens) and self.tokens[self.pos]['text'] != '{': self.pos += 1 if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == '{': self.in_function = True self.in_function_body = False self.brace_depth = 0 self.pos += 1 return param_names def _extract_param_info(self, tokens): """从参数token列表中提取类型和名称""" param_type = [] param_name = None for token in tokens: if token['type'] in ('KEYWORD', 'TYPE') or token['text'] in self.allowed_types: param_type.append(token['text']) elif token['type'] == 'IDENTIFIER' and not token['text'].isupper(): param_name = token['text'] return ' '.join(param_type), param_name def _handle_variable_declaration(self): # 获取变量类型 var_type = self.tokens[self.pos]['text'] self.pos += 1 # 处理指针声明 if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == '*': var_type += '*' self.pos += 1 var_names = [] # 处理变量名 while self.pos < len(self.tokens): token = self.tokens[self.pos] # 标识符 - 变量名 if token['type'] == 'IDENTIFIER' and not token['text'].isupper(): var_name = token['text'] # 跳过宏定义 if var_name not in self.macro_definitions: var_names.append(var_name) self.variable_declarations[var_name] = True self.pos += 1 # 逗号 - 多个变量声明 elif token['text'] == ',': self.pos += 1 # 处理指针声明 if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == '*': self.pos += 1 continue # 结束声明 elif token['text'] == ';': self.pos += 1 break # 数组声明 - 跳过数组大小 elif token['text'] == '[': self.pos += 1 depth = 1 while self.pos < len(self.tokens) and depth > 0: t = self.tokens[self.pos] if t['text'] == '[': depth += 1 elif t['text'] == ']': depth -= 1 self.pos += 1 # 初始化 - 跳过初始化表达式 elif token['text'] == '=': self.pos += 1 depth = 0 while self.pos < len(self.tokens): t = self.tokens[self.pos] if t['text'] in {'(', '['}: depth += 1 elif t['text'] in {')', ']'}: depth -= 1 elif t['text'] in {',', ';'} and depth == 0: break self.pos += 1 # 类型转换 - 跳过 elif token['type'] == 'TYPECAST': self.pos += 1 else: self.pos += 1 break # 添加到局部变量 (跳过宏定义和参数) for var_name in var_names: if var_name in [p['name'] for p in self.parameters] or var_name in self.macro_definitions: continue self.local_vars.append({ 'type': var_type, 'name': var_name, 'line': self.current_line }) def _handle_identifier_use(self, token): var_name = token['text'] line = token['line'] # 跳过宏定义(全大写) if var_name in self.macro_definitions or var_name.isupper(): return # 跳过已声明的变量(局部变量、参数、函数名) if var_name in self.variable_declarations: return # 跳过函数调用(函数名已经在函数调用处理中记录) for call in self.function_calls: if call['name'] == var_name: return # 跳过控制结构 if var_name in self.control_structures: return # 跳过类型别名 if var_name in self.type_aliases: return # 跳过已经记录过的全局变量 if var_name in self.recorded_globals: return # 添加到全局变量 self.global_vars.append({ 'name': var_name, 'line': line }) self.recorded_globals.add(var_name) def _handle_function_call(self): # 提取函数名 func_name = self.tokens[self.pos]['text'] line = self.current_line self.pos += 2 # 跳过函数名和 '(' # 提取参数 params = [] depth = 1 current_param = [] while self.pos < len(self.tokens) and depth > 0: token = self.tokens[self.pos] if token['text'] == '(': depth += 1 elif token['text'] == ')': depth -= 1 if depth == 0: break elif token['text'] == ',' and depth == 1: params.append(''.join([t['text'] for t in current_param]).strip()) current_param = [] self.pos += 1 continue current_param.append(token) self.pos += 1 if current_param: params.append(''.join([t['text'] for t in current_param]).strip()) # 跳过 ')' 如果还在范围内 if self.pos < len(self.tokens) and self.tokens[self.pos]['text'] == ')': self.pos += 1 # 确定返回类型 return_type = "unknown" if func_name.startswith("vd_"): return_type = "void" elif func_name.startswith(("u1_", "u2_", "u4_", "s1_", "s2_", "s4_")): prefix = func_name.split("_")[0] return_type = prefix.upper() # 添加到函数调用列表 self.function_calls.append({ 'name': func_name, 'return_type': return_type, 'type': "function", 'params': ", ".join(params), 'line': line }) class FunctionParserApp: def __init__(self, root): self.root = root self.root.title("C语言函数解析器 (内置解析器版)") self.root.geometry("900x700") self.setup_logging() # 创建输入区域 input_frame = tk.LabelFrame(root, text="输入C语言函数体", padx=5, pady=5) input_frame.pack(fill="both", expand=True, padx=10, pady=5) self.input_text = scrolledtext.ScrolledText(input_frame, width=100, height=20) self.input_text.pack(fill="both", expand=True, padx=5, pady=5) # 按钮区域 btn_frame = tk.Frame(root) btn_frame.pack(fill="x", padx=10, pady=5) # 解析按钮 parse_btn = tk.Button(btn_frame, text="解析函数", command=self.parse_function, bg="#4CAF50", fg="white") parse_btn.pack(side="left", padx=5) # 保存日志按钮 save_log_btn = tk.Button(btn_frame, text="保存日志", command=self.save_logs) save_log_btn.pack(side="right", padx=5) # 进度条 self.progress = ttk.Progressbar(btn_frame, orient="horizontal", length=300, mode="determinate") self.progress.pack(side="left", padx=10, fill="x", expand=True) # 创建输出区域 output_frame = tk.LabelFrame(root, text="解析结果", padx=5, pady=5) output_frame.pack(fill="both", expand=True, padx=10, pady=5) self.output_text = scrolledtext.ScrolledText(output_frame, width=100, height=15) self.output_text.pack(fill="both", expand=True, padx=5, pady=5) self.output_text.config(state=tk.DISABLED) # 日志区域 log_frame = tk.LabelFrame(root, text="日志信息", padx=5, pady=5) log_frame.pack(fill="both", expand=True, padx=10, pady=5) self.log_text = scrolledtext.ScrolledText(log_frame, width=100, height=8) self.log_text.pack(fill="both", expand=True, padx=5, pady=5) self.log_text.config(state=tk.DISABLED) # 添加示例按钮 example_btn = tk.Button(btn_frame, text="加载示例", command=self.load_example) example_btn.pack(side="right", padx=5) # 示例函数体 self.example_code = """static void Diag21_PID_C9(U1 u1_a_num) { U1 u1_t_cmplt; U1 u1_t_cnt; if((U1)DIAG_CNT_ZERO == u1_t_swrstcnt) /* Determine if a software reset is in progress */ { for(u1_t_cnt = (U1)DIAG21_ZERO; u1_t_cnt < (U1)DIAG21_PIDC9_FLAG; u1_t_cnt ++) { u1_t_cmplt = u1_g_InspSoftwareVersion(u4_g_cmd, &u4_g_data, (U1)TRUE); } vd_s_Diag21_U2ToU1(u2_g_buf, u1_g_data, (U1)DIAG21_PIDC9_FLAG); } else { /* Do Nothing */ } }""" def setup_logging(self): """配置日志系统""" self.log_filename = f"parser_{datetime.now().strftime('%Y%m%d_%H%M%S')}.log" logging.basicConfig( filename=self.log_filename, level=logging.INFO, format="%(asctime)s - %(levelname)s - %(message)s" ) logging.info("应用程序启动") def log_to_gui(self, message, level="info"): """将日志信息显示在GUI中""" self.log_text.config(state=tk.NORMAL) timestamp = datetime.now().strftime("%H:%M:%S") self.log_text.insert(tk.END, f"[{timestamp}] {message}\n") self.log_text.see(tk.END) self.log_text.config(state=tk.DISABLED) if level == "info": logging.info(message) elif level == "warning": logging.warning(message) elif level == "error": logging.error(message) def save_logs(self): """保存日志到文件""" try: log_content = self.log_text.get("1.0", tk.END) filename = f"saved_log_{datetime.now().strftime('%H%M%S')}.txt" with open(filename, "w") as f: f.write(log_content) self.log_to_gui(f"日志已保存到: {filename}", "info") messagebox.showinfo("保存成功", f"日志已保存到:\n{filename}") except Exception as e: self.log_to_gui(f"保存日志失败: {str(e)}", "error") messagebox.showerror("保存失败", f"无法保存日志:\n{str(e)}") def update_progress(self, value): """更新进度条""" self.progress['value'] = value self.root.update_idletasks() def load_example(self): """加载示例函数体""" self.input_text.delete(1.0, tk.END) self.input_text.insert(tk.END, self.example_code) self.log_to_gui("已加载示例函数体") def parse_function(self): """使用内置解析器解析C语言函数体""" try: code = self.input_text.get(1.0, tk.END) if not code.strip(): self.log_to_gui("错误: 没有输入函数体", "error") messagebox.showerror("错误", "请输入要解析的C语言函数体") return self.log_to_gui("开始解析函数体...") self.output_text.config(state=tk.NORMAL) self.output_text.delete(1.0, tk.END) self.update_progress(0) # 使用内置词法分析器 self.log_to_gui("执行词法分析...") lexer = SimpleCLexer() tokens = lexer.tokenize(code) self.update_progress(30) # 使用内置语法分析器 self.log_to_gui("执行语法分析...") analyzer = FunctionAnalyzer() analyzer.analyze(tokens) self.update_progress(70) # 显示结果 self.log_to_gui("生成解析报告...") self.display_results( analyzer.local_vars, analyzer.global_vars, analyzer.function_calls, analyzer.function_name, analyzer.parameters ) self.update_progress(100) self.output_text.config(state=tk.DISABLED) self.log_to_gui("解析完成!") messagebox.showinfo("完成", "函数体解析成功完成!") except Exception as e: self.log_to_gui(f"解析错误: {str(e)}", "error") messagebox.showerror("解析错误", f"发生错误:\n{str(e)}") self.update_progress(0) def display_results(self, local_vars, global_vars, function_calls, func_name, func_params): """显示解析结果""" # 显示函数签名 if func_name: self.output_text.insert(tk.END, "=== 函数签名 ===\n") self.output_text.insert(tk.END, f"函数名: {func_name}\n") # 修复参数显示问题 if func_params: param_list = [] for param in func_params: param_list.append(f"{param['type']} {param['name']}") self.output_text.insert(tk.END, f"参数: {', '.join(param_list)}\n\n") else: self.output_text.insert(tk.END, "参数: 无\n\n") else: self.output_text.insert(tk.END, "=== 函数签名 ===\n") self.output_text.insert(tk.END, "警告: 无法识别函数签名\n\n") self.log_to_gui("无法识别函数签名", "warning") # 显示局部变量 if local_vars: self.output_text.insert(tk.END, "=== 局部变量 ===\n") for var in local_vars: self.output_text.insert(tk.END, f"{var['type']} {var['name']} (行号: {var['line']})\n") self.log_to_gui(f"找到局部变量: {var['type']} {var['name']}") self.output_text.insert(tk.END, "\n") else: self.output_text.insert(tk.END, "未找到局部变量\n\n") self.log_to_gui("未找到局部变量", "warning") # 显示使用的全局变量 if global_vars: self.output_text.insert(tk.END, "=== 使用的全局变量 ===\n") for var in global_vars: self.output_text.insert(tk.END, f"{var['name']} (行号: {var['line']})\n") self.log_to_gui(f"找到全局变量: {var['name']}") self.output_text.insert(tk.END, "\n") else: self.output_text.insert(tk.END, "未使用全局变量\n\n") self.log_to_gui("未使用全局变量", "warning") # 显示函数调用 if function_calls: self.output_text.insert(tk.END, "=== 函数调用 ===\n") for func in function_calls: self.output_text.insert(tk.END, f"函数名: {func['name']} (行号: {func['line']})\n") self.output_text.insert(tk.END, f"返回类型: {func['return_type']}\n") self.output_text.insert(tk.END, f"参数: {func['params']}\n") self.output_text.insert(tk.END, "-" * 50 + "\n") self.log_to_gui(f"找到函数调用: {func['name']}") else: self.output_text.insert(tk.END, "未调用任何函数\n\n") self.log_to_gui("未调用任何函数", "warning") # 显示统计信息 self.output_text.insert(tk.END, "=== 解析统计 ===\n") self.output_text.insert(tk.END, f"局部变量数量: {len(local_vars)}\n") self.output_text.insert(tk.END, f"使用的全局变量数量: {len(global_vars)}\n") self.output_text.insert(tk.END, f"函数调用数量: {len(function_calls)}\n") self.output_text.insert(tk.END, "\n") if __name__ == "__main__": root = tk.Tk() app = FunctionParserApp(root) root.mainloop() [11:01:35] 已加载示例函数体 [11:01:36] 开始解析函数体… [11:01:36] 执行词法分析… [11:01:36] 执行语法分析… [11:01:36] 生成解析报告… [11:01:36] 未找到局部变量 [11:01:36] 找到全局变量: u1_t_cmplt [11:01:36] 找到全局变量: u1_t_cnt [11:01:36] 找到全局变量: u1_t_swrstcnt [11:01:36] 找到函数调用: u1_g_InspSoftwareVersion [11:01:36] 找到函数调用: vd_s_Diag21_U2ToU1 [11:01:36] 解析完成! 未找到局部变量 === 使用的全局变量 === u1_t_cmplt (行号: 3) u1_t_cnt (行号: 4) u1_t_swrstcnt (行号: 6) === 函数调用 === 函数名: u1_g_InspSoftwareVersion (行号: 10) 返回类型: U1 参数: u4_g_cmd, &u4_g_data, (U1)TRUE 函数名: vd_s_Diag21_U2ToU1 (行号: 12) 返回类型: void 参数: u2_g_buf, u1_g_data, (U1)DIAG21_PIDC9_FLAG === 解析统计 === 局部变量数量: 0 使用的全局变量数量: 3 函数调用数量: 2
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