在C++程序中添加逻辑流程控制(上)

问题的引出

　　在计算机程序中，除了常见的执行流程控制，还有逻辑流程控制；有时，执行流程即为逻辑流程，但在大多数情况下还是有所区别的，例如，假定有一个Web服务器使用同步套接字读取HTTP请求，那么会编写如下的代码：

void read(HTTP_REQUEST& http_request) { read(http_request.header); read(http_request.body, http_request.header); } void read(HTTP_REQUEST_HEADER& header) { string line = read_line(); parse_request_link(line, header.method, header.uri, header.version); while (TRUE) { line = read_line(); if (line.empty()) break; parse_header_field(line, header); } } void read(BYTE[]& body, HTTP_REQUEST_HEADER& header) { string transfer_encoding = header.fields['Transfer-Encoding']; if (transfer_encoding != b.chunkedb.) body = read_bytes(header.fields['Content-Length']); else { while (TRUE) { string chunk_header = read_line(); DWORD chunk_size = atoi(chunk_header); if (chunk_size == 0) break; BYTE[] chunk_body = read_bytes(chunk_size); body += chunk_body; } } } string read_line() { while (TRUE) { int n = strpos(read_buffer, b.\nb., read_buffer.size()); if (n > 0) break; read_buffer += socket.read(); } return read_buffer.extract(n); } Byte[] read_bytes(int sz) { while (TRUE) { if (sz <= read_buffer.size()) break; read_buffer += socket.read(); } return read_buffer.extract(sz); }

　　在这段代码中，执行流程与逻辑流程是一致的，然而，如果在那些被动接收事件的场合使用了异步套接字，就必须编写下面这样的代码了：

read() { read_buffer += socket.read(); if (state == read_request_line) { if (!read_line(line)) return; parse_request_link(line, method, uri, version); state = read_header_field; } while (state == read_request_line) { if (!read_line(line)) return; if (line.empty()) { transfer_encoding = header.fields['Transfer-Encoding']; if (transfer_encoding != b.chunkedb.) { content_length = header.fields['Content-Length']; state = read_body; } else state = read_chunk_header; } else parse_header_field(line, header, value); } if (state == read_body) { request_body += read_buffer; read_buffer.clear(); if (request_body.size() >= content_length) state = read_finished; return; } if (state == read_chunk_header) { if (!read_line(line)) return; chunk_size = atoi(line); if (chunk_size == 0) { state = read_finished; return; } state = read_body; } if (state == read_chunk_body) { request_body.append(read_buffer, chunk_size); if (chunk_size == 0) state = read_chunk_header; return; } }

　　执行流程完全不同了，但逻辑流程却仍保持不变，因为只能一块一块地接收数据，还必须保存状态值及其他变量，以便在事件发生时进行相应的处理。以上只是一些示范性代码，并不能真正工作，在实际中要编写像这样的函数会更加复杂，也更加容易出错

解决方案

　　为减少麻烦，可在程序主流程之外再创建一个子过程，这个子过程用于执行某些虚拟逻辑，在需要满足某些条件之后才能继续执行时，它可以先停下来，直到主流程告之它再次进行检查。对于上面的示例，可以写成如下的代码：

class Connection { SOCKET socket; Connection(SOCKET s) : socket(s) { FLOW_START conn.flow_start(); } void flow_start() { while (TRUE) { HTTP_REQUEST http_request; try { read(&http_request); } catch (EXCEPTION e) { break; } FILE file; fp = fopen(http_request.uri); if (fp == NULL) { write(fp); fclose(file); } else write(504); } socket.close(); delete this; } void read(HTTP_REQUEST* http_request) { read(&http_request.header); read(&http_request.body, &http_request.header); } void read(HTTP_REQUEST_HEADER* header) { …} void read(BYTE[]& body, HTTP_REQUEST_HEADER& header) { …} string read_line() { while (TRUE) { FLOW_WAIT (m_buffer += ) char* str = strchr(m_buffer, '\n'); if (!str) continue; } string s(m_buffer, 0, str - m_buffer); memcpy(m_buffer, str); buf_avail -= str - m_buffer; return s; } BYTE[] read_bytes(int sz) { while (TRUE) { WAIT (m_buffer += ); if (m_buffer.length < sz) continue; } BYTE[] data = m_buffer.extract(0, sz); return data; } void write(FILE* fp) { int filesize = fp.size(); string header; header << "200 OK\r\nContent-Length: " << filesize << ";\r\n" <<"\r\n"; write(header.c_str(), header.size()); int szBulk; for (int i = 0; i < filesize; i += szBulk) { szBulk = min(filesize - i, 8192); data = fread(fp, szBulk); write(data, szBulk); } } void write(WORD error_status) { string header; header << error_status << " Error\r\n" <<"\r\n"; write(header.c_str(), header.size()); } void write(BYTE[] data, int len) { while (len > 0) { int ret = socket.write(data, len); if (ret > 0) { data += ret; len -= ret; } if (len) { WAIT (bWritable == TRUE); } } } void OnRead() { int avail = socket.avail(); m_buffer += socket.read(avail); } void OnWrite() { bWritable = TRUE; } void OnClose() { delete this; } }; main { Socket listen_socket; listen_socket.listen(http_port); socket_add(listen_socket, NULL); socket_loop(socket_callback); } void socket_callback(void* user_data, SOCKET s, int msg, int lParam, void* pParam) { switch (msg) { case READ: if (user_data == NULL) { SOCKET s2 = accept(s); Connection conn = new Connection(socket); socket_add(s2, conn); break; } ((Connection*)user_data)->OnRead(); break; case WRITE: ((Connection*)user_data)->OnWrite(); break; case EXCEPT: ((Connection*)user_data)->OnExcept(); break; } }

　　这涉及到两个新的原语：一个为FLOW_START，其创建了一个新的子过程；另一个为FLOW_WAIT，其告之系统何时将被调用以继续程序流程。例如，FLOW_WAIT(m_buffer += )意味着m_buffer的操作符+=被执行，FLOW_WAIT (bWritable = TRUE)意味着bWritable被设为TRUE。

　　当一个连接对象创建后，因为FLOW_START这条指令，一个子过程也会被创建，执行流程会沿着此过程执行下去，直至碰到FLOW_WAIT，然后，它会继续执行主流程；当它追加m_buffer或设置bWritable为TRUE时，它将继续执行子过程，直至碰到另一个FLOW_WAIT，此时再返回到主流程当中。