使用 libevent 处理 TCP 粘包问题（基于 Content-Length 或双 \r\n）

在基于 libevent 的 TCP 服务器开发中，处理消息边界是常见需求。以下是两种主流分包方案的完整实现：

一、基于 Content-Length 的分包方案

1.1 数据结构设计

typedef struct {struct bufferevent *bev;int content_length;  // 当前消息的预期长度int received_bytes;  // 已接收字节数char *buffer;        // 消息缓冲区size_t buffer_size;  // 缓冲区大小
} tcp_session_t;

1.2 核心处理逻辑

void read_cb(struct bufferevent *bev, void *arg) {tcp_session_t *session = (tcp_session_t *)arg;struct evbuffer *input = bufferevent_get_input(bev);// 阶段1：读取消息头if (session->content_length == -1) {char *line = evbuffer_readln(input, NULL, EVBUFFER_EOL_CRLF);if (!line) return;  // 不完整的行// 解析Content-Lengthif (strstr(line, "Content-Length:") != NULL) {sscanf(line, "Content-Length: %d", &session->content_length);session->buffer = malloc(session->content_length + 1);}free(line);// 检查是否到达头部结束（空行）line = evbuffer_readln(input, NULL, EVBUFFER_EOL_CRLF);if (line && strlen(line) == 0) {free(line);if (session->content_length == -1) {// 没有Content-Length的简单消息session->content_length = evbuffer_get_length(input);}} else {return;  // 继续等待头部结束}}// 阶段2：读取消息体size_t avail = evbuffer_get_length(input);size_t need = session->content_length - session->received_bytes;size_t to_read = avail < need ? avail : need;evbuffer_remove(input, session->buffer + session->received_bytes, to_read);session->received_bytes += to_read;// 阶段3：完整消息处理if (session->received_bytes == session->content_length) {session->buffer[session->content_length] = '\0';process_complete_message(session->buffer);// 重置状态free(session->buffer);session->buffer = NULL;session->content_length = -1;session->received_bytes = 0;}
}

二、基于双 CRLF 的分包方案

2.1 数据结构设计

typedef struct {struct bufferevent *bev;int header_complete;  // 头部是否解析完成char *header;         // 消息头缓冲区char *body;           // 消息体缓冲区size_t body_len;      // 消息体长度
} tcp_session_t;

2.2 核心处理逻辑

void read_cb(struct bufferevent *bev, void *arg) {tcp_session_t *session = (tcp_session_t *)arg;struct evbuffer *input = bufferevent_get_input(bev);// 阶段1：解析消息头if (!session->header_complete) {char *line = evbuffer_readln(input, NULL, EVBUFFER_EOL_CRLF);if (!line) return;if (session->header == NULL) {session->header = malloc(1024);session->header[0] = '\0';}// 空行表示头部结束if (strlen(line) == 0) {session->header_complete = 1;free(line);// 检查是否有消息体（如POST请求）const char *content_len = strstr(session->header, "Content-Length:");if (content_len) {sscanf(content_len, "Content-Length: %zu", &session->body_len);session->body