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下面为构建LwIP RAW API下HTTP SERVER DEMO的学习记录，内容仅为个人设计与理解，所贴代码均为简化版，不保证适用性与准确性。
本人使用的LwIP版本为2.1.2。LwIP应用层直接接口为ALTCP，是TCP之上包了一个抽象层，也可以通过宏开关LWIP_ALTCP使接口直接对应TCP函数。
一、服务初始化
void http_init(void) { struct altcp_pcb *pcb; //分配协议控制块 pcb = altcp_tcp_new_ip_type(IPADDR_TYPE_ANY); //挂载接口 altcp_bind(pcb, IP_ANY_TYPE, 80); //开启监听 pcb = altcp_listen(pcb); //注册接收回调 altcp_accept(pcb, http_accept); }初始化的内容就是分配协议控制块，绑定到http端口号80上，然后等待客户端连接。
altcp_tcp_new_ip_type函数内调用TCP接口tcp_new_ip_type创建了控制块pcb，然后定义altcp_pcb控制块ret，将pcb裁剪后将几个LwIP关心的部分对接到ret，上层应用只需关心阉割后的ret而无需再看完整的TCP控制块结构体，应用层的callback也只需跟altcp_pcb对接即可。若空间不足，TCP会释放某些非活跃控制块，而altcp只是根据MEMP剩余空间大小不足回复失败
pcb->callback_arg = altcp_pcb;
pcb->recv = altcp_tcp_recv;
pcb->sent = altcp_tcp_sent;
pcb->errf = altcp_tcp_err;
struct altcp_pcb * altcp_tcp_new_ip_type(u8_t ip_type) { /* Allocate the tcp pcb first to invoke the priority handling code if we're out of pcbs */ struct tcp_pcb *tpcb = tcp_new_ip_type(ip_type); if (tpcb != NULL) { struct altcp_pcb *ret = altcp_alloc(); if (ret != NULL) { altcp_tcp_setup(ret, tpcb); return ret; } else { /* altcp_pcb allocation failed -> free the tcp_pcb too */ tcp_close(tpcb); } } return NULL; }其上还有一个altcp_new_ip_type的分配方式，可以用户自定义alloc函数和参数保存在其调用的altcp_allocator_t结构体变量中。
altcp_bind直接操作altcp_tcp_bind，实际使用tcp_bind，将altcp_pcb对应的pcb和http端口号80绑定。TCP遍历已存在的控制块链表，如没有相同的IP和端口号则插入链表。
altcp_listen(conn)启动服务器监听状态，其实际功能函数altcp_tcp_listen调用的也是tcp_listen_with_backlog_and_err。由于TCP监听时会用节省资源的tcp_pcb_listen结构体变量替换tcp_pcb，所以需要更新state元素。再将altcp_tcp_accept与tcp_accept对接，作为客户端连接的回调，其作用是当TCP_EVENT_ACCEPT发生时，开辟一个新的altcp_pcb去对接客户端连接的new_tpcb，并执行arg中预设的accept函数（pcb->callback_arg->accept，即altcp_pcb->accept）
static struct altcp_pcb * altcp_tcp_listen(struct altcp_pcb *conn, u8_t backlog, err_t *err) { struct tcp_pcb *pcb; struct tcp_pcb *lpcb; if (conn == NULL) { return NULL; } ALTCP_TCP_ASSERT_CONN(conn); pcb = (struct tcp_pcb *)conn->state; lpcb = tcp_listen_with_backlog_and_err(pcb, backlog, err); if (lpcb != NULL) { conn->state = lpcb; tcp_accept(lpcb, altcp_tcp_accept); return conn; } return NULL; }altcp_accept是将上一步中accept阶段TCP传递到LwIP的回调注册到应用层的http_accept，在此函数内处理客户端的连接请求，完成完整的连接操作。在用户层的http_accept中开辟一个变量放置应用的状态以及下层回调的参数，并将其与对应的pcb联系起来，之后就是将已经与TCP连接好的LwIP各个altcp回调与应用层具体处理函数对应起来。
#define HTTP_PRIO TCP_PRIO_MIN static err_t http_accept(void *arg, struct altcp_pcb *pcb, err_t err) { struct http_st *st = http_state_alloc(); st->pcb = pcb; altcp_arg(pcb, hs); altcp_setprio(pcb, HTTP_PRIO); altcp_recv(pcb, http_recv); altcp_poll(pcb, http_poll, HTTP_POLL_INT); altcp_sent(pcb, http_sent); altcp_err(pcb, http_err); return ERR_OK; }至此一个服务的初始化过程基本完成，接下来就是应用层的函数编写。
二、应用层函数功能
应用层在http_recv处理接收数据的回调，其中结构体pbuf是协议栈分解出的数据包。在raw api的数据处理中，需要用户手动执行altcp_recved通知协议栈数据正确接收，并且pbuf_free释放数据包缓存
static err_t http_recv(void *arg, struct altcp_pcb *pcb, struct pbuf *p, err_t err) { struct http_st *st = (struct http_st *)arg; if((p == NULL) || (err != ERR_OK) || (st == NULL)) { if(p != NULL) { altcp_recved(pcb, p->tot_len); pbuf_free(p); } http_close_conn(pcb, st); return ERR_OK; } altcp_recved(pcb, p->tot_len); if(st->handle == NULL) { err_t parsed = http_parse_request(p, st, pcb); pbuf_free(p); if(parsed == ERR_OK) { http_send(pcb, st); } else if(parsed == ERR_ARG) { http_close_conn(pcb, hs); } } else { pbuf_free(p); } return ERR_OK; }http_parse_request函数主要功能为解析接收的HTTP协议指令，对正确的“GET”进行响应。http server预先储存了file_handle列表，当client发对应的uri时传输相应的文件
#define CRLF "\r\n" static err_t http_parse_request(struct pbuf *p, struct http_st *st, struct altcp_pcb *pcb) { char *data; u16_t data_len; if((st->handle != NULL) || (st->file != NULL)) { //already started sending return ERR_USE; } //actual data in pbuf data = (char *)p->payload; //length of current buffer data_len = p->len; /* find first \r\n */ if(lwip_strnstr(data, CRLF, data_len) != NULL) { char *sp1, *sp2; int is_09 = 0; u16_t left_len, uri_len; if(strncmp(data, "GET ", 4)) { //unsupported http_find_error_file(st, 501); } sp1 = data + 3; //check URI left_len = (u16_t)(data_len - ((sp1 + 1) - data)); sp2 = lwip_strnstr(sp1 + 1, " ", left_len); if(sp2 == NULL) { sp2 = lwip_strnstr(sp1 + 1, CRLF, left_len); is_09 = 1; } uri_len = (u16_t)(sp2 - (sp1 + 1)); if((sp2 != 0) && (sp2 > sp1)) { //find the end of HTTP headers if(lwip_strnstr(data, CRLF CRLF, data_len) != NULL) { char *uri = sp1 + 1; *sp1 = 0; uri[uri_len] = 0; return http_find_file(st, uri, is_09); } } } return http_find_error_file(st, 400); }static err_t http_find_error_file(struct http_st *st, u16_t error_nr) { const char *uri, *uri1, *uri2, *uri3; if (error_nr == 501) { uri1 = "/501.html"; uri2 = "/501.htm"; uri3 = "/501.shtml"; } else { /* 400 (bad request is the default) */ uri1 = "/400.html"; uri2 = "/400.htm"; uri3 = "/400.shtml"; } if (fs_open(&st->file_handle, uri1) == ERR_OK) { uri = uri1; } else if (fs_open(&st->file_handle, uri2) == ERR_OK) { uri = uri2; } else if (fs_open(&st->file_handle, uri3) == ERR_OK) { uri = uri3; } else { return ERR_ARG; } return http_init_file(st, &st->file_handle, 0, uri, 0, NULL); }根据解析的uri，在本地文件中寻找对应name的文件
#define LWIP_HTTP_MAX_REQUEST_URI_LEN 63 static char http_uri_buf[LWIP_HTTP_MAX_REQUEST_URI_LEN + 1]; static err_t http_find_file(struct http_st *st, const char *uri, int is_09) { size_t loop; struct fs_file *file = NULL; char *params = NULL; err_t err; int i; u8_t tag_check = 0; //check uri size_t uri_len = strlen(uri); if((uri_len > 0) && (uri[uri_len - 1] == '/') && ((uri != http_uri_buf) || (uri_len == 1))) { size_t copy_len = LWIP_MIN(sizeof(http_uri_buf) - 1, uri_len - 1); if(copy_len > 0) { MEMCPY(http_uri_buf, uri, copy_len); http_uri_buf[copy_len] = 0; } for(loop = 0; loop < NUM_DEFAULT_FILENAMES; loop++) { const char *file_name; if(copy_len > 0) { size_t len_left = sizeof(http_uri_buf) - copy_len - 1; if(len_left > 0) { size_t name_len = strlen(httpd_default_filenames[loop].name); size_t name_copy_len = LWIP_MIN(len_left, name_len); MEMCPY(&http_uri_buf[copy_len], httpd_default_filenames[loop].name, name_copy_len); http_uri_buf[copy_len + name_copy_len] = 0; } file_name = http_uri_buf; } else { file_name = httpd_default_filenames[loop].name; } err = fs_open(&st->file_handle, file_name); if(err == ERR_OK) { uri = file_name; file = &st->file_handle; tag_check = httpd_default_filenames[loop].shtml; } } } if(file == NULL) { params = (char *)strchr(uri, '?'); if(params != NULL) { *params = '\0'; params++; } http_cgi_paramcount = -1; if(httpd_num_cgis && httpd_cgis) { for(i = 0; i < httpd_num_cgis; i++) { if(strcmp(uri, httpd_cgis[i].pcCGIName) == 0) { http_cgi_paramcount = extract_uri_parameters(hs, params); uri = httpd_cgis[i].pfnCGIHandler(i, http_cgi_paramcount, st->params, st->param_vals); break; } } } err = fs_open(&st->file_handle, uri); if(err == ERR_OK) { file = &st->file_handle; } else { file = http_get_404_file(st, &uri); } if(file != NULL) { if(file->flags & FS_FILE_FLAGS_SSI) { tag_check = 1; } else { tag_check = http_uri_is_ssi(file, uri); } } } if(file == NULL) { /* None of the default filenames exist so send back a 404 page */ file = http_get_404_file(st, &uri); } return http_init_file(st, file, is_09, uri, tag_check, params); }将找到的文件挂载到状态量st
static err_t http_init_file(struct http_st *st, struct fs_file *file, int is_09, const char *uri, u8_t tag_check, char *params) { if(file != NULL) { if(tag_check) { struct http_ssi_state *ssi = http_ssi_state_alloc(); if(ssi != NULL) { ssi->tag_index = 0; ssi->tag_state = TAG_NONE; ssi->parsed = file->data; ssi->parse_left = file->len; ssi->tag_end = file->data; st->ssi = ssi; } } st->handle = file; st->file = file->data; st->left = (u32_t)file->len; st->retries = 0; if(is_09 && ((st->handle->flags & FS_FILE_FLAGS_HEADER_INCLUDED) != 0)) { /* HTTP/0.9 responses are sent without HTTP header, search for the end of the header. */ char *file_start = lwip_strnstr(st->file, CRLF CRLF, st->left); if(file_start != NULL) { int diff = file_start + 4 - st->file; st->file += diff; st->left -= (u32_t)diff; } } } else { st->handle = NULL; st->file = NULL; st->left = 0; st->retries = 0; } if((st->handle == NULL) || ((st->handle->flags & FS_FILE_FLAGS_HEADER_INCLUDED) == 0)) { get_http_headers(st, uri); } return ERR_OK; }关于如何构建LwIP raw api 中的http server就分享到这里了，希望以上内容可以对大家有一定的帮助，可以学到更多知识。如果觉得文章不错，可以把它分享出去让更多的人看到。
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