如何构建LwIP raw api 中的http server

这篇文章将为大家详细讲解有关如何构建LwIP raw api 中的http server，文章内容质量较高，因此小编分享给大家做个参考，希望大家阅读完这篇文章后对相关知识有一定的了解。

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_MINstatic 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      63static 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。