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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。
