lwip select原理

lwip select原理
英文回答：
The select function in the lwIP (lightweight IP)
library is used for multiplexing I/O operations on multiple sockets. It allows a program to monitor multiple sockets
for activity, such as incoming data or connection requests, and then perform appropriate actions based on the activity.
The select function takes three sets of file descriptors as parameters: the read set, the write set, and the exception set. These sets are used to specify the sockets that the program wants to monitor for read, write, and exception events, respectively. The function also takes a timeout parameter, which specifies the maximum time to wait for an event to occur.
When the select function is called, it checks the status of each socket in the sets and returns the number of sockets that have activity. The program can then use other
functions, such as recv or send, to perform the necessary operations on the active sockets.
Here's an example to illustrate how the select function works:
fd_set read_set;
FD_ZERO(&read_set);
FD_SET(socket1, &read_set);
FD_SET(socket2, &read_set);
int num_sockets = select(max(socket1, socket2) + 1,
&read_set, NULL, NULL, NULL);
if (num_sockets > 0) {。

if (FD_ISSET(socket1, &read_set)) {。

// socket1 has incoming data, read it.
char buffer[1024];
int num_bytes = recv(socket1, buffer, sizeof(buffer), 0);
// process the received data.
}。

if (FD_ISSET(socket2, &read_set)) {。

// socket2 has incoming data, read it.
char buffer[1024];
int num_bytes = recv(socket2, buffer, sizeof(buffer), 0);
// process the received data.
}。

}。

中文回答：
lwIP库中的select函数用于在多个套接字上进行I/O操作的多路复用。

它允许程序监视多个套接字的活动，如传入的数据或连接请求，并根据活动执行相应的操作。

select函数接受三组文件描述符作为参数，读取集合、写入集合和异常集合。

这些集合用于指定程序要监视的套接字的读取、写入和异常事件。

函数还接受一个超时参数，用于指定等待事件发生的最长时间。

当调用select函数时，它会检查集合中每个套接字的状态，并返回有活动的套接字数量。

然后程序可以使用其他函数，如recv或send，在活动的套接字上执行必要的操作。

下面是一个示例，说明select函数的工作原理：
fd_set read_set;
FD_ZERO(&read_set);
FD_SET(socket1, &read_set);
FD_SET(socket2, &read_set);
int num_sockets = select(max(socket1, socket2) + 1, &read_set, NULL, NULL, NULL);
if (num_sockets > 0) {。

if (FD_ISSET(socket1, &read_set)) {。

// socket1有传入数据，读取它。

char buffer[1024];
int num_bytes = recv(socket1, buffer, sizeof(buffer), 0);
// 处理接收到的数据。

}。

if (FD_ISSET(socket2, &read_set)) {。

// socket2有传入数据，读取它。

char buffer[1024];
int num_bytes = recv(socket2, buffer,
sizeof(buffer), 0);
// 处理接收到的数据。

}。

}。

以上就是lwIP库中select函数的原理和使用方法。

通过使用select函数，程序可以同时监视多个套接字的活动，并在需要时执行相应的操作。