Simple usage of I/O multiplexing select function

1. The select function

#include <sys/select.h>

int select(int maxfd
         , fd_set* readset, fd_set* writeset, fd_set* exceptset
         , const struct timeval* timeout);

Description:

• It returns greater than 0 on success, -1 on failure, and 0 on timeout
• maxfd Number of file descriptors to be monitored
• readset registers a collection of file descriptors for reading data
• writeset Register a collection of file descriptors for writing data
• exceptset registers a collection of exception file descriptors
• timeout call select function, program block, set program block timeout time

Role: to monitor multiple file descriptors together.

2. Use

Assignment method of fd_set type object:

FD_ZERO(fd_set* fdset)： 将fd_set变量的所有位初始化为0。
FD_SET（int fd, fd_set* fdset）：在参数fd_set指向的变量中注册文件描述符fd的信息。
FD_CLR(int fd, fd_set* fdset)：参数fd_set指向的变量中清除文件描述符fd的信息。
FD_ISSET(int fd, fd_set* fdset)：若参数fd_set指向的变量中包含文件描述符fd的信息，则返回真。

3. Example

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/select.h>

#define BUF_SIZE 30

int main(int argc, char const *argv[])
{
    
    
    fd_set reads;
    fd_set temps;

    FD_ZERO(&reads);
    FD_SET(0, &reads);   // 0 是控制台标准输入文件描述符，1 是标准输出描述符，2 是异常描述符

    while (1)
    {
    
    
        temps = reads;

        struct timeval timeout;
        timeout.tv_sec = 5;
        timeout.tv_usec = 0;

        int result = select(1, &temps, NULL, NULL, &timeout);
        if (result == -1)   // 发生异常
        {
    
    
            puts("select() error!");
            break;
        }
        else if (result == 0)   // 监听描述符事件超时
        {
    
    
            puts("time-out!");
        }
        else
        {
    
    
            if (FD_ISSET(0, &temps))  // 判断0描述符输入事件是否发生
            {
    
    
                char buf[BUF_SIZE];
                int str_len = read(0, buf, BUF_SIZE);
                buf[str_len] = 0;
                printf("message from console: %s\n", buf);
            }
        }
    }

    return 0;
}