"Linux network development must learn course" 4_ early adopters seclect multiplexing

Question: How to enhance the capability of the server to support multiple clients at the same time?

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Linux Design Philosophy: Everything is a File

What are files in Linux?

• Chivalry:

  • A physical file (logically related collection of data) in a file system

• Broadly:

  • devices, channels, memory, . . .
  • Everything Linux manages

Understanding file descriptors

• The file descriptor is a non-negative integer, essentially a handle
• All resource identifiers that are transparent to users (programmers) can be regarded as handles
• The user interacts with the kernel using file descriptors (handles)
• The kernel operates the data structure of the corresponding resource through the file descriptor

The meaning of everything is a file

• Unify the operation methods of various devices (open, read, write, close)

• Such as:

  • IO devices (command line, display)
  • Network equipment (NIC)
  • ...

Programming Experiment: Manipulating the Command Line as a File

 #include <stdio.h>
#include <unistd.h>

int main()
{
    int iofd = 0;
    char s[] = "D.T.SoftWare\n";
    int len = 0;

    write(iofd, s, sizeof(s));

    len = read(0, s, 5);

    s[len] = 0;

    printf("%s\n", s);

    return 0;
}

output:

 book@100ask:~/Desktop$ ./a.out 
D.T.SoftWare
12345
12345
book@100ask:~/Desktop$ 

...

book@100ask:~/Desktop$ ./a.out 
D.T.SoftWare
12345678
12345                              // 注意这里为什么只输出了 12345
book@100ask:~/Desktop$ 678         // 注意这里为什么输出 678
678: command not found

答：在 a.out 应用程序中输入了"12345678",但应用程序只读取了 5 个字符，即"12345"。当应用程序结束，a.out所在终端取得所有权，得到 "678", 并尝试将其当作命令解析

Classification of event-related functions

• blocking function

  • After the function is called, it needs to wait for an event to occur before returning

• non-blocking function

  • Can return in time after function call (only mark waiting events)
  • After the event occurs, it is passed as a callback

Blocking vs Polling

• Polling refers to a method of sequentially accessing each relevant device to see if it needs service
• Polling can be used to solve the problem of blocking functions that prevent the program from continuing to execute

The magic select() function

• select() is used to monitor whether the specified file character generates events
• The target file can be detected by polling (the flag changes when the event occurs)
• Make specific processing according to the event type (eg: read data)

 int select(int maxfd,                    // maxfd = n(最大的文件描述符 ) + 1，标记监听的描述符范围 [0 - maxfd-1]
    fd_set *readset,                     // 检查可读性
    fd_set *writeset,                    // 检查可写性
    fd_set *exceptset,                   // 检查异常
    const struct timeval *timeout);      // 等待 IO 的时长

How to use the select() function

select() related data types and operations

fd_set 的每一位标识一个文件描述符，当某一位为 1，则表示监听

 * FD_SERO(fd_set *fdset);         // 将 fd_set 变量的所有位设置为0
* FD_SET(int fd, fd_set*fdset);   // 在 fd_set 中指定要监听的 fd
* FD_CLR(int fd, fd_set*fdset);   // 在 fd_set 中剔除 fd, 不再监听
* FD_ISSET(int fd, fd_set*fdset); // 在 fd_set 产看是否包含 fd

Programming experiment: select() first experience

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

int main()
{
    int iofd = 0;
    char s[] = "D.T.SoftWare";
    int len = 0;
    fd_set reads = {0};
    fd_set temps = {0};
    struct timeval timeout = {0};

    FD_ZERO(&reads);
    FD_SET(iofd, &reads);

    while (1) {
        int r = -1;

        temps = reads; // NOTICE !!!

        timeout.tv_sec  = 0;
        timeout.tv_usec = 50000;

        r = select(1, &temps, 0, 0, &timeout);

        if (r > 0) {
            len = read(iofd, s, sizeof(s)-1);

            s[len] = 0;

            printf("Input: %s\n", s);
        }
        else if (r == 0) {
            static int count = 0;

            usleep(10000);

            count++;

            if (count > 100) {
                printf("do something slse\n");

                count = 0; 
            }
        }
        else {
            break;
        }
    }

    return 0;
}

output:

 book@100ask:~/Desktop$ ./a.out
hello word
Input: hello word

do something slse
...
...

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Thinking: Can the server-side functionality be extended using the select() function? If so, how to implement it?