我們已經看過關於伺服器socket建立的教學，為了使進程TCP伺服器端需要執行以下步驟：

建立一個socket使用socket() 系統呼叫.
使用bind()系統呼叫通訊端系結到一個地址。對於網際網路上的伺服器通訊端，地址包括主機的埠號。
使用listen()系統呼叫連線監聽。
accept() 系統呼叫形式接受連線。此呼叫通常會阻塞，直到有用戶端與伺服器連線。
傳送和接收資料，使用read() 和 write() 系統呼叫。

現在要把上面這些步驟的形式寫成原始碼。把這個程式碼寫server.c檔案並用gcc編譯器編譯。

#include <stdio.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

int main( int argc, char *argv[] )
{
    int sockfd, newsockfd, portno, clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;
    int  n;

    /* First call to socket() function */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
    {
        perror("ERROR opening socket");
        exit(1);
    }
    /* Initialize socket structure */
    bzero((char *) &serv_addr, sizeof(serv_addr));
    portno = 5001;
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);
 
    /* Now bind the host address using bind() call.*/
    if (bind(sockfd, (struct sockaddr *) &serv_addr,
                          sizeof(serv_addr)) < 0)
    {
         perror("ERROR on binding");
         exit(1);
    }

    /* Now start listening for the clients, here process will
    * go in sleep mode and will wait for the incoming connection
    */
    listen(sockfd,5);
    clilen = sizeof(cli_addr);

    /* Accept actual connection from the client */
    newsockfd = accept(sockfd, (struct sockaddr *)&cli_addr, 
                                &clilen);
    if (newsockfd < 0) 
    {
        perror("ERROR on accept");
        exit(1);
    }
    /* If connection is established then start communicating */
    bzero(buffer,256);
    n = read( newsockfd,buffer,255 );
    if (n < 0)
    {
        perror("ERROR reading from socket");
        exit(1);
    }
    printf("Here is the message: %s\n",buffer);

    /* Write a response to the client */
    n = write(newsockfd,"I got your message",18);
    if (n < 0)
    {
        perror("ERROR writing to socket");
        exit(1);
    }
    return 0; 
}

處理多個連線：

為了使伺服器能夠同時處理多個連線，我們在上面的程式碼進行以下更改：

將accept語句和下面的程式碼在一個無限迴圈。
建立連線後，呼叫fork()建立一個新的進程。
子進程將關閉sockfd中致電doprocessing函式，通過新的通訊端檔案描述符作為引數。當兩個進程已經完成了他們對話中表示由doprocessing()返回，這個過程簡單地退出。
父進程關閉newsockfd。因為所有這些程式碼是在一個無限迴圈，它接受語句將返回到等待下一個連線。

#include <stdio.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

int main( int argc, char *argv[] )
{
    int sockfd, newsockfd, portno, clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;
    int  n;

    /* First call to socket() function */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
    {
        perror("ERROR opening socket");
        exit(1);
    }
    /* Initialize socket structure */
    bzero((char *) &serv_addr, sizeof(serv_addr));
    portno = 5001;
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);
 
    /* Now bind the host address using bind() call.*/
    if (bind(sockfd, (struct sockaddr *) &serv_addr,
                          sizeof(serv_addr)) < 0)
    {
         perror("ERROR on binding");
         exit(1);
    }
    /* Now start listening for the clients, here 
     * process will go in sleep mode and will wait 
     * for the incoming connection
     */
    listen(sockfd,5);
    clilen = sizeof(cli_addr);
    while (1) 
    {
        newsockfd = accept(sockfd, 
                (struct sockaddr *) &cli_addr, &clilen);
        if (newsockfd < 0)
        {
            perror("ERROR on accept");
            exit(1);
        }
        /* Create child process */
        pid = fork();
        if (pid < 0)
        {
            perror("ERROR on fork");
	    exit(1);
        }
        if (pid == 0)  
        {
            /* This is the client process */
            close(sockfd);
            doprocessing(newsockfd);
            exit(0);
        }
        else
        {
            close(newsockfd);
        }
    } /* end of while */
}

Here is the simple implementation of doprocessing function.

void doprocessing (int sock)
{
    int n;
    char buffer[256];

    bzero(buffer,256);

    n = read(sock,buffer,255);
    if (n < 0)
    {
        perror("ERROR reading from socket");
        exit(1);
    }
    printf("Here is the message: %s\n",buffer);
    n = write(sock,"I got your message",18);
    if (n < 0) 
    {
        perror("ERROR writing to socket");
        exit(1);
    }
}

Unix/Linux Socket 伺服器程式碼範例

處理多個連線：