目錄表

Socket

0x00 前言

C 語言中的 Socket 是在寫網路程式必會碰到的東西，而它牽涉較多參數與函式，不如 python 簡潔，本篇用於紀錄關於網路程式中 socket 相關的細節

0x01 Berkeley Socket

0x02 通訊端 API 函式

socket() 

#include <sys/types.h>
#include <sys/socket.h>
 
int socket(int domain, int type, int protocol);
/* return a file descriptor for the new socket on success, or -1 if error */

建立一個 communication endpoint，並回傳一個 file descriptor

bind() 

#include <sys/socket.h>
 
int bind(int sockfd, const struct sockaddr *address, socklen_t address_len);
/* return 0 for successful; otherwise, -1 if error */
include <netinet/in.h>  
 
struct sockaddr {  
    unsigned short    sa_family;       // 2 bytes address family, AF_xxx  
    char              sa_data[14];     // 14 bytes of protocol address  
};  
 
// IPv4 AF_INET sockets:  
 
struct sockaddr_in {  
    short            sin_family;       // 2 bytes e.g. AF_INET, AF_INET6  
    unsigned short   sin_port;         // 2 bytes e.g. htons(3490)  
    struct in_addr   sin_addr;         // 4 bytes see struct in_addr, below  
    char             sin_zero[8];      // 8 bytes zero this if you want to  
};  
 
struct in_addr {  
    unsigned long s_addr;              // 4 bytes load with inet_pton()  
};

用於 server 端，用來將一個 socket file descriptor 和一個 sockaddr structure 相關聯，sockaddr 結構中會指出這個 socket (sockfd) 所要監聽的 address, port number 等資訊

listen() 

#include <sys/socket.h>
 
int listen(int sockfd, int backlog);
/* return 0 if success, otherwise, -1 for error */

accept() 

#include <sys/socket.h>
 
int accept(int sockfd, struct sockaddr *restrict address, socklen_t *restrict address_len);
/* return the non-negative file descriptor of the accepted socket if success, Otherwise, -1 if error */

接受一個監聽佇列中的連線，回傳指向 client 的 file descriptor

connect() 

#include <sys/socket.h>
 
int connect(int sockfd, const struct sockaddr *address, socklen_t address_len);
/* return 0 if success, otherwise, -1 for error */

用於 client 端，他會透過 sockfd 和 sockaddr structure 向指定的 server 連線

0x03 Example code

/* Server code in C */
 
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
int main(void)
{
    struct sockaddr_in stSockAddr;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
 
    if(-1 == SocketFD)
    {
        perror("can not create socket");
        exit(EXIT_FAILURE);
    }
 
    memset(&stSockAddr, 0, sizeof(struct sockaddr_in));
 
    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    stSockAddr.sin_addr.s_addr = INADDR_ANY;
 
    if(-1 == bind(SocketFD,(const struct sockaddr *)&stSockAddr, sizeof(struct sockaddr_in)))
    {
        perror("error bind failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
    }
 
    if(-1 == listen(SocketFD, 10))
    {
        perror("error listen failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
    }
 
    for(;;)
    {
        int ConnectFD = accept(SocketFD, NULL, NULL);
 
        if(0 > ConnectFD)
        {
            perror("error accept failed");
            close(SocketFD);
            exit(EXIT_FAILURE);
        }
 
        /* perform read write operations ... */
        shutdown(ConnectFD, SHUT_RDWR);
        close(ConnectFD);
    }
 
    close(SocketFD);
    return 0;
}
/* Client code in C */
 
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
int main(void)
{
    struct sockaddr_in stSockAddr;
    int Res;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
 
    if (-1 == SocketFD)
    {
        perror("cannot create socket");
        exit(EXIT_FAILURE);
    }
 
    memset(&stSockAddr, 0, sizeof(struct sockaddr_in));
 
    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    Res = inet_pton(AF_INET, "192.168.1.3", &stSockAddr.sin_addr);
 
    if (0 > Res)
    {
        perror("error: first parameter is not a valid address family");
        close(SocketFD);
        exit(EXIT_FAILURE);
    }
    else if (0 == Res)
    {
        perror("char string (second parameter does not contain valid ipaddress");
        close(SocketFD);
        exit(EXIT_FAILURE);
    }
 
    if (-1 == connect(SocketFD, (const struct sockaddr *)&stSockAddr, sizeof(struct sockaddr_in)))
    {
        perror("connect failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
    }
 
    /* perform read write operations ... */
 
    shutdown(SocketFD, SHUT_RDWR);
 
    close(SocketFD);
    return 0;
}

0x04 參考資料