linux: 初试网络编程

#include <netinet/in.h>

struct sockaddr 
{
    unsigned short sa_family;      /*地址族*/
    char sa_data[14];              /*14字节的协议地址，包含该socket的IP地址和端口号。*/
};
struct sockaddr_in 
{
    short int sa_family;           /*地址族*/
    unsigned short int sin_port;   /*端口号*/
    struct in_addr sin_addr;       /*IP地址*/
    unsigned char sin_zero[8];     /*填充0 以保持与struct sockaddr同样大小*/
};
struct in_addr
{
        unsigned long int  s_addr;    /* 32位IPv4地址，网络字节序 */
};
#include <netinet/in.h>

tips

sa_family:
AF_INET   -> IPv4协议  
AF_INET6  -> IPv6协议

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <arpa/inet.h>
             
             
       int inet_aton(const char *cp, struct in_addr *inp);/* 注意，参数inp为传出参数 */
       char *inet_ntoa(struct in_addr in);

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

int main(int argc, char *argv[])
{
    char ip_buf[] = "180.97.33.107";
    struct in_addr my_addr;
    inet_aton(ip_buf,&my_addr);
    printf("ip : %s \n", ip_buf);
    printf("net: %x \n", my_addr.s_addr);
    return 0;
}

[purple@localhost 0827]$ gcc -o test test.c -Wall
[purple@localhost 0827]$ ./test
ip : 180.97.33.107
net: 6b2161b4

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

int main(int argc, char *argv[])
{
    struct in_addr my_addr;
    my_addr.s_addr = 0xb461216b;
    printf("net: %x \n", my_addr.s_addr);
    printf("ip : %s \n", inet_ntoa(my_addr));
    return 0;
}

[purple@localhost 0827]$ gcc -o test1 test1.c -Wall
[purple@localhost 0827]$ ./test1
net: b461216b
ip : 107.33.97.180

SYNOPSIS
       #include <netdb.h>
       struct hostent *gethostbyname(const char *name);

The hostent structure is defined in <netdb.h> as follows:

           struct hostent {
               char  *h_name;            /* official name of host */
               char **h_aliases;         /* alias list */
               int    h_addrtype;        /* host address type */
               int    h_length;          /* length of address */
               char **h_addr_list;       /* list of addresses */
           }
           #define h_addr h_addr_list[0] /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name of the host.

       h_aliases
              An array of alternative names for the host, terminated by a NULL
              pointer.

       h_addrtype
              The type of address; always AF_INET or AF_INET6 at present.

       h_length
              The length of the address in bytes.

       h_addr_list
              An  array of pointers to network addresses for the host (in net-
              work byte order), terminated by a NULL pointer.

       h_addr The first address in h_addr_list for backward compatibility.

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
int main(int argc, char* argv[])// exe hostname
{
    struct hostent* p ;
    p = gethostbyname(argv[1]) ;
    /*
    struct hostent {
        char  *h_name;          
        char **h_aliases;     
        int    h_addrtype;   
        int    h_length;    
        char **h_addr_list;
    }
#define h_addr h_addr_list[0]
 */ 
    printf("host name: %s \n", p ->h_name);
    
    int index ;
    char** pp  = p -> h_aliases ;
    for(index = 0 ; pp[index] != NULL; index ++ )
    {
        printf("alias : %s \n", pp[index]);
    }
    
    printf("ip type : %d\n", p ->h_addrtype);
    
    printf("addr len : %d \n", p ->h_length);
    
    pp = p ->h_addr_list ;
    for(index = 0; pp[index] != NULL ; index ++)
    {
        /* 由于h_addr_list是一个字符串指针数组，数组中存放的指针指向一个网络字节序 
            但是系统函数inet_ntoa需要传入的参数是一个结构体，因此需要进行转换。
            pp[index]是一个char*类型的指针，先将其转换为struct in_addr*类型的指针，
            接着去引用，即得到结构体。                                                 */
        printf("ip : %s \n", inet_ntoa( *(struct in_addr *)pp[index] ) );
    }
    
    return 0 ;
}

[purple@localhost 0827]$ gcc -o myhost my_host.c -Wall
[purple@localhost 0827]$ ./myhost www.baidu.com
host name: www.a.shifen.com
alias : www.baidu.com
ip type : 2
addr len : 4
ip : 180.97.33.107
ip : 180.97.33.108

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int a = 0x61;//97    内存中的情况是：小端 61 00 00 00 如果是大端机 00 00 00 61 两种情况
    printf("%x\n",(char*)(&a)[0]);  把&a转化为char*是为了只指向一个字节 int*指向的是4个字节 
}