原理：ftp协议是基于tcpip协议，用socket实现tcpip协议发送ftp指令、内容
IP地址转换函数
inet_pton是一个IP地址转换函数，可以在将点分文本的IP地址转换为二进制网络字节序”的IP地址，而且inet_pton和inet_ntop这2个函数能够处理ipv4和ipv6。
strerror:
C语言错误提示函数:strerror和errno 相关头文件: string.h errno.h 函数原型: char*strerror(interrnum); errnum是错误的编号,要通过“errno”来获得
代码实现功能：socket连接ftp服务器21端口，发送登录账号密码命令，登录成功
代码：
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<errno.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#define IPV4_PROTO AF_INET
#define SOCKET_TYPE SOCK_STREAM
#define protocol IPPROTO
#define FTP_PORT 21
#define BUFF_LEN 4096
//return -1 connect failed -2 send failed -3 recv failed
//usage pratise socket ftp
int main(int argc, char **argv)
{
int sockFd = 0;
int ret = 0;
char buf[BUFF_LEN]={0};
struct sockaddr_in serverAddr;
if(argc != 2)
{
return -1;
}
for(int i = 0; i < argc; i++)
{
printf("%s ", argv[i]);
}
printf("\n");
if( (sockFd = socket(AF_INET, SOCK_STREAM, 0)) < 0){
printf("create socket error: %s(errno: %d)\n", strerror(errno),errno);
exit(0);
}
memset(&serverAddr, 0, sizeof(serverAddr));
serverAddr.sin_family = AF_INET;
serverAddr.sin_port = htons(21);
if( inet_pton(AF_INET, argv[1], &serverAddr.sin_addr) <= 0){
printf("inet_pton error for %s\n",argv[1]);
return -1;
}
if( connect(sockFd, (struct sockaddr*)&serverAddr, sizeof(serverAddr)) < 0)
{
printf("connect error: %s(errno: %d)\n",strerror(errno),errno);
return -1;
}
if((ret = recv(sockFd, buf, BUFF_LEN,0)) == -1)
{
perror("recv error");
return -3;
}
buf[ret] = '\0';
printf("Received : %s ",buf);
printf("send msg to server: \n");
char user[] = "user ftpusr\r\n";
char passwd[] = "pass 123456aB\r\n";
if( send(sockFd, user, strlen(user), 0) < 0)
{
printf("send msg error: %s(errno: %d)\n", strerror(errno), errno);
return -2;
}
printf("send : %s\n",user);
if((ret = recv(sockFd, buf, BUFF_LEN,0)) == -1)
{
perror("recv error");
return -3;
}
buf[ret] = '\0';
printf("Received : %s ",buf);
if( send(sockFd, passwd, strlen(passwd), 0) < 0)
{
printf("send msg error: %s(errno: %d)\n", strerror(errno), errno);
return -2;
}
printf("send : %s\n",passwd);
if((ret = recv(sockFd, buf, BUFF_LEN,0)) == -1)
{
perror("recv error");
return -3;
}
buf[ret] = '\0';
printf("Received : %s ",buf);
close(sockFd);
return 0;
}