ftp 客户端从服务器读取不完整的消息

Question

这是我的代码简单的 ftp 客户端，我可以连接到服务器登录，但我的程序从服务器读取随机不完整的消息，有时它们是完整的，但有时它只读取几个字节，我不知道该怎么做请帮助我

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/socket.h>

#define BUF_SIZE 5000

int main()
{
  struct addrinfo hints;
  struct addrinfo *result, *rp;
  int my_socket, passive_socket, sock, len, m = 0, bytes;
  char buf[BUF_SIZE], i[BUF_SIZE];

  memset(&hints, 0, sizeof(struct addrinfo));
  hints.ai_family = AF_UNSPEC;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_protocol = 0;

  if ((sock = getaddrinfo("ftp.linux.cz", "ftp", &hints, &result)) != 0)
  {
    fprintf(stderr, "%s\n", gai_strerror(sock));
    exit(EXIT_FAILURE);
  }

  for (rp = result; rp != NULL ; rp = rp->ai_next)
  {
    if ((my_socket = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol))
        == -1)
      continue;
    if (connect(my_socket, rp->ai_addr, rp->ai_addrlen) != -1)
      break;
    close(my_socket);
  }

  freeaddrinfo(result);

  read(my_socket, buf, BUF_SIZE);
  printf("%s", buf);

  len = strlen("USER anonymous\r\n");
  write(my_socket, "USER anonymous\r\n", len);

  read(my_socket, buf, BUF_SIZE);
  printf("%s", buf);

  len = strlen("PASS secret\r\n");
  write(my_socket, "PASS secret\r\n", len);

  read(my_socket, buf, BUF_SIZE);
  printf("%d\n", bytes);
  for (int n = 0; n < bytes; n++)
    printf("%c", buf[n]);

  len = strlen("EPSV\r\n");
  write(my_socket, "EPSV\r\n", len);

  read(my_socket, buf, BUF_SIZE);
  //printf( "%s", buf );  

  for (int n = 39; n > 0; n++)
  {
    if (buf[n] >= 48 && buf[n] <= 57)
    {
      i[m] = buf[n];
      m++;
    }
    else
    {
      i[m] = '\0';
      n = -1;
    }
  }

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

#if 0
  if ((sock = getaddrinfo("ftp.fit.vutbr.cz", i, &hints, &result)) != 0)
  {
    fprintf(stderr, "%s\n", gai_strerror(sock));
    exit(EXIT_FAILURE);
  }

  for (rp = result; rp != NULL ; rp = rp->ai_next)
  {
    if ((passive_socket = socket(rp->ai_family, rp->ai_socktype,
        rp->ai_protocol)) == -1)
      continue;
    if (connect(passive_socket, rp->ai_addr, rp->ai_addrlen) != -1)
      break;
    close(passive_socket);
  }

  read(passive_socket, buf, BUF_SIZE);
  printf("%s", buf);

  len = strlen("NLST\r\n");
  write(my_socket, "NLST\r\n", len);

  read(passive_socket, buf, BUF_SIZE);
  printf("%s", buf);
#endif

  return 0;
}

Answer 1

您不能假设 read 将一次读取整个响应，它可以读取更少，它可以读取 BUF_SIZE 字节，但在一次调用期间不能读取更多。

您的程序显然缺少一个干净的解析器。您应该对您正在阅读的内容做出适当的反应，这可能是信息性消息、警告、错误或其他任何内容。

如果你想为FTP 协议滚动你自己的解析器，我能给你的最好建议是查看State Machine Pattern，然后查看Lexical Analyzer 和properly built parsers 以便执行所以。

通过这种方式，您最终将能够构建功能强大且健壮的网络应用程序。