C Socket 未连接并返回退出值 255

Question

我有一段 C 代码应该连接到 www.google.com 并发出 HTTP GET 请求，但是当我运行它时，它会在“正在连接..”上停留大约 30 秒，然后返回“连接失败”和退出返回值为 255。我做错了什么？

#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define PORT 8000
struct hostent *hostinfo;

int main(void) {
  int sock = 0, valread;
  struct sockaddr_in serv_addr;
  char *hostname = "www.google.com";
  char *request = "GET / HTTP/1.1\r\nHost: www.google.com\r\n\r\n";
  hostinfo = gethostbyname(hostname);
  
  char *ip = inet_ntoa(*(struct in_addr*)hostinfo->h_addr_list[0]);

  char buffer[1024] = {0};
  printf("Creating socket...\n");
  if((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0){
    printf("\n Socket creation error \n");
    return -1;
  }
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_port = htons(PORT);
  printf("Checking address...\n");
  if(inet_pton(AF_INET, ip, &serv_addr.sin_addr) <= 0){
    printf("\n Invalid IP/Address not supported \n");
    return -1;
  }
  printf("Connecting to host %s...\n", ip);
  if(connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0){
    printf("\n Connection Failed \n");
    return -1;
  }
  send(sock, request, strlen(request), 0);
  printf("Message sent\n");
  valread = read(sock, buffer, 1024);
  printf("%s\n", buffer);
  return 0;
}

Answer 1

我看到了两个主要问题。

• 您使用了错误的端口。为http 使用端口80。
• 您的read 和printf 是一个危险的组合，很容易导致访问越界（以及未定义的行为）。您从套接字读取的内容不会以空值终止。您可以改为执行以下操作：

  ...
  printf("Message sent\n");
  while((valread = read(sock, buffer, sizeof(buffer))) > 0) {
    fwrite(buffer, valread, 1, stdout);
  }
  

  但是，当所有内容都已读取时，这将阻塞。查看非阻塞 I/O 或考虑使用 select、epoll 或 poll 等待套接字上的可用数据。

如果您只想获取响应然后断开连接，则可以在服务器发送响应后使用Connection: close 关闭连接。完整代码如下：

#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define PORT 80

int main(void) {
  int sock = 0, valread;
  struct hostent *hostinfo;
  struct sockaddr_in serv_addr;
  const char *hostname = "www.google.com";
  const char *request = "GET / HTTP/1.1\r\n"
                        "Host: www.google.com\r\n"
                        "Connection: close\r\n\r\n";   // <- added
  hostinfo = gethostbyname(hostname);
  
  char *ip = inet_ntoa(*(struct in_addr*)hostinfo->h_addr_list[0]);

  char buffer[1024] = {0};
  printf("Creating socket...\n");
  if((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0){
    printf("\n Socket creation error \n");
    return -1;
  }
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_port = htons(PORT);
  printf("Checking address...\n");
  if(inet_pton(AF_INET, ip, &serv_addr.sin_addr) <= 0){
    printf("\n Invalid IP/Address not supported \n");
    return -1;
  }
  printf("Connecting to host %s...\n", ip);
  if(connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0){
    perror("connect()");
    return -1;
  }
  send(sock, request, strlen(request), 0);
  printf("Message sent\n");
  while((valread = read(sock, buffer, sizeof(buffer))) > 0) {
    fwrite(buffer, valread, 1, stdout);
  }
}