c - 使用 gethostbyname 时出现段错误

Question

我正在尝试创建一个代理服务器，它接受来自客户端的请求，并在收到来自服务器的消息后将响应发送回客户端。但程序出现段错误。

我的代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
int main(int argc,char *argv[])
{
    struct sockaddr_in server,client;
    struct hostent *host;
    int s1,s2,s3,len;
    int port;
    char ip[100];
    char path[100];
    int p;
    int n;
    char buffer[500];
    char site[500];
    port = atoi(argv[1]);

    bzero((char *)&server,sizeof(server));
    server.sin_port = htons(port);
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_family = AF_INET;
    s1 = socket(AF_INET,SOCK_STREAM,0);

    if(bind(s1,(struct sockaddr *)&server,sizeof(server)) == -1) {
        perror("can't bind");
        exit(1);
    }

    if(listen(s1,10) == -1) {
        perror("coudnt't listen");
        exit(1);
    }
    len = sizeof(client);
    s2 = accept(s1,(struct sockaddr *)&client,&len);
    if(s2 == -1) {
        perror("coudnt aceept");
        exit(1);
    }
    memset(buffer,0,sizeof(buffer));
    n = recv(s2,buffer,500,0);
    if(n < 0) {
        perror("not recieved");
        exit(1);
    }

    sscanf(buffer, "http://%99[^:]:%99d/%99[^\n]", ip, &p, path);
    bzero((char *)&client,sizeof(client));
    client.sin_port = htons(80);
    host = gethostbyname(ip);
    bcopy((char *)host->h_addr,(char *)&client.sin_addr.s_addr,host->h_length);
    client.sin_family = AF_INET;
    s3 = socket(AF_INET,SOCK_STREAM,0);


    if(connect(s3,(struct sockaddr *)&client,sizeof(client)) == -1) {
        perror("can't connect\n");
        exit(1);
    }
    sprintf(site,"GET http://%s/ HTTP/1.0\n\n",ip);
    n = send(s3,site,strlen(site),0);
    if(n < 0) {
        perror("message not sent");
        exit(1);
    }
    while(1) {
        memset(site,0,sizeof(site));
        n = recv(s3,site,500,0);
        if(n < 0) {
            perror("coudnot read");
            exit(1);
        }
        site[n] = '\0';
        send(s2,site,strlen(site),0);
    }
    close(s1);
    close(s2);
    return 0;

}

错误似乎来自gethostbyname(ip)因为当我使用gethostbyname("www.iiita.ac.in")时，它有效。

Answer 1

gethostbyname 在发生错误时返回空指针，因此 host->h_addr 是空取消引用。

使用前需要检查host是否为NULL。

参见http://man7.org/linux/man-pages/man3/gethostbyname.3.html