C gethostbyname 段错误

Question

我正在尝试修改 Linux tracepath (traceroute) 程序以使其处理三个不同的 ip 地址。这三个 ip 地址是:

1. "11.11.11.11";
2. "144.133.133.133";
3. “202.202.202.202”；

应该将它们转换为套接字地址结构和无符号整数值等，然后再转换回字符串，但我一直遇到段错误。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/types.h>
#include <linux/errqueue.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <netinet/in.h>
//#include <resolv.h>
#include <sys/time.h>
#include <sys/uio.h>
#include <arpa/inet.h>
#include <getopt.h> 

char target_ip_string[INET_ADDRSTRLEN] = {0}; // first address  written out string
char alternate_ip_1[INET_ADDRSTRLEN] = {0}; // second address  written out string
char alternate_ip_2[INET_ADDRSTRLEN] = {0}; // third address  written out string

struct sockaddr_in target_host; // first address
struct sockaddr_in target_alt1; // second address
struct sockaddr_in target_alt2; // third address

/**
 * This example should convert three distict ip addresses from three distinct 
 * strings to three distinct integer values.
 */
int
main(int argc, char **argv) {
    setvbuf(stdout, NULL, _IONBF, 0); // auto flushing.

    struct hostent *hostname_target_entry;
    struct hostent *hostname_alt1_entry;
    struct hostent *hostname_alt2_entry;

    target_host.sin_family = AF_INET;
    target_alt1.sin_family = AF_INET;
    target_alt2.sin_family = AF_INET;

    char * target_hostname = (char *) "11.11.11.11";
    char * alternate_ip_1_local = (char *) "144.133.133.133";
    char * alternate_ip_2_local = (char *) "202.202.202.202";

    printf("The first ip is: %s \n", target_hostname);
    printf("The second ip is: %s \n", alternate_ip_1_local);
    printf("The third ip is: %s \n\n", alternate_ip_2_local);

    hostname_target_entry = gethostbyname(target_hostname);
    hostname_alt1_entry = gethostbyname(alternate_ip_1_local);
    hostname_alt2_entry = gethostbyname(alternate_ip_2_local);

    memcpy(&target_host.sin_addr, hostname_target_entry->h_addr_list[0], 4);
    memcpy(&target_alt1.sin_addr, hostname_alt1_entry->h_addr_list[0], 4);
    memcpy(&target_alt2.sin_addr, hostname_alt2_entry->h_addr_list[0], 4);

    // These should be three different ip addresses
    printf("First ip as an integer: %u \n", target_host.sin_addr.s_addr); // Why are these all the same?
    printf("Second ip as an integer: %u \n", target_alt1.sin_addr.s_addr); // Why are these all the same?
    printf("Third ip as an integer: %u \n\n", target_alt2.sin_addr.s_addr); // Why are these all the same?

    printf("No seg fault yet. \n\n");

    // This conversion should yield "11.11.11.11"
    const char * conversion1 = inet_ntop(AF_INET, &(target_host.sin_addr), target_ip_string, INET_ADDRSTRLEN);

    printf("Result of inet_ntop: %s \n", conversion1); // Why is this "202.202.202.202" and not "11.11.11.11"?

    printf("Seg fault here. \n\n");

    // This conversion should yield "144.133.133.133".
    const char * conversion2 = inet_ntop(AF_INET, &(target_alt1.sin_addr), alternate_ip_1, INET_ADDRSTRLEN);

    // This conversion should yield "202.202.202.202".
    const char * conversion3 = inet_ntop(AF_INET, &(target_alt2.sin_addr), alternate_ip_2, INET_ADDRSTRLEN);

    return 0;
}

终端输出:

The first ip is: 11.11.11.11 
The second ip is: 144.133.133.133 
The third ip is: 202.202.202.202 

First ip as an integer: 3402287818 
Second ip as an integer: 3402287818 
Third ip as an integer: 3402287818 

No seg fault yet. 

Result of inet_ntop: 202.202.202.202 
Seg fault here. 

为什么我在 string 和 hostent/sockaddr_in 之间的转换总是失败？

Answer 1

gethostbyname 函数返回一个指向静态数据的指针，因此如果您不复制给定调用的结果，它将被下一次调用覆盖。

由于您只转换 IP 地址而不转换主机名，因此最好改用 inet_addr:

target_host.sin_addr.s_addr = inet_addr("11.11.11.11");
target_alt1.sin_addr.s_addr = inet_addr("144.133.133.133");
target_alt2.sin_addr.s_addr = inet_addr("202.202.202.202");