C- addrinfo -指针分割错误

Question

C- addrinfo -指针分割错误

我有一个分割错误；我做了一个奇怪的修复，但我仍然很担心，因为我不知道问题是什么。任何洞察力都是惊人的！在最后一个代码块中工作“修复”！

试图使用target_addrinfo绑定时发生的分段错误：

struct addrinfo *my_server_res;
struct addrinfo *their_server_res;

int main(int argc, char **argv) {
//........
    set_addrinfo();
    handle_binding();
}

void set_addrinfo() {
    int status;
    struct addrinfo hints;
    struct addrinfo *target_addrinfo; // fix (1/3): **target_addrinfo
    char ipstr[INET6_ADDRSTRLEN];

    // THIS IS TRUE, takes my_server_res
    // fix(2/3): (l ? &my_server_res : &their_server_res)
    target_addrinfo = (l ? my_server_res : their_server_res); 

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    char *node;
    node = (l ? "localhost" : host_name);

    // fix (3/3) 3rd argument: target_addrinfo
    if ((status = getaddrinfo(node, port, &hints, &target_addrinfo)) != 0) {
        fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
        exit(1);
    }

}

void handle_binding() {
    struct addrinfo *target_addrinfo;
    // THIS IS TRUE, takes my_server_res
    target_addrinfo = (l ? my_server_res : their_server_res);
    int b_res = bind(sockfd, target_addrinfo->ai_addr, target_addrinfo->ai_addrlen);
    if (b_res == -1) {
        perror("__Bind");
        exit(1);
    }
}

堆栈跟踪:再次尝试使用my_server_res时出现的问题？：

Program received signal SIGSEGV, Segmentation fault.
0x0000000000401e16 in handle_binding () at ncTh.c:385
385     int b_res = bind(sockfd, target_addrinfo->ai_addr, target_addrinfo->ai_addrlen);
(gdb) backtrace
#0  0x0000000000401e16 in handle_binding () at ncTh.c:385
#1  0x0000000000401cad in setup_socket () at ncTh.c:330
#2  0x0000000000401ad0 in main (argc=6, argv=0x7fffffffdea8) at ncTh.c:266
(gdb) frame 0
#0  0x0000000000401e16 in handle_binding () at ncTh.c:385
385     int b_res = bind(sockfd, target_addrinfo->ai_addr, target_addrinfo->ai_addrlen);
(gdb) 

工作“修复”，更改为set_addrinfo()：

struct addrinfo **target_addrinfo; // was *target_addrinfo
char ipstr[INET6_ADDRSTRLEN];

target_addrinfo = (l ? &my_server_res : &their_server_res); // was my_server_res : their_server_res

memset(&hints, 0, sizeof hints);
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;

char *node;
node = (l ? "localhost" : host_name);
                               // 3rd argument was &target_addrinfo :
if ((status = getaddrinfo(node, port, &hints, target_addrinfo)) != 0) {
    fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
    exit(1);
}

Answer 1

在原代码中：

struct addrinfo *target_addrinfo;
target_addrinfo = (l ? my_server_res : their_server_res);

这会将指针my_server_res的值复制到target_addrinfo中。

getaddrinfo(node, port, &hints, &target_addrinfo)

这会将指向target_addrinfo的指针(指向指针的指针)传递给函数getaddrinfo，然后函数getaddrinfo通过执行等效的*&target_addrinfo = new_ptr为其分配一个新的值。这个新值是指向getaddrinfo正在为您检索的信息的指针。

所有这一切对my_server_res没有任何影响，它仍然保持原值，很可能为NULL。

在新的守则中：

struct addrinfo **target_addrinfo;
target_addrinfo = (l ? &my_server_res : &their_server_res);

这会将指向my_server_res的指针复制到target_addrinfo (现在是指向指针的指针)。

getaddrinfo(node, port, &hints, target_addrinfo)

这会将target_addrinfo (当前是指向my_server_res的指针)的值传递给函数getaddrinfo，然后getaddrinfo通过执行与*target_addrinfo = new_ptr等价的操作为其分配一个新的值。my_server_res现在包含指向getaddrinfo正在为您检索的信息的指针。

Answer 2

您可能会陷入target_addrinfo问题的取消引用，我发布了以下代码。(总比不迟到好：)

struct addrinfo *my_server_res;
struct addrinfo *their_server_res;

struct addrinfo **target_addrinfo; //this should be global 

int main(int argc, char **argv) {
//........
    set_addrinfo();
    handle_binding();
}

void set_addrinfo() {
    int status;
    struct addrinfo hints;

    char ipstr[INET6_ADDRSTRLEN];

    // THIS IS TRUE, takes my_server_res
    target_addrinfo = (l ? &my_server_res : &their_server_res); //set it once here

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    char *node;
    node = (l ? "localhost" : host_name);
    if ((status = getaddrinfo(node, port, &hints, &target_addrinfo)) != 0) {
        fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(status));
        exit(1);
    }

}

void handle_binding() {
    //target_addrinfo should be initialized in set_addrinfo()
    int b_res = bind(sockfd, 
        (*target_addrinfo)->ai_addr,    // don't forget to dereference target_addrinfo, here
        (*target_addrinfo)->ai_addrlen); // and here

    if (b_res == -1) {
        perror("__Bind");
        exit(1);
    }
}