替换gethostbyname

gethostbyname

为了获取域名的ip地址，我写了如下代码

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std::string host2ip(const std::string& host) {
    if (host.empty()) return {};
    std::regex valid_ipv4_re(R"(^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$)");
    std::smatch ipv4_match;
    if (std::regex_match(host, ipv4_match, valid_ipv4_re)) {
        return host;
    }
    struct hostent *server = gethostbyname(host.c_str());
    if (!server) {
        LOG(ERROR) << "gethostbyname fail, host: " << host;
        return {};
    }
    char addr[128];
    inet_ntop(server->h_addrtype, server->h_addr, addr, sizeof(addr));
    return addr;
}

如果参数是个ipv4地址，则直接返回，否则通过gethostbyname函数查询。

服务运行了一个半月左右，登上机器一看发现有两次coredump（有脚本辅助重启，客户端没感知到），堆栈如下

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#0  inet_ntop4 (size=128, dst=0x7f864afcc870 "\020", src=0x0) at ./resolv/inet_ntop.c:85
#1  __GI_inet_ntop (af=2, src=0x0, dst=0x7f864afcc870 "\020", size=128) at ./resolv/inet_ntop.c:57
#2  0x000055f656081cab in liph::host2ip(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
    ()

src参数为null，再查看下gethostbyname函数手册，发现是不可重入的。可重入版本是gethostbyname_r，但是POSIX规范中这些函数都被标记为deprecated，我们有更好的选择。

getaddrinfo

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#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>

struct addrinfo {
    int ai_flags;
    int ai_family;
    int ai_socktype;
    int ai_protocol;
    socklen_t ai_addrlen;
    struct sockaddr *ai_addr;
    char *ai_canonname;
    struct addrinfo *ai_next;
};

int getaddrinfo(const char *restrict node, const char *restrict service, const struct addrinfo *restrict hints, struct addrinfo **restrict res); // 线程安全
void freeaddrinfo(struct addrinfo *res);
const char *gai_strerror(int errcode);

替换如下

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std::string host2ip(const std::string& host) {
    struct addrinfo hints;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;  // IPv4 or IPv6
    struct addrinfo *result = nullptr;
    int r = getaddrinfo(host.c_str(), nullptr, &hints, &result);
    if (r != 0) {
        LOG(ERROR) << "host: " << host;
        LOG(ERROR) << "getaddrinfo fail: " << gai_strerror(r);
        return {};
    }
    std::unique_ptr<struct addrinfo, decltype([](struct addrinfo *ptr) { if (ptr) freeaddrinfo(ptr); })> res(result);
    if (res) {  // do not need ai_next
        void *addr = nullptr;
        switch (res->ai_family) {
        case AF_INET:
            addr = &(reinterpret_cast<struct sockaddr_in *>(res->ai_addr)->sin_addr);
            break;
        case AF_INET6:
            addr = &(reinterpret_cast<struct sockaddr_in6 *>(res->ai_addr)->sin6_addr);
            break;
        default:
            return {};
        }
        if (addr) {
            char buffer[128];
            inet_ntop(res->ai_family, addr, buffer, sizeof(buffer));
            return buffer;
        }
        return {};
    }
    return {};
}