c++ - 为什么 select 挂起 => 两个 UDP 服务器(广播)

Question

我正在尝试实现一个服务器，如果在 Wlan 或 eth 中访问(在同一端口(强制))，该服务器将提供不同的数据，因此我使用 select 构建此代码，但即使有数据传入，我的 select 也会无限期挂起(使用仅使用 1 个 udp 服务器的同一应用程序进行测试，并且不会“看到”差异)。知道我做错了什么吗？ (我很绝望)

#include<stdio.h>   
#include<string.h> 
#include<stdlib.h> 
#include<arpa/inet.h>
#include<sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <errno.h>
#include <fstream>
#include <iostream>
#include <vector>

#define BUFLEN 1024
#define PORT 8000   
using namespace std;
char *ipAddrFromInterface(char *apInterfaceName) //this function is not from me
{
    char *if_name = (char *) apInterfaceName;
    struct ifreq ifr;
    size_t if_name_len=strlen(if_name);
    if (if_name_len<sizeof(ifr.ifr_name)) {
        memcpy(ifr.ifr_name,if_name,if_name_len);
        ifr.ifr_name[if_name_len]=0;
    } else {
       printf("interface name is too long\n");
    }int fd=socket(AF_INET,SOCK_DGRAM,0);
    if (fd==-1) {
        printf("A => %s\n",strerror(errno));
    }if (ioctl(fd,SIOCGIFADDR,&ifr)==-1) {
        int temp_errno=errno;
        close(fd);
        printf("B => %s\n",strerror(temp_errno));
    }
    if (ioctl(fd,SIOCGIFADDR,&ifr)==-1) {
        int temp_errno=errno;
        close(fd);
       printf("C => %s\n",strerror(temp_errno));
    }
    close(fd);

    struct sockaddr_in* ipaddr = (struct sockaddr_in*)&ifr.ifr_addr;
    return inet_ntoa(ipaddr->sin_addr);
}

struct socketData
{
    int sock;
    sockaddr_in  socket;
    char *interfaceName;
};

void print(int i)
{
    printf("%d\n", i);
    fflush(stdout);
}

void server_receive_thread(vector<char*> aInterfaceList)
{
    int socketIndex = 0;
    struct sockaddr_in  localSock;
    int socketDescriptor; int socketLength;

    vector<socketData> aSockets;
    for(; socketIndex < aInterfaceList.size(); socketIndex++)
    {
        socketData socketD;
        char *apInterfaceName = aInterfaceList.at(socketIndex);
        if((socketDescriptor = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
        {
            printf("can't listen on interface %s... sleeping\n", apInterfaceName);
        }
        else
        {
            memset(&localSock, 0,  sizeof(localSock));
            localSock.sin_family = AF_INET;
            localSock.sin_port = htons(PORT);

            inet_aton(ipAddrFromInterface(apInterfaceName), &localSock.sin_addr);

            setsockopt(socketDescriptor, SOL_SOCKET, SO_BINDTODEVICE, apInterfaceName, sizeof(apInterfaceName));
            if(bind(socketDescriptor, (struct sockaddr *) &localSock, sizeof(localSock)) == -1)
            {
                printf("can't bind interface %s to listen on port %d... sleeping\n", apInterfaceName, PORT);
            }
            else
            {
                printf("bound to interface %s on port %d\n", apInterfaceName, PORT);
                socketD.sock = socketDescriptor;
                socketD.socket = localSock;
                socketD.interfaceName = apInterfaceName;
                aSockets.push_back(socketD);
            }
        }
    }

            fd_set master;
        int fdMax = 0;
        while(1)
        {
                FD_ZERO(&master);
                for(int iSock = 0; iSock < aSockets.size(); iSock++)
                {
                        socketData d = aSockets.at(iSock);
                        FD_SET(d.sock, &master);
                        if(fdMax = 0 || d.sock > fdMax)
                        {
                                fdMax = d.sock;
                        }
                }
                printf("fdmax is : ");
                print(fdMax);
                if(select(fdMax+1, &master, NULL, NULL, NULL) == -1)
                        print(2);
                print(200);
                for(int iSock = 0; iSock < aSockets.size(); iSock++)
                {
                        socketData d = aSockets.at(iSock);
                        if(FD_ISSET(d.sock, &master))
                        {
                                print(3);
                        }
                }
                print(1);
        }


}

int main()
{
std::vector<char*>  interfaceList;
char *wlan = "wlan0";
interfaceList.push_back("wlan0");
interfaceList.push_back("eth0");
server_receive_thread(interfaceList);
return 0;
}

我正在构建: g++ -lpthread -g 文件名.cpp在树莓派上

谢谢

编辑

(我将打开一个关于它的新线程)好吧，我发现了部分问题，我必须绑定(bind)的不是IP地址，而是广播(255.255.255.255)(很抱歉我之前忘记提到这一点)...但是，虽然这有效，但它无法识别每个接口(interface)，因此应用程序不知道如何解释它

实际上:

#include<stdio.h>   
#include<string.h> 
#include<stdlib.h> 
#include<arpa/inet.h>
#include<sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <errno.h>
#include <fstream>
#include <iostream>
#include <vector>

#define BUFLEN 1024
#define PORT 8000   
using namespace std;
char *ipAddrFromInterface(char *apInterfaceName) //this function is not from me
{
     return "255.255.255.255";
    /*char *if_name = (char *) apInterfaceName;
    struct ifreq ifr;
    size_t if_name_len=strlen(if_name);
    if (if_name_len<sizeof(ifr.ifr_name)) {
        memcpy(ifr.ifr_name,if_name,if_name_len);
        ifr.ifr_name[if_name_len]=0;
    } else {
       printf("interface name is too long\n");
    }int fd=socket(AF_INET,SOCK_DGRAM,0);
    if (fd==-1) {
        printf("A => %s\n",strerror(errno));
    }if (ioctl(fd,SIOCGIFADDR,&ifr)==-1) {
        int temp_errno=errno;
        close(fd);
        printf("B => %s\n",strerror(temp_errno));
    }
    if (ioctl(fd,SIOCGIFADDR,&ifr)==-1) {
        int temp_errno=errno;
        close(fd);
       printf("C => %s\n",strerror(temp_errno));
    }
    close(fd);

    struct sockaddr_in* ipaddr = (struct sockaddr_in*)&ifr.ifr_addr;
    return inet_ntoa(ipaddr->sin_addr);*/
}

struct socketData
{
    int sock;
    sockaddr_in  socket;
    char *interfaceName;
};

void print(int i)
{
    printf("%d\n", i);
    fflush(stdout);
}

void server_receive_thread(vector<char*> aInterfaceList)
{
    int socketIndex = 0;
    struct sockaddr_in  localSock;
    int socketDescriptor; int socketLength;

    vector<socketData> aSockets;
    for(; socketIndex < aInterfaceList.size(); socketIndex++)
    {
        socketData socketD;
        char *apInterfaceName = aInterfaceList.at(socketIndex);
        if((socketDescriptor = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
        {
            printf("can't listen on interface %s... sleeping\n", apInterfaceName);
        }
        else
        {
            memset(&localSock, 0,  sizeof(localSock));
            localSock.sin_family = AF_INET;
            localSock.sin_port = htons(PORT);

            inet_aton(ipAddrFromInterface(apInterfaceName), &localSock.sin_addr);

            setsockopt(socketDescriptor, SOL_SOCKET, SO_BINDTODEVICE, apInterfaceName, sizeof(apInterfaceName));
            if(bind(socketDescriptor, (struct sockaddr *) &localSock, sizeof(localSock)) == -1)
            {
                printf("can't bind interface %s to listen on port %d... sleeping\n", apInterfaceName, PORT);
            }
            else
            {
                printf("bound to interface %s on port %d\n", apInterfaceName, PORT);
                socketD.sock = socketDescriptor;
                socketD.socket = localSock;
                socketD.interfaceName = apInterfaceName;
                aSockets.push_back(socketD);
            }
        }
    }

            fd_set master;
        int fdMax = -1;
        while(1)
        {
                FD_ZERO(&master);
                for(int iSock = 0; iSock < aSockets.size(); iSock++)
                {
                        socketData d = aSockets.at(iSock);
                        FD_SET(d.sock, &master);
                        if(d.sock > fdMax)
                        {
                                fdMax = d.sock;
                        }
                }
                printf("fdmax is : ");
                print(fdMax);
                if(select(fdMax+1, &master, NULL, NULL, NULL) == -1)
                        print(2);
                print(200);
                for(int iSock = 0; iSock < aSockets.size(); iSock++)
                {
                        socketData d = aSockets.at(iSock);
                        if(FD_ISSET(d.sock, &master))
                        {
                                print(3);
                        }
                }
                print(1);
        }


}

int main()
{
std::vector<char*>  interfaceList;
interfaceList.push_back("wlan0");
interfaceList.push_back("eth0");
server_receive_thread(interfaceList);
return 0;
}

Answer 1

嗯，这一行肯定会导致观察到的不当行为:

if(fdMax = 0 || d.sock > fdMax)

由于它使用赋值运算符 (=) 而不是相等测试运算符 (==)，因此每次计算 if 子句时都会将 fdMax 设置回零。这意味着 fdMax 计算不正确，因此太小，因此您的某些套接字未包含在 select 的 watch-sockets-set 中。

您实际上需要做的就是:

if (d.sock > fdMax) fdMax = d.sock;

另一个(小)错误有点早，在这里:

    int fdMax = 0;

应该是:

    int fdMax = -1;

(这是次要的，因为它只会在 aSockets 列表为空的情况下导致实际的不当行为，这可能是不太可能发生的情况，但为了完整起见，最好使其正确)