C windows 套接字在第一次执行时永远阻塞

Question

我正在编写一个 C 软件以通过 UDP 与电机控制器进行交互。我正在使用：

Win 7 专业版 64 位
日食月神
minGW

目前我有一个问题，似乎与 socket/wsa 有关：我的程序的执行永远停留在 recvfrom() 上（所以显然控制器没有按预期响应）。使用附带的软件，这只会在第一次执行时发生（或在未执行大约 3 分钟后），其他程序连续 3-4 次出现此问题。查看 wireshark 发现，第一个（或暂停 3 分钟后）导致传输 "ARP" 包而不是我的 UDP 消息。这是为什么呢（好像是在寻找目的地）？我该如何避免 "crashing" 我的软件因此而受到影响？

我是不是忘记初始化什么了？

我的代码如下所示：

#include <stdio.h>
#include <conio.h>
#include <winsock2.h>

#define Y_AXIS_IP "XXX.XXX.XXX.XX"
#define Y_AXIS_PORT 880X

int startWinsock(void);

int main() {
//Start the winsocket (needed to create sockets)
long ws = startWinsock();
if (ws) {
    printf("ERROR: Failed to init Winsock API! Code: %ld\n", ws);
    getch();
    exit(EXIT_FAILURE);
}
//Create an UDP Socket
SOCKET UDPsocket = socket(AF_INET, SOCK_DGRAM, 0);
if (UDPsocket == INVALID_SOCKET) {
    printf("ERROR: Socket could not be created! Code: %d\n",
            WSAGetLastError());
    getch();
    exit(EXIT_FAILURE);
}
//Create a struct to use with the socket (this gives information about type (AF_INET = Internet Protocol) which port and which IP to use)
SOCKADDR_IN addrY;
memset(&addrY, 0, sizeof(addrY));
addrY.sin_family = AF_INET; //Assert Type
addrY.sin_port = htons(Y_AXIS_PORT);    //Assert Port
addrY.sin_addr.S_un.S_addr = inet_addr(Y_AXIS_IP);  //assert IP Address

char message[] = "0000MTX     00000000OR:1:000F\r";

int buffersize = 100;
char *recvbuf = malloc(buffersize); //None of the replys can get larger than 100 chars

if (recvbuf == NULL) {
    printf("Out of memory!\n");
    getch();
    exit(EXIT_FAILURE);
}

//clear the receive buffer and prepare the address size
memset(recvbuf, '[=11=]', buffersize);
int addrsize = sizeof(addrY);

//Send the message to the device
if (sendto(UDPsocket, message, strlen(message), 0,
        (struct sockaddr *) &addrY, sizeof(addrY)) == SOCKET_ERROR) {
    printf("sendto() failed with error code : %d", WSAGetLastError());
    getch();
    exit(EXIT_FAILURE);
}

//Receive from device (blocks program until recv event)
if (recvfrom(UDPsocket, recvbuf, buffersize, 0, (struct sockaddr *) &addrY,
        &addrsize) == SOCKET_ERROR) {
    //If not timed out Display the Error
    printf("recvfrom() failed with error code : %d", WSAGetLastError());
    getch();
    exit(EXIT_FAILURE);
}

printf("%s\n", recvbuf);
getch();

free(recvbuf);

return 0;
}

int startWinsock(void) {
WSADATA wsa;
return WSAStartup(MAKEWORD(2, 2), &wsa);
}

如果您有任何想法或建议，我将非常高兴。提前致谢！

Answer 1

在计算机可以将数据包发送到另一台主机之前，它必须解析其 MAC 地址。这是通过 ARP 请求完成的。它由操作系统透明地处理。
recvfrom 会阻塞。这不是错误。当没有传入数据时，基本上有三种方法可以避免 recvfrom 永远阻塞：
1. 使套接字异步，即非阻塞
2. 设置超时（参见 Set timeout for winsock recvfrom）
3. 使用 select (Setting winsock select timeout accurately)

Answer 2

所以我终于找到了解决这个问题的方法：

1. catch the timeout that occurs at recvfrom
2. save the sockets address in a temporary socket
3. close the original socket
4. end the WSA (call WSACleanup())
5. start the WSA (call WSAStartup())
6. create a new socket at the address of the former socket
7. transmit the message again

似乎工作正常（另见下面的代码）

如果您发现我的代码有任何错误或危险，请随时发表评论并帮助我提高技能。

感谢您的帮助和想法，

塞巴斯蒂安

代码：

if (recvfrom(*UDPsocket, buffer, buffersize, 0, (struct sockaddr *) &addr,
        &addrsize) == SOCKET_ERROR) {

    int WSAerror = WSAGetLastError();

    if (WSAerror == 10060) {

        printf("[WARNING] recvfrom() timed out!\n");
        //Store the address of the socket
        SOCKET *tempsocket = NULL;
        tempsocket = UDPsocket;
        //destroy the old socket
        closesocket(*UDPsocket);
        WSACleanup();
        //create a new socket
        startWinsock();
        *tempsocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

        //Send the message to the device
        if (sendto(*tempsocket, Frame.pcompletemsg,
                strlen(Frame.pcompletemsg), 0, (struct sockaddr *) &addr,
                sizeof(addr)) == SOCKET_ERROR) {
            printf("[ERROR] sendto() failed with error code : %d\n",
                    WSAGetLastError());
            getch();
            WSACleanup();
            exit(EXIT_FAILURE);
        }
        if (recvfrom(*tempsocket, buffer, buffersize, 0,
                (struct sockaddr *) &addr, &addrsize) == SOCKET_ERROR) {

            int WSAerror = WSAGetLastError();
            printf("[ERROR] recvfrom() failed with error code : %d",
                    WSAerror);
            getch();
            WSACleanup();
            exit(EXIT_FAILURE);
        }

    } else {
        printf("[ERROR] recvfrom() failed with error code : %d", WSAerror);
        getch();
        WSACleanup();
        exit(EXIT_FAILURE);
    }
}

C windows 套接字在第一次执行时永远阻塞

C windows socket forever blocked on first execution

c

sockets

windows

winsock