How to get socket port number if it's system assigned

启动服务端,让系统自动分配端口后,并把这个端口号告诉客户端,以便于客户端连接.

 

在bind(...)函数后使用getsockname(...)就可以获取此端口号, 代码如下:

 

#include <iostream>
#include <windows.h>
  
using namespace std;
  
#define  PORT 0
#define  IP_ADDRESS "127.0.0.1"


#if 1
int WINAPI WinMain(          HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR lpCmdLine,
    int nCmdShow)
#else
int main(int argc, char * argv)
#endif

{
    WSADATA  wd;
    SOCKET theSock;
    struct sockaddr_in theSockAddr;
    int ret = 0;
    int addrLen = 0;

    //Init Windows Socket
    if ( WSAStartup(MAKEWORD(2,2), &wd) != 0 )
    {
        cout << "Init Windows Socket Failed::" << GetLastError() << endl;
        return -1;
    }

    //Create Socket
    theSock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if ( theSock == INVALID_SOCKET )
    {
        cout << "Create Socket Failed::" << GetLastError() << endl;
        return -1;
    }

    theSockAddr.sin_family = AF_INET;
    theSockAddr.sin_addr.s_addr = inet_addr(IP_ADDRESS);
    theSockAddr.sin_port = htons(PORT);
    memset(theSockAddr.sin_zero, 0x00, 8);

    //Bind Socket
    ret = bind(theSock, (struct sockaddr*)&theSockAddr, sizeof(theSockAddr));
    if ( ret != 0 )
    {
        cout << "Bind Socket Failed::" << GetLastError() << endl;
        return -1;
    }

    // Get bind port number
    {
        struct sockaddr_in anotherSockAddr;
        int len = sizeof(anotherSockAddr);
        getsockname(theSock,(sockaddr *)&anotherSockAddr,&len);

        unsigned short newPort = ntohs(anotherSockAddr.sin_port);
        cout << "Bind Socket value is " << newPort << endl;
    }
   
    closesocket(theSock);
    WSACleanup();
    return 0;
}