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Asked: 2026-05-11T19:37:42+00:00

I’m trying to list available interfaces using the WSAIoctl function. I have to pass

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I’m trying to list available interfaces using the WSAIoctl function. I have to pass in a buffer to hold the complete list. I want to get a count of the interfaces before I allocate memory to hold the interface details but if I pass in a NULL pointer the call just fails (I dont get a valid count returned). Any way to get this count befor I have to allocate memory?

Background is that I am starting a load of processes/threads on on Windows machine which all connect to a single server. I want the server to see these individual connections as coming from different IP addresses and I have added a load of aliases to the test machine to allow for this (lots). The WSAIoct does correctly find all the ones I added.

Cheers…

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1 Answer

  1. Editorial Team

    From the msdn documentation for WSAIoctl:

    Note: If the output buffer is not
    large enough to contain the address
    list, SOCKET_ERROR is returned as the
    result of this IOCTL and
    WSAGetLastError returns WSAEFAULT. The
    required size, in bytes, for the
    output buffer is returned in the
    lpcbBytesReturned parameter in this
    case. Note the WSAEFAULT error code is
    also returned if the lpvInBuffer,
    lpvOutBuffer, or lpcbBytesReturned
    parameter is not completely contained
    in a valid part of the user address
    space.

    So you have to call the WSAIoctl function twice. The first time with an arbitrary buffer and then check for the error codes mentioned in the documentation. Then use the the size returned in lpcbBytesReturned to allocate the buffer and call the WSAIoctl function a second time.

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