将全局变量成员的指针发送到 WinHttpQueryHeaders (WinHTTP API) 不会改变它的值

Question

我有以下代码片段：

        // Using HttpQueryInfo to obtain the size of the buffer into dwSize.
        if (!WinHttpQueryHeaders(hRequest,
            WINHTTP_QUERY_RAW_HEADERS_CRLF,
            WINHTTP_HEADER_NAME_BY_INDEX, NULL, &st.dwSize, WINHTTP_NO_HEADER_INDEX))
        {
            // An ERROR_INSUFFICIENT_BUFFER is expected because you
            // are looking for the size of the headers.  If any other
            // error is encountered, display error information.
            DWORD dwErr = GetLastError();
            if (dwErr != ERROR_INSUFFICIENT_BUFFER)
            {
                DEBUG_PRINT(("Error %d encountered.", dwErr));
                return;
            } else {
                // enters here and prints '0' (initial value)
                DEBUG_PRINT(("size of buffer: ", &st.dwSize));
            }
        }

而 st 是具有成员 dwSize 的全局对象。

当我运行这部分处于调试模式时，我看到 st.dwSize 在调用 WinHttpQueryHeaders 后没有改变它的值。

但是如果我创建一个本地变量 DWORD dwSize = 0 并将 &dwSize 发送到 WinHttpQueryHeaders，它会获取缓冲区大小并成功更改其值。

为什么我不应该将全局对象成员的指针发送到 WinHttpQueryHeaders 或任何其他外部 API 函数？

Answer 1

WinHttpQueryHeaders 没有成功更改的原因 st.dwSize 是因为我将 st declered 作为 static 全局变量。

static WinHttpSubtransport st;

如Scope rules of the "persistent" variables in C中所写：

A static global variable is a global variable that can only be accessed by functions in the same C program file as the variable.

将全局变量成员的指针发送到 WinHttpQueryHeaders (WinHTTP API) 不会改变它的值

sending a pointer of global variable's member to WinHttpQueryHeaders (WinHTTP API) does not change it's value

winhttp

c++11