Question

Is there a concrete way to determine the exact buffer size required for a REG_BINARY value for RegQueryValueEx in C? If possible, please provide code demonstrating method.

Answer 1

Do the winAPI double-call dance, which applies to numerous win APIs besides the Reg functions: call RegQueryValueEx once, passing NULL as lpData, and a pointer to lpcpData, args 5 and 6. The function will return ERROR_SUCCESS and fill lpcpData with the required buffer size. Call again with a buffer of that size.

You can also begin with passing a buffer, and if it’s too small the function will return ERROR_MORE_DATA and fill lpcpData with the required buffer size.

Consider the following general pseudo-code for a winapi loop which employs the second approach:

// alloc space for buffer, i.e. buffer is non-NULL

do
{
    winret = SomeWinApi(...args..., buffer, &bufferSize, ...more args...);
    if (winret != ERROR_SUCCESS)
    {
        if (winret == ERROR_MORE_DATA)
            // alloc more space
        else
        {
            // more error handling
            break;
        }
    }
    else bufferIncomplete = FALSE;

} while (bufferIncomplete);

This also handles cases where the buffer might change size between calls, i.e. a shared buffer or a registry key that’s being updated.