Why does WTSFreeMemoryExA always return ERROR_INVALID_PARAMETER when passed a WTSTypeClass of WTSTypeSessionInfoLevel1?

Question

According to the documentation, WTSFreeMemoryExA can be used to free a WTS_SESSION_INFO_1A structure by passing a WTS_TYPE_CLASS of WTSTypeSessionInfoLevel1. However, any attempt to do so fails with error code 87 (ERROR_INVALID_PARAMETER, "The parameter is incorrect").

How to get WTSFreeMemoryExA to work?

Answer 1

This appears to be a bug in Windows (at least in Windows 10 version 2004). Contrary to the documentation, the WTSFreeMemoryExA function does not accept WTSTypeSessionInfoLevel1, whereas WTSFreeMemoryExW does. This means that instead of using the WTSEnumerateSessionsExA function which returns WTS_SESSION_INFO_1A structures, you need to instead use the WTSEnumerateSessionsExW function which returns WTS_SESSION_INFO_1W.

This bug effectively makes WTSEnumerateSessionsExA unusable, unless you don't care about the memory leak caused by the inability to free its results. This bug appears to have been known about for some time. (Hopefully, some day, Microsoft will fix this.)

Some reports claim that even using WTSEnumerateSessionsExW and WTSFreeMemoryExW appears to leak memory, which implies that WTSEnumerateSessions combined with WTSQuerySessionInformation may be the better approach. However, I myself have been unable to reproduce that issue. I suspect it was a real issue at one point, but has been fixed by Microsoft in more recent Windows versions.

Answer 2

thank you for raising this question.

We checked the relevant source code and found the source code related to WTSFreeMemoryA. It accepts the first parameter WTSTypeClass as WTSTypeProcessInfoLevel0 or WTSTypeProcessInfoLevel1, but it doesn’t accept the value WTSTypeSessionInfoLevel1 and therefore return the ERROR_INVALID_PARAMETER error on this call.

This is different from the description in the document, we will submit this issue. And you can try to use WTSFreeMemoryW to avoid this issue.