Operating System: Windows XP 64 bit, SP2.

I have an unusual problem. I am porting some code from 32 bit to 64 bit. The 32 bit code works just fine. But when I call CreateThread() for the 64 bit version the call fails. I have three places where this fails. 2 call CreateThread(). 1 calls beginthreadex() which calls CreateThread().

All three calls fail with error code 0x3E6, "Invalid access to memory location".

The problem is all the input parameters are correct.

HANDLE  h;
DWORD   threadID;

h = CreateThread(0,            // default security
                 0,            // default stack size
                 myThreadFunc, // valid function to call
                 myParam,      // my param
                 0,            // no flags, start thread immediately
                 &threadID);

All three calls to CreateThread() are made from a DLL I've injected into the target program at the start of the program execution (this is before the program has got to the start of main()/WinMain()). If I call CreateThread() from the target program (same params) via say a menu, it works. Same parameters etc. Bizarre.

If I pass NULL instead of &threadID, it still fails.

If I pass NULL as myParam, it still fails.

I'm not calling CreateThread from inside DllMain(), so that isn't the problem. I'm confused and searching on Google etc hasn't shown any relevant answers.

If anyone has seen this before or has any ideas, please let me know.

Thanks for reading.