Question

This is the working original code:

// ...
unsigned __int64 num = 57;
sprintf_s(buffer, sizeof(buffer), "%llu", num);

However, when I try to extract this part into this function:

void addBuffered(void** attributeValue, char* format)
{
    sprintf_s(buffer, sizeof(buffer), format, *attributeValue);
}

by calling:

addBuffered((void**)&num, "%d");

I have to change the format parameter in sprintf_s from %llu to %d to get the correct value. Can somebody explain why this happens and if the change of the parameter to %d can be a problem? Thanks!

Answer 1

This happens because sizeof(void *) == 4 in your case. And you implicitly cast __int64 to void * by function call. So if you use %llu format, you print some garbage from memory.

I suggest you to rewrite function if it is possible:

template <typename T>
void addBuffered(T *attributeValue, char* format)
{
    sprintf_s(buffer, sizeof(buffer), format, *attributeValue);
}

call example:

addBuffered(&num, "%luu");