SDL_GetRowStretchCode, SDL_NewRowStretchCode — (/vol/8/src/cvs/workspace/SDL_stretch/SDL_stretchcode.h)
TheRowStretchCode is a shared buffer between Stretch-routines that use no extra buffer-argument. You should call SDL_SetRowStretchCode to fill this internal buffer and set a "call"-operation for your target cpu to execute this static buffer. That is done for effiency as the RowStretch is often called in a tight loop for each Row in a rectengular stretch and it is best to not use a variable argument with an indirect call or a function call that would build up a callframe and release that callframe later.
If you do not need that effiency, use PutRowStretchCode and RunRowStretchCode which are also good in a multithreading environment. To allocate a new buffer for usage with Put/Run you can use the NewRowStretchCode routine which is also used on NX machines (e.g. AMD64) where the data segment is set to be not-executable (in that case it will allocate from heap and use mprotect). if the argument is 0 then a buffer of the default size is allocated. If the buffer allocation (or mprotect) fails it will return NULL and SDL_SetError.