I have one question I can not find an answer to.

sword OCIEnvCreate ( OCIEnv **envhpp,
ub4 mode,
CONST dvoid *ctxp,
CONST dvoid *(*malocfp)
(dvoid *ctxp,
size_t size),
CONST dvoid *(*ralocfp)
(dvoid *ctxp,
dvoid *memptr,
size_t newsize),
CONST void (*mfreefp)
(dvoid *ctxp,
dvoid *memptr))
size_t xtramemsz,
dvoid **usrmempp );

What is the signifigance of xtramemsz and usrmempp? The burning question is how do I properly determine what I should pass in for xtramemsz?