get_starting_head
head = get_starting_head(shead, HDR_SZ * 2, GET_SF_RESV_SIZE(sf), NULL);
head = get_starting_head(shead, need_sz, resv_sz, &offset);
head = get_starting_head(shead, HDR_SZ, GET_SF_RESV_SIZE(sf), NULL);
head = get_starting_head(shead, sizeof(struct fpsimd_context) + HDR_SZ,
struct _aarch64_ctx *get_starting_head(struct _aarch64_ctx *shead,