Function drmgr_insert_write_cls_field 

pub unsafe extern "C" fn drmgr_insert_write_cls_field(
    drcontext: *mut c_void,
    idx: c_int,
    ilist: *mut instrlist_t,
    where_: *mut instr_t,
    reg: reg_id_t,
    scratch: reg_id_t,
) -> bool_

Inserts into \p ilist prior to \p where meta-instruction(s) to write the general-purpose full-size register \p reg to the user-controlled drcontext field for the current (at execution time) callback and index. Writes to the same field as drmgr_set_cls_field(). The register \p scratch will be overwritten. \return whether successful.