Informix Error -438

Reasons / Common Causes

-438 is the mirror-image restriction of -436: when the time-out value is -1 (indefinite wait, or effectively "unregister"), the callback function argument must be NULL — a non-null function pointer combined with -1 is rejected, just as a bounded time-out with a missing callback is rejected by -436.

1. A callback function pointer supplied alongside a time-out of -1 — the direct, only cause, typically when unregistering a callback.
2. Application code that reuses a previous bounded-time-out registration call's arguments when actually intending to unregister (switch to -1), without also clearing the callback argument to NULL.

Solutions / Resolution

1. Pass NULL for the callback function whenever the time-out value is -1, per the official guidance.
2. When unregistering a callback (switching to -1), clear the callback argument to NULL in the same change, rather than reusing a prior bounded-time-out call's arguments.

Examples

Correctly unregistering with NULL

int timeout = -1;
/* register_callback(timeout, NULL); */

The disallowed combination

int timeout = -1;
/* register_callback(timeout, callback_fn); */
/* -438: callback_fn must be NULL when timeout is -1 */

Diagnostic Checks

1. Check whether the callback argument is NULL whenever the time-out value is -1.

Related Errors / Related Topics

• -436 — "Call back function must be defined when time-out value is 0 or greater." The mirror-image restriction — together these two errors define the valid combinations for this time-out/callback mechanism.
• -435 — "Time-out value must be -1 or greater." The base restriction on the time-out argument's valid range.

Time-out -1 always pairs with a NULL callback — clear the callback argument explicitly when switching a registration to indefinite wait / unregistering.