Informix Error -1258: Cannot attach to shared memory used to communicate with back end.

Reasons / Common Causes

-1258 is flagged by the official text as an internal error indicating a failure to connect to the database server's shared memory — per the official guidance, this is closely related to -1251/-1252's shared-memory-creation failures, but at the attach step rather than creation.

1. A system limit on shared-memory attachments reached, per the official guidance's general framing — no more specific application-level cause is named.
2. A local system problem (permissions, stale shared-memory segments) preventing the attach.

Solutions / Resolution

1. Check for operating-system messages that may provide additional detail, per the official guidance.
2. Verify that no system limits or local problems exist, per the official guidance.
3. Contact IBM Informix Technical Support with full documentation of the circumstances, per the official guidance.

Examples

Checking shared-memory segment state

$ ipcs -m

Diagnostic Checks

1. Check shared-memory limits and existing segments for anything stale or conflicting.
2. Check the operating-system error log for the specific attach failure reason.

Related Errors / Related Topics

• -1251 — "Unable to create shared memory. semget failed." A related shared-memory error, at the creation step rather than attach.
• -1252 — "Unable to create shared memory. shmget failed." Another related shared-memory creation error.
• -1257 — "Operating system cannot fork process for back end." A related back-end connection error, about process creation rather than shared memory.

Couldn't attach to the back end's shared memory — check OS-level limits and existing segments, and escalate to IBM Informix Technical Support if nothing local explains it.