pre-existing shared memory block (key n, ID n) is still in use

Symptoms

A statement failed with SQLSTATE F0001 (lock_file_exists), reported at severity FATAL. This is a Configuration File Error condition: PostgreSQL emits the message pre-existing shared memory block (key n, ID n) is still in use.

• The client receives SQLSTATE F0001 (lock file exists).
• The operation is rejected at FATAL level; the statement does not complete.

What the server log shows

FATAL:  pre-existing shared memory block (key n, ID n) is still in use
HINT:  Terminate any old server processes associated with data directory "…".

Why PostgreSQL raises this

Class F0 (Configuration File Error) is raised when a configuration file is malformed or a parameter value is invalid.

As described in PostgreSQL’s Section 19.1 Setting Parameters and Appendix A (PostgreSQL Error Codes), SQLSTATE F0001 carries the condition name lock_file_exists in class Configuration File Error. (Paraphrased — see the linked reference for the exact wording.)

Common causes

• A syntax error in postgresql.conf or an included file.
• An invalid or out-of-range parameter value.
• A parameter that cannot be set in the given context.

How to fix it

1. Fix the offending line and reload (pg_ctl reload).
2. Validate parameter values against the documented range.
3. Check the log for the exact file and line reported.

Version applicability

This message is present in PostgreSQL 15, 16, 17, 18 and 19.

Related & next steps

• SQLSTATE F0001 — Lock File Exists (full reference)
• PostgreSQL error code index

Reference: PostgreSQL Section 19.1 Setting Parameters.