EC
2039
MySQLERRORNotableConnectionHIGH confidence

Error from fcntl() on network socket

Production Risk

Medium — connections may fail under resource pressure.

What this means

MySQL encountered an error from the fcntl() system call when setting socket flags (e.g., non-blocking mode). This is typically an OS-level issue.

Why it happens
  1. 1OS-level file descriptor limit reached (too many open files).
  2. 2System call interrupted by signal.
  3. 3Kernel bug or resource exhaustion affecting socket operations.
How to reproduce
trigger — this will error
trigger — this will error
-- Observed in MySQL error log; no specific SQL triggers it

expected output

ERROR 2039 (HY000): Got an error from fcntl().

Fix 1

Increase the open file limit for the MySQL process

Increase the open file limit for the MySQL process
-- In /etc/security/limits.conf:
mysql soft nofile 65535
mysql hard nofile 65535

Why this works

Higher file descriptor limits prevent EMFILE errors from fcntl().

Fix 2

Review the MySQL error log for the underlying errno

Review the MySQL error log for the underlying errno
-- Check error log for errno after the fcntl error

Why this works

The errno value identifies the specific OS-level failure.

Sources
Official documentation ↗

MySQL 8.0 — 2039 ER_NET_FCNTL_ERROR

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