MySQL cannot perform the fcntl() file-control call on a network socket, interrupting the client-server connection.
MySQL Error 1155: ER_NET_FCNTL_ERROR arises when the server fails to run the fcntl() system call on a socket, breaking the connection. Check OS file-descriptor limits, firewall rules, and TCP keepalive settings, then restart MySQL to restore stable connectivity.
Got an error from fcntl()
The error appears when MySQL invokes the fcntl() system call on a socket or file descriptor and receives a non-zero errno. MySQL translates that failure into SQLSTATE 08S01 (communication link failure) and returns error 1155 to the client.
The problem lies outside SQL syntax.
It reflects an operating-system or network-layer issue that prevents MySQL from updating socket flags, usually O_NONBLOCK or FD_CLOEXEC.
Error 1155 typically surfaces during connection establishment, replication handshakes, or long-running queries that exhaust file descriptors.
It can also show up in high-concurrency workloads where MySQL opens and closes thousands of sockets per second.
Because the call fails deep in the network stack, clients often experience sudden disconnects with the same message echoed in the MySQL error log.
Unaddressed, the error breaks client sessions, stalls replication, and triggers application downtime.
Continuous retries increase server load and may mask the root cause, leading to cascading failures under traffic spikes.
Operating-system limits, firewall interference, abrupt socket closure, or kernel bugs frequently trigger fcntl() failures. The underlying errno provides the exact reason, such as EMFILE or EBADF.
Identify the errno in the MySQL error log, raise ulimit -n, inspect SELinux/AppArmor rules, and validate firewall timeouts.
Restart MySQL after adjustments to clear lingering sockets.
High-traffic APIs often hit file-descriptor exhaustion. Batch jobs with many short-lived connections encounter FD leaks. Container platforms mis-configured on low open-file limits trigger the error at startup.
Set open-file-limit in my.cnf, monitor with Prometheus exporters, and enable connection pooling.
Galaxy’s editor highlights failing queries and allows quick parameter tweaks to reproduce and validate fixes.
Errors 1158 (ER_NET_PACKET_TOO_LARGE) and 2006 (MySQL server has gone away) also indicate network failures but stem from packet size or timeout issues, not fcntl() system calls.
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No data is lost on the server, but uncommitted transactions from disconnected clients roll back.
Check the MySQL error log; the line with “fcntl() failed” includes errno in parentheses.
You can raise ulimit live on some systems, but a restart guarantees MySQL adopts new limits.
Galaxy surfaces connection errors instantly, letting engineers test higher open_files_limit values and share fixes via Collections.
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