How to fix ERROR 1155: Got error from fcntl() in MySQL

First, determine what the system's open file descriptor limit is set to. Run this command on the MySQL server:

ulimit -n

This shows the current limit. The default is often 1024, which may be too low for production MySQL servers handling many connections.

If the limit is too low, increase it to accommodate more connections. You can temporarily increase it for the current session:

ulimit -n 65536

To make this permanent, edit /etc/security/limits.conf and add:

mysql soft nofile 65536
mysql hard nofile 65536
mysql soft nproc 65536
mysql hard nproc 65536

Then restart the MySQL server for the changes to take effect.

sudo systemctl restart mysql

Review the MySQL error log for more details about when and why the fcntl error occurs. The log location depends on your MySQL configuration, typically at:

/var/log/mysql/error.log

Or check using MySQL:

SHOW VARIABLES LIKE 'log_error';

Look for patterns in the errors to identify if they coincide with high connection counts or specific times.

Ensure the MySQL server socket is properly configured and accessible. Check your MySQL configuration file (/etc/mysql/my.cnf or /etc/my.cnf):

[mysqld]
socket=/var/run/mysqld/mysqld.sock

Verify the socket file exists and has proper permissions:

ls -la /var/run/mysqld/mysqld.sock

The mysql user should own the socket with read/write permissions.

If connections are failing from remote hosts, verify network connectivity:

# Test basic connectivity
ping mysql_server_ip

# Test port accessibility
nc -zv mysql_server_ip 3306

# Or use telnet
telnet mysql_server_ip 3306

If remote connections fail, check MySQL is listening on the correct interface:

SHOW VARIABLES LIKE 'bind_address';

If bound to localhost only, change it to accept remote connections (if appropriate).

Check the current number of connections and the max limit:

SHOW STATUS LIKE 'Threads_connected';
SHOW VARIABLES LIKE 'max_connections';

If connections are consistently near the max, increase the limit:

SET GLOBAL max_connections = 1000;

Make it permanent by adding to your MySQL configuration file:

[mysqld]
max_connections=1000

Also review application connection pooling to ensure connections are being reused and not exhausted.

After making configuration changes, restart the MySQL service:

# On Linux with systemd
sudo systemctl restart mysql

# Or
sudo service mysql restart

# On macOS
mysql.server restart

Monitor the error log after restart to confirm fcntl errors are resolved.