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4.10 Unix Signal Handling in MySQL
On Unix and Unix-like systems, a process can be the recipient of
signals sent to it by root
or the account that
owns the process. Signals can be sent using the
kill command. Some command interpreters
associate certain key sequences with signals, such as
Control+C to send a SIGINT
signal. This section describes how the MySQL server and client
programs respond to signals.
Server Response to Signals
mysqld responds to signals as follows:
SIGTERM
causes the server to shut down. This is like executing aSHUTDOWN
statement without having to connect to the server (which for shutdown requires an account that has theSHUTDOWN
privilege).SIGHUP
causes the server to reload the grant tables and to flush tables, logs, the thread cache, and the host cache. These actions are like various forms of theFLUSH
statement. Prior to MySQL 8.0.20, the server also writes a status report to the error log that has this format:Status information: Current dir: /var/mysql/data/ Running threads: 0 Stack size: 196608 Current locks: Key caches: default Buffer_size: 8388600 Block_size: 1024 Division_limit: 100 Age_limit: 300 blocks used: 0 not flushed: 0 w_requests: 0 writes: 0 r_requests: 0 reads: 0 handler status: read_key: 0 read_next: 0 read_rnd 0 read_first: 1 write: 0 delete 0 update: 0 Table status: Opened tables: 5 Open tables: 0 Open files: 7 Open streams: 0 Alarm status: Active alarms: 1 Max used alarms: 2 Next alarm time: 67
As of MySQL 8.0.19,
SIGUSR1
causes the server to flush the error log, general query log, and slow query log. One use forSIGUSR1
is to implement log rotation without having to connect to the server (which to flush logs requires an account that has theRELOAD
privilege). See Section 5.4.6, “Server Log Maintenance”.The server response to
SIGUSR1
is a subset of the response toSIGHUP
, enablingSIGUSR1
to be used as a more “lightweight” signal that flushes certain logs without the otherSIGHUP
effects such as flushing the thread and host caches and writing a status report to the error log.SIGINT
normally is ignored by the server. Starting the server with the--gdb
option installs an interrupt handler forSIGINT
for debugging purposes. See Section 29.5.1.4, “Debugging mysqld under gdb”.
MySQL client programs respond to signals as follows:
The mysql client interprets
SIGINT
(typically the result of typing Control+C) as instruction to interrupt the current statement if there is one, or to cancel any partial input line otherwise. This behavior can be disabled using the--sigint-ignore
option to ignoreSIGINT
signals.Client programs that use the MySQL client library block
SIGPIPE
signals by default. These variations are possible:Client can install their own
SIGPIPE
handler to override the default behavior. See Section 28.7.3.3, “Writing C API Threaded Client Programs”.Clients can prevent installation of
SIGPIPE
handlers by specifying theCLIENT_IGNORE_SIGPIPE
option tomysql_real_connect()
at connect time. See Section 28.7.6.54, “mysql_real_connect()”.
Document created the 26/06/2006, last modified the 26/10/2018
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