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MySQL 5.6 Reference Manual  /  MySQL Programs  /  Unix Signal Handling in MySQL

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 the root system account or the system 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.

  • 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 the FLUSH statement. Sending the signal enables the flush operations to be performed without having to connect to the server, which requires a MySQL account that has privileges sufficient for those operations. The server also writes a status report to the error log that has this format:

    Status information:
    
    Current dir: /var/mysql/data/
    Running threads: 4  Stack size: 262144
    Current locks:
    lock: 0x7f742c02c0e0:
    
    lock: 0x2cee2a20:
    :
    lock: 0x207a080:
    
    Key caches:
    default
    Buffer_size:       8388608
    Block_size:           1024
    Division_limit:        100
    Age_limit:             300
    blocks used:             4
    not flushed:             0
    w_requests:              0
    writes:                  0
    r_requests:              8
    reads:                   4
    
    handler status:
    read_key:           13
    read_next:           4
    read_rnd             0
    read_first:         13
    write:               1
    delete               0
    update:              0
    
    Table status:
    Opened tables:        121
    Open tables:          114
    Open files:            18
    Open streams:           0
    
    Memory status:
    <malloc version="1">
    <heap nr="0">
    <sizes>
      <size from="17" to="32" total="32" count="1"/>
      <size from="33" to="48" total="96" count="2"/>
      <size from="33" to="33" total="33" count="1"/>
      <size from="97" to="97" total="6014" count="62"/>
      <size from="113" to="113" total="904" count="8"/>
      <size from="193" to="193" total="193" count="1"/>
      <size from="241" to="241" total="241" count="1"/>
      <size from="609" to="609" total="609" count="1"/>
      <size from="16369" to="16369" total="49107" count="3"/>
      <size from="24529" to="24529" total="98116" count="4"/>
      <size from="32689" to="32689" total="32689" count="1"/>
      <unsorted from="241" to="7505" total="7746" count="2"/>
    </sizes>
    <total type="fast" count="3" size="128"/>
    <total type="rest" count="84" size="195652"/>
    <system type="current" size="690774016"/>
    <system type="max" size="690774016"/>
    <aspace type="total" size="690774016"/>
    <aspace type="mprotect" size="690774016"/>
    </heap>
    :
    <total type="fast" count="85" size="5520"/>
    <total type="rest" count="116" size="316820"/>
    <total type="mmap" count="82" size="939954176"/>
    <system type="current" size="695717888"/>
    <system type="max" size="695717888"/>
    <aspace type="total" size="695717888"/>
    <aspace type="mprotect" size="695717888"/>
    </malloc>
    
    Events status:
    LLA = Last Locked At  LUA = Last Unlocked At
    WOC = Waiting On Condition  DL = Data Locked
    
    Event scheduler status:
    State      : INITIALIZED
    Thread id  : 0
    LLA        : n/a:0
    LUA        : n/a:0
    WOC        : NO
    Workers    : 0
    Executed   : 0
    Data locked: NO
    
    Event queue status:
    Element count   : 0
    Data locked     : NO
    Attempting lock : NO
    LLA             : init_queue:95
    LUA             : init_queue:103
    WOC             : NO
    Next activation : never
  • SIGINT normally is ignored by the server. Starting the server with the --gdb option installs an interrupt handler for SIGINT for debugging purposes. See Section 5.8.1.4, “Debugging mysqld under gdb”.

Client Response to Signals

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 ignore SIGINT signals.

  • Client programs that use the MySQL client library block SIGPIPE signals by default. These variations are possible: