( Threaded clients

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 22.2.15 How to Make a Threaded Client
 The client library is almost thread-safe. The biggest problem is that
 the subroutines in `net.c' that read from sockets are not interrupt
 safe.  This was done with the thought that you might want to have your
 own alarm that can break a long read to a server.  If you install
 interrupt handlers for the `SIGPIPE' interrupt, the socket handling
 should be thread-safe.
 New in 4.0.16: To not abort the program when a connection terminates,
 MySQL blocks `SIGPIPE' on the first call to `mysql_server_init'(),
 `mysql_init()' or `mysql_connect()'.  If you want to have your own
 `SIGPIPE' handler, you should first call `mysql_server_init()' and then
 install your handler. In older versions of MySQL `SIGPIPE' was blocked,
 but only in the thread safe client library, for every call to
 In the older binaries we distribute on our Web site
 (`'), the client libraries are not normally
 compiled with the thread-safe option (the Windows binaries are by
 default compiled to be thread-safe). Newer binary distributions should
 have both a normal and a thread-safe client library.
 To get a threaded client where you can interrupt the client from other
 threads and set timeouts when talking with the MySQL server, you should
 use the `-lmysys', `-lmystrings', and `-ldbug' libraries and the
 `net_serv.o' code that the server uses.
 If you don't need interrupts or timeouts, you can just compile a
 thread-safe client library `(mysqlclient_r)' and use this.   MySQL
 C API C.  In this case, you don't have to worry about the `net_serv.o'
 object file or the other MySQL libraries.
 When using a threaded client and you want to use timeouts and
 interrupts, you can make great use of the routines in the `thr_alarm.c'
 file.  If you are using routines from the `mysys' library, the only
 thing you must remember is to call `my_init()' first!  C Thread
 All functions except `mysql_real_connect()' are by default thread-safe.
 The following notes describe how to compile a thread-safe client
 library and use it in a thread-safe manner.  (The notes below for
 `mysql_real_connect()' actually apply to `mysql_connect()' as well, but
 because `mysql_connect()' is deprecated, you should be using
 `mysql_real_connect()' anyway.)
 To make `mysql_real_connect()' thread-safe, you must recompile the
 client library with this command:
      shell> ./configure --enable-thread-safe-client
 This will create a thread-safe client library `libmysqlclient_r'.
 (Assuming that your OS has a thread-safe `gethostbyname_r()' function.)
 This library is thread-safe per connection.  You can let two threads
 share the same connection with the following caveats:
    * Two threads can't send a query to the MySQL server at the same
      time on the same connection.  In particular, you have to ensure
      that between a `mysql_query()' and `mysql_store_result()' no other
      thread is using the same connection.
    * Many threads can access different result sets that are retrieved
      with `mysql_store_result()'.
    * If you use `mysql_use_result', you have to ensure that no other
      thread is using the same connection until the result set is closed.
      However, it really is best for threaded clients that share the same
      connection to use `mysql_store_result()'.
    * If you want to use multiple threads on the same connection, you
      must have a mutex lock around your `mysql_query()' and
      `mysql_store_result()' call combination.  Once
      `mysql_store_result()' is ready, the lock can be released and other
      threads may query the same connection.
    * If you program with POSIX threads, you can use
      `pthread_mutex_lock()' and `pthread_mutex_unlock()' to establish
      and release a mutex lock.
 You need to know the following if you have a thread that is calling
 MySQL functions which did not create the connection to the MySQL
 When you call `mysql_init()' or `mysql_connect()', MySQL will create a
 thread-specific variable for the thread that is used by the debug
 library (among other things).
 If you call a MySQL function, before the thread has called
 `mysql_init()' or `mysql_connect()', the thread will not have the
 necessary thread-specific variables in place and you are likely to end
 up with a core dump sooner or later.
 The get things to work smoothly you have to do the following:
   1. Call `my_init()' at the start of your program if it calls any
      other MySQL function before calling `mysql_real_connect()'.
   2. Call `mysql_thread_init()' in the thread handler before calling
      any MySQL function.
   3. In the thread, call `mysql_thread_end()' before calling
      `pthread_exit()'.  This will free the memory used by MySQL
      thread-specific variables.
 You may get some errors because of undefined symbols when linking your
 client with `libmysqlclient_r'. In most cases this is because you
 haven't included the thread libraries on the link/compile line.
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