27.7.3.3 Writing C API Threaded Client Programs
This section provides guidance for writing client programs that use the thread-related functions in the MySQL C API. For further information about these functions, see Section 27.7.11, “C API Threaded Function Descriptions”. For examples of source code that uses them, look in the client
directory of a MySQL source distribution:
The source for mysqlimport uses threading in the code associated with the
--use-threads
option.The source for mysqlslap uses threads to set up simultaneous workloads, to test server operation under high load.
If undefined-reference errors occur when linking a threaded program against the MySQL client library, the most likely cause is that you did not include the thread libraries on the link/compile command.
The client library is almost thread-safe. The biggest problem is that the subroutines in sql/net_serv.cc
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, socket handling should be thread-safe.
To avoid aborting the program when a connection terminates, MySQL blocks SIGPIPE
on the first call to mysql_library_init()
, mysql_init()
, or mysql_connect()
. To use your own SIGPIPE
handler, first call mysql_library_init()
, then install your handler.
The client library is thread-safe per connection. Two threads can share the same connection with the following caveats:
Multiple threads cannot send a query to the MySQL server at the same time on the same connection. In particular, you must ensure that between calls to
mysql_query()
andmysql_store_result()
in one thread, no other thread uses the same connection. To do this, use a mutex lock around your pair ofmysql_query()
andmysql_store_result()
calls. Aftermysql_store_result()
returns, the lock can be released and other threads may query the same connection.If you use POSIX threads, you can use
pthread_mutex_lock()
andpthread_mutex_unlock()
to establish and release a mutex lock.NoteIf you examine programs in a MySQL source distribution, instead of calls to
pthread_mutex_lock()
andpthread_mutex_unlock()
, you will see calls tonative_mutex_lock()
andnative_mutex_unlock()
. The latter functions are defined in thethr_mutex.h
header file and map to platform-specific mutex functions.Multiple threads can access different result sets that are retrieved with
mysql_store_result()
.To use
mysql_use_result()
, you must ensure that no other thread uses the same connection until the result set is closed. However, it really is best for threaded clients that share the same connection to usemysql_store_result()
.
If a thread does not create the connection to the MySQL database but calls MySQL functions, take the following into account:
When you call mysql_init()
, MySQL creates 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()
, the thread does not have the necessary thread-specific variables in place and you are likely to end up with a core dump sooner or later. To avoid problems, you must do the following:
Call
mysql_library_init()
before any other MySQL functions. It is not thread-safe, so call it before threads are created, or protect the call with a mutex.Arrange for
mysql_thread_init()
to be called early in the thread handler before calling any MySQL function. (If you callmysql_init()
, it callsmysql_thread_init()
for you.)In the thread, call
mysql_thread_end()
before callingpthread_exit()
. This frees the memory used by MySQL thread-specific variables.
The preceding notes regarding mysql_init()
also apply to mysql_connect()
, which calls mysql_init()
.