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.
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(). If you want to
use your own SIGPIPE handler, you should
first call mysql_library_init()
and then install your handler.
If “undefined symbol” errors occur when linking
against the libmysqlclient client library, in
most cases this is because you have not included the thread
libraries on the link/compile command.
The client 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 calls to
mysql_query()andmysql_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 must 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 usemysql_store_result().If you want to use multiple threads on the same connection, you must have a mutex lock around your pair of
mysql_query()andmysql_store_result()calls. Oncemysql_store_result()is ready, 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.
You need to know the following if you have a thread that did not create the connection to the MySQL database but is calling MySQL functions:
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 will callmysql_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().