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
Chapter 16, C API Threaded Function Descriptions. For examples of source
code that uses them, look in the
directory of a MySQL source distribution:
The source for mysqlimport uses threading in the code associated with the
The source for mysqlslap uses threads to set up simultaneous workloads, to test server operation under high load.
As an alternative to thread programming, applications may find the asynchronous (nonblocking) C API functions useful. These functions enable applications to submit multiple outstanding requests to the server and determine when each has finished using polling. For more information, see Chapter 12, C API Asynchronous Interface.
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
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,
SIGPIPE on the first call to
mysql_connect(). To use your own
SIGPIPE handler, first call
install your handler.
The client library is thread-safe per connection. Two threads can share the same connection with the following caveats:
Unless you are using the asynchronous C API functions mentioned previously, 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_store_result()in one thread, no other thread uses the same connection. To do this, use a mutex lock around your pair of
mysql_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_unlock()to establish and release a mutex lock.Note
If you examine programs in a MySQL source distribution, instead of calls to
pthread_mutex_unlock(), you will see calls to
native_mutex_unlock(). The latter functions are defined in the
thr_mutex.hheader file and map to platform-specific mutex functions.
Multiple threads can access different result sets that are retrieved with
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 use
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 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:
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.
In the thread, call
pthread_exit(). This frees the memory used by MySQL thread-specific variables.