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
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.
If “undefined symbol” errors occur when linking
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:
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. You must
have a mutex lock around your pair of
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.
Many threads can access different result sets that are
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 use
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 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:
before any other MySQL functions. It is not thread-safe, so
call it before threads are created, or protect the call with
In the thread, call
pthread_exit(). This frees the
memory used by MySQL thread-specific variables.