">BIO_read(3)</a> functions are thread safe. However it would not be thread safe to call BIO_write() from one thread while calling BIO_read() in another where both functions are passed the same <b>BIO</b> object since both of them may attempt to make changes to the same <b>BIO</b> object.</p>

<p>There are exceptions to these rules. A small number of functions are not thread safe at all. Where this is the case this restriction should be noted in the documentation for the function. Similarly some data struct