# Thread Safety

Per libcurl thread safety documentation, libcurl is thread-safe but has no internal thread synchronization.

For Python programs using PycURL, this means:

- Accessing the same PycURL object from different threads is OK when this object is not involved in active transfers, as Python internally has a Global Interpreter Lock and only one operating system thread can be executing Python code at a time.

- Accessing a PycURL object that is involved in an active transfer from Python code inside a libcurl callback for the PycURL object in question is OK, because PycURL takes out the appropriate locks.

- Accessing a PycURL object that is involved in an active transfer from Python code outside of a libcurl callback for the PycURL object in question is unsafe.

PycURL handles the necessary SSL locks for OpenSSL/LibreSSL/BoringSSL, GnuTLS, NSS, mbedTLS and wolfSSL.

A special situation exists when libcurl uses the standard C library name resolver (i.e., not threaded nor c-ares resolver). By default libcurl uses signals for timeouts with the C library resolver, and signals do not work properly in multi-threaded programs. When using PycURL objects from multiple Python threads `NOSIGNAL` option must be given.