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The igraph library is considered thread-safe if it has been compiled with thread-local storage enabled, i.e. the IGRAPH_ENABLE_TLS setting was toggled to ON and the current platform supports this feature. To check whether an igraph build is thread-safe, use the IGRAPH_THREAD_SAFE macro. When linking to external versions of igraph's dependencies, it is the responsibility of the user to check that these dependencies were also compiled to be thread-safe.

Note that igraph is only thread-safe if it was built with the internal ARPACK library, i.e. the one that comes with igraph. The standard ARPACK library is not thread-safe.

The default random number generator that igraph uses is not guaranteed to be thread-safe. You need to set a different random number generator instance for every thread that you want to use igraph from. This is especially important if you set the seed of the random number generator to ensure reproducibility; sharing a random number generator between threads would break reproducibility as the order in which the various threads are scheduled is random, and therefore they would still receive random numbers in an unpredictable order from the shared random number generator.