&errno as thread identifier
By chys on March 13th, 2010
Thread identifiers may have different types in different systems – pthread_t under POSIX; uintptr_t or HANDLE under Windows. They are usually integers or pointers, but POSIX actually allows structures though not usually found. This leads to a little difficulty in portability.
A trick, used by OpenSSL (if not overridden by calling CRYPTO_set_id_callback), is to use &errno as the thread identifier.
Per C standard, errno must be a modifiable lvalue, and thus we can safely take its address – &errno. And under any practically usable thread implementation, errno must be thread-local, which means &errno is different in different threads.
Note: It seems OpenSSL defaults to &errno only since 0.9.8m. Earlier versions always require the user to call CRYPTO_set_id_callback.
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Tags: multithread, portability
