What will happen if I call pthread_join() on an unused pthread_t?

Question

Suppose I have a declared pthread_t struct, like the following:

pthread_t newThread;

And then I call:

pthread_join(&newThread, NULL, myMethod, NULL);

What will pthread_join() do?

Answer 1

According to ISO C, the newThread variable is an "indeterminately valued object", the use of which triggers undefined behavior. It could have a "trap representation" which triggers a CPU exception.

Or it may just be interpreted as a random value of that type, which the API could handle in one of two ways: either there is no such thread, and ESRCH is returned, or by fluke there is such a thread. Then various cases arise: is it joinable or not, etc.