I am having some trouble figuring out how pthread_key_t and pthread_key_create work. From my understand, each thread has TLS (thread local storage) and that a key is used to access the thread local storage. What I do not get is when a key is created, does every thread get to use it? Lets say Thread 0 creates key 0, can Thread 1 then use key 0? If Thread 1 used key 0, would it access its own TLS or Thread 0's TLS?
Is there some global array or something that keeps track of all the keys being used?
pthread_keys are just what you said, thread local storage referred to by a common key. So multiple threads use the same key, but get different storage space (per thread).
A quick example (contrived too), say you were building an asynchronous server (like IMAP). You could keep track of client connections in an array, with each having a key for the current task/request. So when a request comes in a new thread is spun up and the thread stores in the Client_Connection->WhatAmIDoing key a pointer to the "request" structure. The thread now wouldn't have to pass around that pointer because any function that thread executes could simply call the pthread_getspecific() function and get the pointer to what it's supposed to be doing.