The pthread_setspecific function associates a thread-specific value with a key obtained via a previous call to pthread_key_create. Different threads can bind different values to the same key. These values are typically pointers to blocks of dynamically allocated memory that have been reserved for use by the calling thread.
The effect of calling pthread_setspecific with a key value not obtained from pthread_key_create or after key has been deleted with pthread_key_delete is undefined.
The pthread_setspecific function may be called from a thread-specific data destructor function, however this may result in lost storage or infinite loops.
If successful, the pthread_setspecific function will return zero. Otherwise an error number will be returned to indicate the error.
The pthread_setspecific function will fail if: