perthread

Perthread, or per-thread storage, is data that is allocated separately for each thread in a multi-threaded program. Each thread has its own copy, which is invisible to other threads unless explicitly accessed through thread-local handles. Perthread data persists for the lifetime of the thread and is automatically isolated from other threads.

Common implementations rely on thread-local storage (TLS). In C++, the thread_local keyword declares a variable with thread storage duration. In C, compilers typically provide __thread or thread_local. Java uses ThreadLocal to hold per-thread values. Python provides threading.local to create per-thread namespaces. Operating systems offer APIs like pthread_key_t and TlsAlloc on Windows to allocate thread-specific keys and store data via pthread_setspecific or TlsSetValue.

Use cases include per-thread caches, per-thread logging buffers, per-thread errno-like state, and avoiding synchronization when shared data is mostly immutable or read-mostly. These patterns help reduce contention by keeping thread-specific state out of shared data paths.

Advantages include reduced contention and avoidance of locks for thread-specific data; however, per-thread data increases overall memory usage and can complicate cleanup, particularly on thread termination or in thread pools. Portability considerations arise where TLS semantics differ or where threads are pooled rather than created/destroyed per task.

Related concepts include thread-local storage, thread-specific data, and the various language and OS APIs for TLS.