Operating System

(0) User Thread

•Managed by user-level Threads Library

–No support from the kernel (The kernel is not aware of the existence of threads)
–Fast to create and manage
–Block all threads for a blocking system call if the kernel is single threaded–Cannot take advantage of multi-processors

•Examples
–POSIX Pthreads
–Mach C-threads
–Solaris UI-threads

a user-level thread includes a set of registers and a stack, and shares the entire address space with the other threads in the enclosing process. however, a user-level thread is handled entirely in user code, usually by a special library that provides at least start, swap and suspend calls. Because the OS is unaware of a user-level thread's existence, a user-level thread can not separately receive signals or use operating system scheduling calls such as sleep(). Many implementations of user-level threads exist, including:
GNU Portable Threads (Pth)
FreeBSD's userland threads
QuickThreads and those developed by us for the Charm++ system.