futex - fast user-space locking
The Linux kernel provides futexes ("Fast user-space mutexes") as a building block for fast user-space locking and semaphores. Futexes are very basic and lend themselves well for building higher-level locking abstractions such as mutexes, condition variables, read-write locks, barriers, and semaphores. Most programmers will in fact not be using futexes directly but will instead rely on system libraries built on them, such as the Native POSIX Thread Library (NPTL) (see pthreads(7)). A futex is identified by a piece of memory which can be shared between processes or threads. In these different processes, the futex need not have identical addresses. In its bare form, a futex has semaphore semantics; it is a counter that can be incremented and decremented atomically; processes can wait for the value to become positive. Futex operation occurs entirely in user space for the noncontended case. The kernel is involved only to arbitrate the contended case. As any sane design will strive for noncontention, futexes are also optimized for this situation. In its bare form, a futex is an aligned integer which is touched only by atomic assembler instructions. This integer is four bytes long on all platforms. Processes can share this integer using mmap(2), via shared memory segments, or because they share memory space, in which case the application is commonly called multithreaded. Semantics Any futex operation starts in user space, but it may be necessary to communicate with the kernel using the futex(2) system call. To "up" a futex, execute the proper assembler instructions that will cause the host CPU to atomically increment the integer. Afterward, check if it has in fact changed from 0 to 1, in which case there were no waiters and the operation is done. This is the noncontended case which is fast and should be common. In the contended case, the atomic increment changed the counter from -1 (or some other negative number). If this is detected, there are waiters. User space should now set the counter to 1 and instruct the kernel to wake up any waiters using the FUTEX_WAKE operation. Waiting on a futex, to "down" it, is the reverse operation. Atomically decrement the counter and check if it changed to 0, in which case the operation is done and the futex was uncontended. In all other circumstances, the process should set the counter to -1 and request that the kernel wait for another process to up the futex. This is done using the FUTEX_WAIT operation. The futex(2) system call can optionally be passed a timeout specifying how long the kernel should wait for the futex to be upped. In this case, semantics are more complex and the programmer is referred to futex(2) for more details. The same holds for asynchronous futex waiting.
Initial futex support was merged in Linux 2.5.7 but with different semantics from those described above. Current semantics are available from Linux 2.5.40 onward.
To reiterate, bare futexes are not intended as an easy-to-use abstraction for end users. Implementors are expected to be assembly literate and to have read the sources of the futex user-space library referenced below. This man page illustrates the most common use of the futex(2) primitives; it is by no means the only one.
clone(2), futex(2), get_robust_list(2), set_robust_list(2), set_tid_address(2), pthreads(7) Fuss, Futexes and Furwocks: Fast Userlevel Locking in Linux (proceedings of the Ottawa Linux Symposium 2002), futex example library, futex-*.tar.bz2 ⟨ftp://ftp.kernel.org/pub/linux/kernel/people /rusty/⟩.
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.