Linux utime manual#

To use futexes the kernel maintains a futex (i.e., the kernel. Note that no explicit initialization or destruction is necessary See futex(7) for more detail onīesides the basic wait and wake-up futex functionality, there areįurther futex operations aimed at supporting more complex use The lock flag used as a futex word (this can be further optimized Lock, a thread has to first reset the lock state to not acquiredĪnd then execute a futex operation that wakes threads blocked on Word still matches the 'acquired state'). Only if the lock is still acquired (i.e., the value in the futex Representing the acquired state as the expected value to aįutex() wait operation. May pass the lock's flag as a futex word and the value Lock because it is already acquired by another thread. State.) On the other hand, a thread may be unable to acquire a Mode, and the kernel maintains no information about the lock (Such instructions are performed entirely in user Instructions, for example atomically changing it from notĪcquired to acquired using an atomic compare-and-exchange In the uncontendedĬase, a thread can access or modify the lock state with atomic Lock (i.e., acquired or not acquired) can be represented as anĪtomically accessed flag in shared memory. One use of futexes is for implementing locks. Potentially changes shared memory, blocking via a futex is an User space with the implementation of blocking by the kernel.Īnalogously to an atomic compare-and-exchange operation that Thus, the futex word is used to connect the synchronization in

Operations performed by other threads on the same futex word. With the expected value, and the actual blocking will happenĪtomically and will be totally ordered with respect to concurrent Loading of the futex word's value, the comparison of that value The calling thread supplied (as one of the arguments of theįutex() call) as the expected value of the futex word. The kernel will block only if the futex word has the value that When executing a futex operation that requests to block a thread, Sufficient to place the futex word in a global variable shared by Location in physical memory.) In a multithreaded program, it is (Thus, the futex word may have different virtual addresses inĭifferent processes, but these addresses all refer to the same

Shared memory, created using (for example) mmap(2) or shmat(2). In order to share aįutex between processes, the futex is placed in a region of (Futexes are 32īits in size on all platforms, including 64-bit systems.) Allįutex operations are governed by this value. Wake any processes or threads waiting for a particular condition.Ī futex is a 32-bit value-referred to below as a futex word-whoseĪddress is supplied to the futex() system call. Likely that the program has to block for a longer time until theĬondition becomes true. Space program employs the futex() system call only when it is Synchronization operations are performed in user space. It is typically used as aīlocking construct in the context of shared-memory The futex() system call provides a method for waiting until aĬertain condition becomes true. #include long syscall(SYS_futex, uint32_t * uaddr, int futex_op, uint32_t val, const struct timespec * timeout, /* or: uint32_t val2 */ uint32_t * uaddr2, uint32_t val3 ) Note: glibc provides no wrapper for futex(), necessitating the #include /* Definition of SYS_* constants */ SYNOPSIS top #include /* Definition of FUTEX_* constants */

Linux utime manual#

Futex(2) System Calls Manual futex(2) NAME top