Quick Navigator

Search Site

Linux VPS
A - Starter
B - Basic
C - Preferred
D - Commercial
MPS - Dedicated
Previous VPSs
* Sign Up! *

Contact Us
Online Help
Domain Status
Man Pages

Virtual Servers

Topology Map

Server Agreement
Year 2038

USA Flag



Man Pages

Manual Reference Pages  - SX (9)


sx, sx_init, sx_destroy, sx_slock, sx_xlock, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock, sx_try_upgrade, sx_downgrade, sx_assert, sx_unlock, sx_xlocked, SX_SYSINIT - kernel shared/exclusive lock


     Nm Ss utility macros
     Kernel options
See Also


.In sys/param.h
.In sys/lock.h
.In sys/sx.h void sx_init struct sx *sx const char *description void sx_destroy struct sx *sx void sx_slock struct sx *sx void sx_xlock struct sx *sx int sx_try_slock struct sx *sx int sx_try_xlock struct sx *sx void sx_sunlock struct sx *sx void sx_xunlock struct sx *sx int sx_try_upgrade struct sx *sx void sx_downgrade struct sx *sx void sx_assert struct sx *sx int what int sx_xlocked struct sx *sx

    Nm Ss utility macros

sx_unlock struct sx *sx SX_SYSINIT name struct sx *sx const char *description

    Kernel options

.Cd options INVARIANTS


Shared/exclusive locks are used to protect data that are read far more often than they are written. Mutexes are inherently more efficient than shared/exclusive locks, so shared/exclusive locks should be used prudently.

Shared/exclusive locks are created with sx_init, where sx is a pointer to space for a
.Vt struct sx , and description is a pointer to a null-terminated character string that describes the shared/exclusive lock. Shared/exclusive locks are destroyed with sx_destroy. Threads acquire and release a shared lock by calling sx_slock or sx_try_slock and sx_sunlock or sx_unlock. Threads acquire and release an exclusive lock by calling sx_xlock or sx_try_xlock and sx_xunlock or sx_unlock. A thread can attempt to upgrade a currently held shared lock to an exclusive lock by calling sx_try_upgrade. A thread that has an exclusive lock can downgrade it to a shared lock by calling sx_downgrade.

sx_try_slock and sx_try_xlock will return 0 if the shared/exclusive lock cannot be acquired immediately; otherwise the shared/exclusive lock will be acquired and a non-zero value will be returned.

sx_try_upgrade will return 0 if the shared lock cannot be upgraded to an exclusive lock immediately; otherwise the exclusive lock will be acquired and a non-zero value will be returned.

When compiled with
.Cd options INVARIANTS and
.Cd options INVARIANT_SUPPORT , the sx_assert function tests sx for the assertions specified in what, and panics if they are not met. The following assertions are supported:
SX_LOCKED Assert that the current thread has either a shared or an exclusive lock on the
.Vt sx lock pointed to by the first argument.
SX_SLOCKED Assert that the current thread has a shared lock on the
.Vt sx lock pointed to by the first argument.
SX_XLOCKED Assert that the current thread has an exclusive lock on the
.Vt sx lock pointed to by the first argument.
  Assert that the current thread has no lock on the
.Vt sx lock pointed to by the first argument.

sx_xlocked will return non-zero if the current process holds the exclusive lock; otherwise, it will return zero.

For ease of programming, sx_unlock is provided as a macro frontend to the respective functions, sx_sunlock and sx_xunlock. Algorithms that are aware of what state the lock is in should use either of the two specific functions for a minor performance benefit.

The SX_SYSINIT macro is used to generate a call to the sx_sysinit routine at system startup in order to initialize a given sx lock. The parameters are the same as sx_init but with an additional argument, name, that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine.

A thread may not hold both a shared lock and an exclusive lock on the same lock simultaneously; attempting to do so will result in deadlock.


A thread may hold a shared or exclusive lock on an sx lock while sleeping. As a result, an sx lock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an sx lock while another thread blocked on the same sx lock after acquiring a mutex, then the second thread would effectively end up sleeping while holding a mutex, which is not allowed.


condvar(9), mtx_pool(9), mutex(9), panic(9), sema(9)


Currently there is no way to assert that a lock is not held. This is not possible in the non- WITNESS case for asserting that this thread does not hold a shared lock. In the non- WITNESS case, the SX_LOCKED and SX_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.
Search for    or go to Top of page |  Section 9 |  Main Index

January 5, 2005 SX (9)

Powered by GSP Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.