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.Dd February 26, 2001
.Dt SX 9
.Os
.Sh NAME
.Nm sx ,
.Nm sx_init ,
.Nm sx_destroy ,
.Nm sx_slock ,
.Nm sx_xlock ,
.Nm sx_try_slock ,
.Nm sx_try_xlock ,
.Nm sx_sunlock ,
.Nm sx_xunlock ,
.Nm sx_try_upgrade ,
.Nm sx_downgrade
.Nd kernel shared/exclusive lock
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/lock.h>
.Fd #include <sys/mutex.h>
.Fd #include <sys/sx.h>
.Ft void
.Fn sx_init "struct sx *sx" "const char *description"
.Ft void
.Fn sx_destroy "struct sx *sx"
.Ft void
.Fn sx_slock "struct sx *sx"
.Ft void
.Fn sx_xlock "struct sx *sx"
.Ft int
.Fn sx_try_slock "struct sx *sx"
.Ft int
.Fn sx_try_xlock "struct sx *sx"
.Ft void
.Fn sx_sunlock "struct sx *sx"
.Ft void
.Fn sx_xunlock "struct sx *sx"
.Ft int
.Fn sx_try_upgrade "struct sx *sx"
.Ft void
.Fn sx_downgrade "struct sx *sx"
.Sh DESCRIPTION
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.
.Pp
Shared/exclusive locks are created with
.Fn sx_init ,
where
.Fa sx
is a pointer to space for a
.Vt struct sx ,
and
.Fa description
is a pointer to a null-terminated character string that describes the
shared/exclusive lock.
Shared/exclusive locks are destroyed with
.Fn sx_destroy .
Threads acquire and release a shared lock by calling
.Fn sx_slock
or
.Fn sx_try_slock
and
.Fn sx_sunlock .
Threads acquire and release an exclusive lock by calling
.Fn sx_xlock
or
.Fn sx_try_xlock
and
.Fn sx_xunlock .
A thread can attempt to upgrade a currently owned shared lock to an exclusive
lock by calling
.Fn sx_try_upgrade .
A thread that owns an exclusive lock can downgrade it to a shared lock by
calling
.Fn sx_downgrade .
.Pp
.Fn sx_try_slock
and
.Fn 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.
.Pp
.Fn 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.
.Pp
A thread may not own a shared lock and an exclusive lock simultaneously;
attempting to do so will result in deadlock.
.Sh SEE ALSO
.Xr mutex 9