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$FreeBSD: head/share/man/man9/critical_enter.9 84306 2001-10-01 16:09:29Z ru $
.Dd March 22, 2001 .Dt CRITICAL_ENTER 9 .Os .Sh NAME .Nm critical_enter , .Nm critical_exit .Nd enter and exit a critical region .Sh SYNOPSIS n sys/types.h n sys/systm.h .Ft critical_t .Fn critical_enter "void" .Ft void .Fn critical_exit "critical_t savecrit" .Sh DESCRIPTION These functions are used to prevent preemption in a critcal region of code. All that is guaranteed is that the current CPU will not be preempted by an external interrupt. The current CPU may still trigger faults and exceptions during a critical section. The .Fn critical_enter function returns an opaque value of type .Vt critical_t . This value must be passed to the .Fn critical_exit function when leaving the critical region.
p Note that these functions are not required to provide any inter-CPU synchronization, data protection, or memory ordering guarantees and thus should .Em not be used to protect shared data structures.
p These functions should be used with care as an infinite loop within a critical region will deadlock the CPU. .Sh RETURN VALUES The .Fn critical_enter function returns an opaque value that must be passed to a corresponding call to .Fn critical_exit . .Sh EXAMPLES This example demonstrates the use of .Fn critical_enter and .Fn critical_exit to guarantee atomic access to the DMA controller. d -literal -offset indent int isa_dmastatus(int chan) { u_long cnt = 0; int ffport, waport; u_long low1, high1, low2, high2; critical_t savecrit; ... savecrit = critical_enter(); outb(ffport, 0); low1 = inb(waport); high1 = inb(waport); outb(ffport, 0); low2 = inb(waport); high2 = inb(waport); critical_exit(savecrit); ... } .Ed .Sh HISTORY These functions were introduced in .Fx 5.0 .