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.Dd February 28, 2010
.Dt KVM_GETPCPU 3
.Os
.Sh NAME
.Nm kvm_dpcpu_setcpu
.Nm kvm_getmaxcpu ,
.Nm kvm_getpcpu
.Nd access per-CPU data
.Sh LIBRARY
.Lb libkvm
.Sh SYNOPSIS
.In sys/param.h
.In sys/pcpu.h
.In sys/sysctl.h
.In kvm.h
.Ft int
.Fn kvm_dpcpu_setcpu "kvm_t *kd" "u_int cpu"
.Ft int
.Fn kvm_getmaxcpu "kvm_t *kd"
.Ft void *
.Fn kvm_getpcpu "kvm_t *kd" "int cpu"
.Sh DESCRIPTION
The
.Fn kvm_dpcpu_setcpu ,
.Fn kvm_getmaxcpu ,
and
.Fn kvm_getpcpu
functions are used to access the per-CPU data of active processors in the
kernel indicated by
.Fa kd .
Per-CPU storage comes in two flavours: data stored directly in a
.Vt "struct pcpu"
associated with each CPU, and dynamic per-CPU storage (DPCPU), in which a
single kernel symbol refers to different data depending on what CPU it is
accessed from.
.Pp
The
.Fn kvm_getmaxcpu
function returns the maximum number of CPUs supported by the kernel.
.Pp
The
.Fn kvm_getpcpu
function returns a buffer holding the per-CPU data for a single CPU.
This buffer is described by the
.Vt "struct pcpu"
type.
The caller is responsible for releasing the buffer via a call to
.Xr free 3
when it is no longer needed.
If
.Fa cpu
is not active, then
.Dv NULL
is returned instead.
.Pp
Symbols for dynamic per-CPU data are accessed via
.Xr kvm_nlist 3
as with other symbols.
.Nm libkvm
maintains a notion of the "current CPU", set by
.Xr kvm_dpcpu_setcpu ,
which defaults to 0.
Once another CPU is selected,
.Xr kvm_nlist 3
will return pointers to that data on the appropriate CPU.
.Sh CACHING
.Fn kvm_getmaxcpu
and
.Fn kvm_getpcpu
cache the nlist values for various kernel variables which are
reused in successive calls.
You may call either function with
.Fa kd
set to
.Dv NULL
to clear this cache.
.Sh RETURN VALUES
On success, the
.Fn kvm_getmaxcpu
function returns the maximum number of CPUs supported by the kernel.
If an error occurs,
it returns -1 instead.
.Pp
On success, the
.Fn kvm_getpcpu
function returns a pointer to an allocated buffer or
.Dv NULL .
If an error occurs,
it returns -1 instead.
.Pp
On success, the
.Fn kvm_dpcpu_setcpu
call returns 0; if an error occurs, it returns -1 instead.
.Pp
If any function encounters an error,
then an error message may be retrieved via
.Xr kvm_geterr 3 .
.Sh SEE ALSO
.Xr free 3 ,
.Xr kvm 3