---
name: "kvm_counter_u64_fetch"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/kvm_counter_u64_fetch"
---

# kvm\_counter\_u64\_fetch(3)

access per-CPU data

## NAME

**kvm\_dpcpu\_setcpu**, **kvm\_getmaxcpu**, **kvm\_getpcpu** – access per-CPU data

## LIBRARY

Kernel Data Access Library (libkvm, -lkvm)

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/pcpu.h\>**

**#include \<sys/sysctl.h\>**

**#include \<kvm.h\>**

*int*\

**kvm\_dpcpu\_setcpu**(*kvm\_t \*kd*, *u\_int cpu*);

*int*\

**kvm\_getmaxcpu**(*kvm\_t \*kd*);

*int*\

**kvm\_getncpus**(*kvm\_t \*kd*);

*void* *\**\

**kvm\_getpcpu**(*kvm\_t \*kd*, *int cpu*);

*ssize\_t*\

**kvm\_read\_zpcpu**(*kvm\_t \*kd*, *u\_long base*, *void \*buf*, *size\_t size*, *int cpu*);

*uint64\_t*\

**kvm\_counter\_u64\_fetch**(*kvm\_t \*kd*, *u\_long base*);

## DESCRIPTION

The **kvm\_dpcpu\_setcpu**(), **kvm\_getmaxcpu**(), and **kvm\_getpcpu**() functions are used to access the per-CPU data of active processors in the kernel indicated by *kd*. Per-CPU storage comes in two flavours: data stored directly in a *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.

The **kvm\_getmaxcpu**() function returns the maximum number of CPUs supported by the kernel.

The **kvm\_getncpus**() function returns the current number of CPUs in the kernel.

The **kvm\_getpcpu**() function returns a buffer holding the per-CPU data for a single CPU. This buffer is described by the *struct pcpu* type. The caller is responsible for releasing the buffer via a call to [free(3)](/free/3.md) when it is no longer needed. If *cpu* is not active, then NULL is returned instead.

The **kvm\_read\_zpcpu**() function is used to obtain private per-CPU copy from a UMA\_ZONE\_PCPU [zone(9)](/zone/9.md). It takes *base* argument as base address of an allocation and copyies *size* bytes into *buf* from the part of allocation that is private to *cpu*.

The **kvm\_counter\_u64\_fetch**() function fetches value of a [counter(9)](/counter/9.md) pointed by *base* address.

Symbols for dynamic per-CPU data are accessed via [kvm\_nlist(3)](/kvm_nlist/3.md) as with other symbols. **libkvm** maintains a notion of the "current CPU", set by **kvm\_dpcpu\_setcpu**(), which defaults to 0. Once another CPU is selected, [kvm\_nlist(3)](/kvm_nlist/3.md) will return pointers to that data on the appropriate CPU.

## CACHING

**kvm\_getmaxcpu**() and **kvm\_getpcpu**() cache the nlist values for various kernel variables which are reused in successive calls. You may call either function with *kd* set to NULL to clear this cache.

## RETURN VALUES

On success, the **kvm\_getmaxcpu**() function returns the maximum number of CPUs supported by the kernel. If an error occurs, it returns -1 instead.

On success, the **kvm\_getpcpu**() function returns a pointer to an allocated buffer or NULL. If an error occurs, it returns -1 instead.

On success, the **kvm\_dpcpu\_setcpu**() call returns 0; if an error occurs, it returns -1 instead.

On success, the **kvm\_read\_zpcpu**() function returns number of bytes copied. If an error occurs, it returns -1 instead.

If any function encounters an error, then an error message may be retrieved via [kvm\_geterr(3)](/kvm_geterr/3.md).

## SEE ALSO

[free(3)](/free/3.md), [kvm(3)](/kvm/3.md), [counter(9)](/counter/9.md), [zone(9)](/zone/9.md)