---
name: "counter"
section: "9"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/SYSCTL_ADD_COUNTER_U64"
---

# counter(9)

SMP-friendly kernel counter implementation

Part of [SYSCTL\_ADD\_COUNTER\_U64(9)](/SYSCTL_ADD_COUNTER_U64/9.md).

## NAME

**counter** – SMP-friendly kernel counter implementation

## SYNOPSIS

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

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

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

*counter\_u64\_t*\

**counter\_u64\_alloc**(*int wait*);

*void*\

**counter\_u64\_free**(*counter\_u64\_t c*);

*void*\

**counter\_u64\_add**(*counter\_u64\_t c*, *int64\_t v*);

*void*\

**counter\_enter**();

*void*\

**counter\_exit**();

*void*\

**counter\_u64\_add\_protected**(*counter\_u64\_t c*, *int64\_t v*);

*uint64\_t*\

**counter\_u64\_fetch**(*counter\_u64\_t c*);

*void*\

**counter\_u64\_zero**(*counter\_u64\_t c*);

*struct* *counter\_rate* *\**\

**counter\_rate\_alloc**(*int flags*, *int period*);

*int64\_t*\

**counter\_ratecheck**(*struct counter\_rate \*cr*, *int64\_t limit*);

*uint64\_t*\

**counter\_rate\_get**(*struct counter\_rate \*cr*);

*void*\

**counter\_rate\_free**(*struct counter\_rate \*cr*);

**COUNTER\_U64\_SYSINIT**(*counter\_u64\_t c*);

**COUNTER\_U64\_DEFINE\_EARLY**(*counter\_u64\_t c*);

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

**SYSCTL\_COUNTER\_U64**(*parent*, *nbr*, *name*, *access*, *ptr*, *descr*);

**SYSCTL\_ADD\_COUNTER\_U64**(*ctx*, *parent*, *nbr*, *name*, *access*, *ptr*, *descr*);

**SYSCTL\_COUNTER\_U64\_ARRAY**(*parent*, *nbr*, *name*, *access*, *ptr*, *len*, *descr*);

**SYSCTL\_ADD\_COUNTER\_U64\_ARRAY**(*ctx*, *parent*, *nbr*, *name*, *access*, *ptr*, *len*, *descr*);

## DESCRIPTION

**counter** is a generic facility to create counters that can be utilized for any purpose (such as collecting statistical data). A **counter** is guaranteed to be lossless when several kernel threads do simultaneous updates. However, **counter** does not block the calling thread, also no [atomic(9)](/atomic/9.md) operations are used for the update, therefore the counters can be used in any non-interrupt context. Moreover, **counter** has special optimisations for SMP environments, making **counter** update faster than simple arithmetic on the global variable. Thus **counter** is considered suitable for accounting in the performance-critical code paths.

- **counter\_u64\_alloc**(*wait*)

  Allocate a new 64-bit unsigned counter. The *wait* argument is the [malloc(9)](/malloc/9.md) wait flag, should be either *M\_NOWAIT* or *M\_WAITOK*. If *M\_NOWAIT* is specified the operation may fail and return NULL.

- **counter\_u64\_free**(*c*)

  Free the previously allocated counter *c*. It is safe to pass NULL.

- **counter\_u64\_add**(*c*, *v*)

  Add *v* to *c*. The KPI does not guarantee any protection from wraparound.

- **counter\_enter**()

  Enter mode that would allow the safe update of several counters via **counter\_u64\_add\_protected**(). On some machines this expands to [critical(9)](/critical/9.md) section, while on other is a nop. See IMPLEMENTATION DETAILS.

- **counter\_exit**()

  Exit mode for updating several counters.

- **counter\_u64\_add\_protected**(*c*, *v*)

  Same as **counter\_u64\_add**(), but should be preceded by **counter\_enter**().

- **counter\_u64\_fetch**(*c*)

  Take a snapshot of counter *c*. The data obtained is not guaranteed to reflect the real cumulative value for any moment.

- **counter\_u64\_zero**(*c*)

  Clear the counter *c* and set it to zero.

- **counter\_rate\_alloc**(*flags*, *period*)

  Allocate a new struct counter\_rate. *flags* is passed to [malloc(9)](/malloc/9.md). *period* is the time over which the rate is checked.

- **counter\_ratecheck**(*cr*, *limit*)

  The function is a multiprocessor-friendly version of **ppsratecheck**() which uses **counter** internally. Returns non-negative value if the rate is not yet reached during the current period, and a negative value otherwise. If the limit was reached during the previous period, but was just reset back to zero, then **counter\_ratecheck**() returns number of events since previous reset.

- **counter\_rate\_get**(*cr*)

  The number of hits to this check within the current period.

- **counter\_rate\_free**(*cr*)

  Free the *cr* counter.

- **COUNTER\_U64\_SYSINIT**(*c*)

  Define a [SYSINIT(9)](/SYSINIT/9.md) initializer for the global counter *c*.

- **COUNTER\_U64\_DEFINE\_EARLY**(*c*)

  Define and initialize a global counter *c*. It is always safe to increment *c*, though updates prior to the SI\_SUB\_COUNTER [SYSINIT(9)](/SYSINIT/9.md) event are lost.

- **SYSCTL\_COUNTER\_U64**(*parent*, *nbr*, *name*, *access*, *ptr*, *descr*)

  Declare a static [sysctl(9)](/sysctl/9.md) oid that would represent a **counter**. The *ptr* argument should be a pointer to allocated *counter\_u64\_t*. A read of the oid returns value obtained through **counter\_u64\_fetch**(). Any write to the oid zeroes it.

- **SYSCTL\_ADD\_COUNTER\_U64**(*ctx*, *parent*, *nbr*, *name*, *access*, *ptr*, *descr*)

  Create a [sysctl(9)](/sysctl/9.md) oid that would represent a **counter**. The *ptr* argument should be a pointer to allocated *counter\_u64\_t*. A read of the oid returns value obtained through **counter\_u64\_fetch**(). Any write to the oid zeroes it.

- **SYSCTL\_COUNTER\_U64\_ARRAY**(*parent*, *nbr*, *name*, *access*, *ptr*, *len*, *descr*)

  Declare a static [sysctl(9)](/sysctl/9.md) oid that would represent an array of **counter**. The *ptr* argument should be a pointer to allocated array of *counter\_u64\_t's*. The *len* argument should specify number of elements in the array. A read of the oid returns len-sized array of *uint64\_t* values  obtained through **counter\_u64\_fetch**(). Any write to the oid zeroes all array elements.

- **SYSCTL\_ADD\_COUNTER\_U64\_ARRAY**(*ctx*, *parent*, *nbr*, *name*, *access*, *ptr*, *len*, *descr*)

  Create a [sysctl(9)](/sysctl/9.md) oid that would represent an array of **counter**. The *ptr* argument should be a pointer to allocated array of *counter\_u64\_t's*. The *len* argument should specify number of elements in the array. A read of the oid returns len-sized array of *uint64\_t* values obtained through **counter\_u64\_fetch**(). Any write to the oid zeroes all array elements.

## IMPLEMENTATION DETAILS

On all architectures **counter** is implemented using per-CPU data fields that are specially aligned in memory, to avoid inter-CPU bus traffic due to shared use of the variables between CPUs. These are allocated using *UMA\_ZONE\_PCPU* [uma(9)](/uma/9.md) zone. The update operation only touches the field that is private to current CPU. Fetch operation loops through all per-CPU fields and obtains a snapshot sum of all fields.

On amd64 a **counter** update is implemented as a single instruction without lock semantics, operating on the private data for the current CPU, which is safe against preemption and interrupts.

On i386 architecture, when machine supports the cmpxchg8 instruction, this instruction is used. The multi-instruction sequence provides the same guarantees as the amd64 single-instruction implementation.

On some architectures updating a counter require a [critical(9)](/critical/9.md) section.

## EXAMPLES

The following example creates a static counter array exported to userspace through a sysctl:

```
#define MY_SIZE 8
static counter_u64_t array[MY_SIZE];
SYSCTL_COUNTER_U64_ARRAY(_debug, OID_AUTO, counter_array, CTLFLAG_RW,
    &array[0], MY_SIZE, "Test counter array");
```

## SEE ALSO

[atomic(9)](/atomic/9.md), [critical(9)](/critical/9.md), [locking(9)](/locking/9.md), [malloc(9)](/malloc/9.md), [ratecheck(9)](/ratecheck/9.md), [sysctl(9)](/sysctl/9.md), [SYSINIT(9)](/SYSINIT/9.md), [uma(9)](/uma/9.md)

## HISTORY

The **counter** facility first appeared in FreeBSD 10.0.

## AUTHORS

The **counter** facility was written by Gleb Smirnoff and Konstantin Belousov.