---
name: "perfmonctl"
section: "2"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/perfmonctl"
---

# perfmonctl(2)

interface to IA-64 performance monitoring unit

## NAME

perfmonctl - interface to IA-64 performance monitoring unit

## SYNOPSIS

```
#include <syscall.h>
#include <perfmon.h>
```

**long perfmonctl(**int narg;                 **int** *fd***, int** *cmd***, void** *arg***\[***narg***\], int** *narg***);**

*Note*: There is no glibc wrapper for this system call; see HISTORY.

## DESCRIPTION

The IA-64-specific **perfmonctl**() system call provides an interface to the PMU (performance monitoring unit). The PMU consists of PMD (performance monitoring data) registers and PMC (performance monitoring control) registers, which gather hardware statistics.

**perfmonctl**() applies the operation *cmd* to the input arguments specified by *arg*. The number of arguments is defined by *narg*. The *fd* argument specifies the perfmon context to operate on.

Supported values for *cmd* are:

- **PFM\_CREATE\_CONTEXT**

  ```
  perfmonctl(int fd, PFM_CREATE_CONTEXT, pfarg_context_t *ctxt, 1);
  ```

  Set up a context.

-

  The *fd* parameter is ignored. A new perfmon context is created as specified in *ctxt* and its file descriptor is returned in *ctxt-\>ctx\_fd*.

-

  The file descriptor can be used in subsequent calls to **perfmonctl**() and can be used to read event notifications (type *pfm\_msg\_t*) using [**read**(2)](/read/2.md). The file descriptor is pollable using [**select**(2)](/select/2.md), [**poll**(2)](/poll/2.md), and [**epoll**(7)](/epoll/7.md).

-

  The context can be destroyed by calling [**close**(2)](/close/2.md) on the file descriptor.

- **PFM\_WRITE\_PMCS**

  ```
  perfmonctl(int fd, PFM_WRITE_PMCS, pfarg_reg_t *pmcs, n);
  ```

  Set PMC registers.

- **PFM\_WRITE\_PMDS**

  ```
  perfmonctl(int fd, PFM_WRITE_PMDS, pfarg_reg_t *pmds, n);
  ```

  Set PMD registers.

- **PFM\_READ\_PMDS**

  ```
  perfmonctl(int fd, PFM_READ_PMDS, pfarg_reg_t *pmds, n);
  ```

  Read PMD registers.

- **PFM\_START**

  ```
  perfmonctl(int fd, PFM_START, NULL, 0);
  ```

  Start monitoring.

- **PFM\_STOP**

  ```
  perfmonctl(int fd, PFM_STOP, NULL, 0);
  ```

  Stop monitoring.

- **PFM\_LOAD\_CONTEXT**

  ```
  perfmonctl(int fd, PFM_LOAD_CONTEXT, pfarg_load_t *largs, 1);
  ```

  Attach the context to a thread.

- **PFM\_UNLOAD\_CONTEXT**

  ```
  perfmonctl(int fd, PFM_UNLOAD_CONTEXT, NULL, 0);
  ```

  Detach the context from a thread.

- **PFM\_RESTART**

  ```
  perfmonctl(int fd, PFM_RESTART, NULL, 0);
  ```

  Restart monitoring after receiving an overflow notification.

- **PFM\_GET\_FEATURES**

  ```
  perfmonctl(int fd, PFM_GET_FEATURES, pfarg_features_t *arg, 1);
  ```

- **PFM\_DEBUG**

  ```
  perfmonctl(int fd, PFM_DEBUG, val, 0);
  ```

  If *val* is nonzero, enable debugging mode, otherwise disable.

- **PFM\_GET\_PMC\_RESET\_VAL**

  ```
  perfmonctl(int fd, PFM_GET_PMC_RESET_VAL, pfarg_reg_t *req, n);
  ```

  Reset PMC registers to default values.

## RETURN VALUE

**perfmonctl**() returns zero when the operation is successful. On error, -1 is returned and *errno* is set to indicate the error.

## STANDARDS

Linux on IA-64.

## HISTORY

Linux 2.4. Removed in Linux 5.10.

This system call was broken for many years, and was ultimately removed in Linux 5.10.

glibc does not provide a wrapper for this system call; on kernels where it exists, call it using [**syscall**(2)](/syscall/2.md).

## SEE ALSO

[**gprof**(1)](/gprof/1.md)

The perfmon2 interface specification