---
name: "kcmp"
section: "2"
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/kcmp"
---

# kcmp(2)

compare two kernel objects

## NAME

**kcmp** – compare two kernel objects

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

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

*int*\

**kcmp**(*pid\_t pid1*, *pid\_t pid2*, *int type*, *uintptr\_t idx1*, *uintptr\_t idx2*);

## DESCRIPTION

The **kcmp** system call allows the caller to determine whether the two processes with PIDs *pid1* and *pid2* reference the same kernel object. The *type* parameter specifies the type of object, and *idx1* and *idx2* are identifiers which refer to some object in the context of process *pid1* and *pid2*, respectively.

The following values for *type* may be specified:

- KCMP\_FILE

  Compare two file descriptions referred to by file descriptors *idx1* and *idx2*. They may be equivalent if, for example, one of the descriptors was created by applying [dup(2)](/dup/2.md) to the other descriptor.

- KCMP\_FILEOBJ

  Perform a “deep comparison” of the file descriptions referred to by file descriptors *idx1* and *idx2*. This tests whether the underlying object referred to by the file descriptions is the same. For example, if the same filesystem path is opened twice, the kernel will create two separate file descriptions to back the two file descriptors, but they will refer to the same underlying object, a [vnode(9)](/vnode/9.md). When compared using the KCMP\_FILE type, these descriptors will be different, but using the KCMP\_FILEOBJ type, they will be equal (assuming that the path was not unlinked in between the two opens).

- KCMP\_FILES

  Determine whether the two processes share the same file descriptor table. This will be the case if one of the processes was created by [rfork(2)](/rfork/2.md) without specifying the RFFDG flag. The *idx1* and *idx2* parameters are ignored.

- KCMP\_SIGHAND

  Determine whether the two processes share the same signal handler table. This will be the case if one of the processes was created using the RFSIGSHARE flag to [rfork(2)](/rfork/2.md). The *idx1* and *idx2* parameters are ignored.

- KCMP\_VM

  Determine whether the two processes share a virtual memory address space. This may be the case if one of the processes created the other using [vfork(2)](/vfork/2.md) or [rfork(2)](/rfork/2.md) with the RFMEM flag. The *idx1* and *idx2* parameters are ignored.

The caller of **kcmp** must have permission to debug both processes, otherwise the system call will fail.

## RETURN VALUES

If *idx1* and *idx2* refer to the same object, **kcmp** returns 0. If the object referred to by *pid1* and *idx1* is less or greater than the object referred to by *pid2* and *idx2*, **kcmp** returns the values 1 and 2, respectively. The order is defined internally by the kernel and is stable until the system reboots. If the two objects cannot be compared for some reason, **kcmp** returns 3. For example, if *type* is KCMP\_FILEOBJ and *idx1* and *idx2* are different descriptor types, e.g., a socket and a file, then **kcmp** will return 3.

If an error occurs, the value -1 is returned and the global variable *errno* is set to indicate the error.

## ERRORS

**kcmp** may fail with the following errors:

- \[ENODEV\]

  KCMP\_FILEOBJ was specified and *idx1* refers to a file descriptor which does not implement a comparison operator.

- \[EINVAL\]

  The value of *type* is invalid.

- \[EBADF\]

  One of the file descriptors referred to by *idx1* or *idx2* is not valid.

- \[ESRCH\]

  One of the processes referred to by *pid1* or *pid2* does not exist or is not visible (e.g., due to jail restrictions).

- \[EPERM\]

  The caller does not have permission to access one of the processes referred to by *pid1* or *pid2*.

## SEE ALSO

[dup(2)](/dup/2.md), [fcntl(2)](/fcntl/2.md), [fork(2)](/fork/2.md), [rfork(2)](/rfork/2.md), [vfork(2)](/vfork/2.md)

## STANDARDS

The **kcmp** system call originated in Linux. This implementation aims to be source-compatible with the Linux implementation. FreeBSD implements only a subset of the possible values for *type* supported in Linux. More values may be added in the future. The KCMP\_FILEOBJ type is a FreeBSD extension.

## HISTORY

The **kcmp** function was introduced in FreeBSD 14.1.