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KCMP(2) Linux Programmer's Manual KCMP(2)

NAME
kcmp - compare two processes to determine if they share a kernel resource

SYNOPSIS
#include <linux/kcmp.h>

int kcmp(pid_t pid1, pid_t pid2, int type,
unsigned long idx1, unsigned long idx2);

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

DESCRIPTION
The kcmp() system call can be used to check whether the two processes identified by pid1
and pid2 share a kernel resource such as virtual memory, file descriptors, and so on.

The type argument specifies which resource is to be compared in the two processes. It has
one of the following values:

KCMP_FILE
Check whether a file descriptor idx1 in the process pid1 refers to the same open
file description (see open(2)) as file descriptor idx2 in the process pid2.

KCMP_FILES
Check whether the process share the same set of open file descriptors. The argu‐
ments idx1 and idx2 are ignored.

KCMP_FS
Check whether the processes share the same filesystem information (i.e., file mode
creation mask, working directory, and filesystem root). The arguments idx1 and
idx2 are ignored.

KCMP_IO
Check whether the processes share I/O context. The arguments idx1 and idx2 are
ignored.

KCMP_SIGHAND
Check whether the processes share the same table of signal dispositions. The argu‐
ments idx1 and idx2 are ignored.

KCMP_SYSVSEM
Check whether the processes share the same list of System V semaphore undo opera‐
tions. The arguments idx1 and idx2 are ignored.

KCMP_VM
Check whether the processes share the same address space. The arguments idx1 and
idx2 are ignored.

Note the kcmp() is not protected against false positives which may occur if tasks are run‐
ning. One should stop tasks by sending SIGSTOP (see signal(7)) prior to inspection with
this system call to obtain meaningful results.

RETURN VALUE
The return value of a successful call to kcmp() is simply the result of arithmetic compar‐
ison of kernel pointers (when the kernel compares resources, it uses their memory
addresses).

The easiest way to explain is to consider an example. Suppose that v1 and v2 are the
addresses of appropriate resources, then the return value is one of the following:

0 v1 is equal to v2; in other words, the two processes share the resource.

1 v1 is less than v2.

2 v1 is greater than v2.

3 v1 is not equal to v2, but ordering information is unavailable.

On error, -1 is returned, and errno is set appropriately.

kcmp() was designed to return values suitable for sorting. This is particularly handy if
one needs to compare a large number of file descriptors.

ERRORS
EBADF type is KCMP_FILE and fd1 or fd2 is not an open file descriptor.

EINVAL type is invalid.

EPERM Insufficient permission to inspect process resources. The CAP_SYS_PTRACE capabil‐
ity is required to inspect processes that you do not own.

ESRCH Process pid1 or pid2 does not exist.

VERSIONS
The kcmp() system call first appeared in Linux 3.5.

CONFORMING TO
kcmp() is Linux-specific and should not be used in programs intended to be portable.

NOTES
Glibc does not provide a wrapper for this system call; call it using syscall(2).

This system call is available only if the kernel was configured with CONFIG_CHECK‐
POINT_RESTORE. The main use of the system call is for the checkpoint/restore in user
space (CRIU) feature. The alternative to this system call would have been to expose suit‐
able process information via the proc(5) filesystem; this was deemed to be unsuitable for
security reasons.

See clone(2) for some background information on the shared resources referred to on this
page.