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
arguments 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 arguments idx1 and idx2 are ignored.
- KCMP_SYSVSEM
- Check whether the processes share the same list of System V
semaphore undo operations. 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 running. 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 comparison 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 capability 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.
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_CHECKPOINT_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 suitable 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.
SEE ALSO¶
clone(2),
unshare(2)
COLOPHON¶
This page is part of release 3.74 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
http://www.kernel.org/doc/man-pages/.