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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 file system information (i.e., file mode
	      creation mask, working directory, and file system 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 have place if tasks are
       running.  Which means one should stop tasks being inspected with this  syscall  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) file system; 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 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-01-27					  KCMP(2)
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