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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 descrip-
              tor 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 file system  informa-
              tion (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 dis-
              positions.  The arguments idx1 and idx2 are ignored.
       KCMP_SYSVSEM
              Check whether the processes share the same list of System V sem-
              aphore  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 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 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.
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_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) file system; this was deemed to be unsuitable for secu-
       rity 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)