kcmp

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  filesystem  informa-
              tion  (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 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
       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.

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) filesystem; 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 4.04 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/.

Linux                             2013-12-08                           KCMP(2)
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