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

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

       #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.

       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:

              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.

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

              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.

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

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

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

              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.

       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

       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.

       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.

       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.

       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

       clone(2), unshare(2)

       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/.

Linux                                       2013-12-08                                    KCMP(2)

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