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


       #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 arguments 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 ignored.

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

              Check whether the processes share  the  same  list  of  System V
              semaphore  undo  operations.   The  arguments  idx1 and idx2 are

              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.


       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

           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


       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.


       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

       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.


       clone(2), unshare(2)


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