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       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
              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

              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 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 have place if tasks are
       running.  Which means one should stop tasks being inspected with this  syscall  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

       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
              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 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 security reasons.

       See clone(2) for some background information on the shared resources referred to  on  this


       clone(2), unshare(2)


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