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 descriptor 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 information (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  dispositions.   The
              arguments idx1 and idx2 are ignored.

       KCMP_SYSVSEM
              Check  whether  the  processes  share  the  same  list  of  System V semaphore 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 security 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/.