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  have  place  if  tasks  are  running.
       Which means one should stop tasks being inspected with this syscall 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 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2013-01-27                                            KCMP(2)