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.

   Permission  to  employ  kcmp()  is  governed  by  ptrace  access   mode
   PTRACE_MODE_READ_REALCREDS  checks  against  both  pid1  and  pid2; see

   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 the processes are currently  running.   One  should  stop  the
   processes  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.  Other ptrace limitations may also apply, such
          as          CONFIG_SECURITY_YAMA,          which,           when
          /proc/sys/kernel/yama/ptrace_scope  is 2, limits kcmp() to child
          processes; see ptrace(2).

   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.


   The program below uses kcmp() to test whether pairs of file descriptors
   refer  to  the same open file description.  The program tests different
   cases for the file  descriptor  pairs,  as  described  in  the  program
   output.  An example run of the program is as follows:

       $ ./a.out
       Parent PID is 1144
       Parent opened file on FD 3

       PID of child of fork() is 1145
            Compare duplicate FDs from different processes:
                 kcmp(1145, 1144, KCMP_FILE, 3, 3) ==> same
       Child opened file on FD 4
            Compare FDs from distinct open()s in same process:
                 kcmp(1145, 1145, KCMP_FILE, 3, 4) ==> different
       Child duplicated FD 3 to create FD 5
            Compare duplicated FDs in same process:
                 kcmp(1145, 1145, KCMP_FILE, 3, 5) ==> same

   Program source

   #define _GNU_SOURCE
   #include <sys/syscall.h>
   #include <sys/wait.h>
   #include <sys/stat.h>
   #include <stdlib.h>
   #include <stdio.h>
   #include <unistd.h>
   #include <fcntl.h>
   #include <linux/kcmp.h>

   #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                           } while (0)

   static int
   kcmp(pid_t pid1, pid_t pid2, int type,
        unsigned long idx1, unsigned long idx2)
       return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);

   static void
   test_kcmp(char *msg, id_t pid1, pid_t pid2, int fd_a, int fd_b)
       printf("\t%s\n", msg);
       printf("\t\tkcmp(%ld, %ld, KCMP_FILE, %d, %d) ==> %s\n",
               (long) pid1, (long) pid2, fd_a, fd_b,
               (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
                           "same" : "different");

   main(int argc, char *argv[])
       int fd1, fd2, fd3;
       char pathname[] = "/tmp/kcmp.test";

       fd1 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
       if (fd1 == -1)

       printf("Parent PID is %ld\n", (long) getpid());
       printf("Parent opened file on FD %d\n\n", fd1);

       switch (fork()) {
       case -1:

       case 0:
           printf("PID of child of fork() is %ld\n", (long) getpid());

           test_kcmp("Compare duplicate FDs from different processes:",
                   getpid(), getppid(), fd1, fd1);

           fd2 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
           if (fd2 == -1)
           printf("Child opened file on FD %d\n", fd2);

           test_kcmp("Compare FDs from distinct open()s in same process:",
                   getpid(), getpid(), fd1, fd2);

           fd3 = dup(fd1);
           if (fd3 == -1)
           printf("Child duplicated FD %d to create FD %d\n", fd1, fd3);

           test_kcmp("Compare duplicated FDs in same process:",
                   getpid(), getpid(), fd1, fd3);




   clone(2), unshare(2)


   This  page  is  part of release 4.09 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


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