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26 .\" Kernel commit d97b46a64674a267bc41c9e16132ee2a98c3347d
28 .TH KCMP 2 2017-03-13 "Linux" "Linux Programmer's Manual"
30 kcmp \- compare two processes to determine if they share a kernel resource
33 .B #include <linux/kcmp.h>
35 .BI "int kcmp(pid_t " pid1 ", pid_t " pid2 ", int " type ,
36 .BI " unsigned long " idx1 ", unsigned long " idx2 );
40 There is no glibc wrapper for this system call; see NOTES.
44 system call can be used to check whether the two processes identified by
48 share a kernel resource such as virtual memory, file descriptors,
53 is governed by ptrace access mode
54 .B PTRACE_MODE_READ_REALCREDS
64 argument specifies which resource is to be compared in the two processes.
65 It has one of the following values:
68 Check whether a file descriptor
72 refers to the same open file description (see
78 The existence of two file descriptors that refer to the same
79 open file description can occur as a result of
83 or passing file descriptors via a domain socket (see
87 Check whether the processes share the same set of open file descriptors.
93 See the discussion of the
99 Check whether the processes share the same filesystem information
100 (i.e., file mode creation mask, working directory, and filesystem root).
106 See the discussion of the
112 Check whether the processes share I/O context.
118 See the discussion of the
124 Check whether the processes share the same table of signal dispositions.
130 See the discussion of the
136 Check whether the processes share the same
137 list of System\ V semaphore undo operations.
143 See the discussion of the
149 Check whether the processes share the same address space.
155 See the discussion of the
162 is not protected against false positives which may occur if
163 the processes are currently running.
164 One should stop the processes by sending
168 prior to inspection with this system call to obtain meaningful results.
170 The return value of a successful call to
172 is simply the result of arithmetic comparison
173 of kernel pointers (when the kernel compares resources, it uses their
176 The easiest way to explain is to consider an example.
181 are the addresses of appropriate resources, then the return value
182 is one of the following:
188 in other words, the two processes share the resource.
201 but ordering information is unavailable.
204 On error, \-1 is returned, and
206 is set appropriately.
209 was designed to return values suitable for sorting.
210 This is particularly handy if one needs to compare
211 a large number of file descriptors.
222 is not an open file descriptor.
229 Insufficient permission to inspect process resources.
232 capability is required to inspect processes that you do not own.
233 Other ptrace limitations may also apply, such as
234 .BR CONFIG_SECURITY_YAMA ,
236 .I /proc/sys/kernel/yama/ptrace_scope
252 system call first appeared in Linux 3.5.
255 is Linux-specific and should not be used in programs intended to be portable.
257 Glibc does not provide a wrapper for this system call; call it using
260 This system call is available only if the kernel was configured with
261 .BR CONFIG_CHECKPOINT_RESTORE .
262 The main use of the system call is for the
263 checkpoint/restore in user space (CRIU) feature.
264 The alternative to this system call would have been to expose suitable
265 process information via the
267 filesystem; this was deemed to be unsuitable for security reasons.
271 for some background information on the shared resources
272 referred to on this page.
274 The program below uses
276 to test whether pairs of file descriptors refer to
277 the same open file description.
278 The program tests different cases for the file descriptor pairs,
279 as described in the program output.
280 An example run of the program is as follows:
286 Parent opened file on FD 3
288 PID of child of fork() is 1145
289 Compare duplicate FDs from different processes:
290 kcmp(1145, 1144, KCMP_FILE, 3, 3) ==> same
291 Child opened file on FD 4
292 Compare FDs from distinct open()s in same process:
293 kcmp(1145, 1145, KCMP_FILE, 3, 4) ==> different
294 Child duplicated FD 3 to create FD 5
295 Compare duplicated FDs in same process:
296 kcmp(1145, 1145, KCMP_FILE, 3, 5) ==> same
303 #include <sys/syscall.h>
304 #include <sys/wait.h>
305 #include <sys/stat.h>
310 #include <linux/kcmp.h>
312 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
316 kcmp(pid_t pid1, pid_t pid2, int type,
317 unsigned long idx1, unsigned long idx2)
319 return syscall(SYS_kcmp, pid1, pid2, type, idx1, idx2);
323 test_kcmp(char *msg, id_t pid1, pid_t pid2, int fd_a, int fd_b)
325 printf("\\t%s\\n", msg);
326 printf("\\t\\tkcmp(%ld, %ld, KCMP_FILE, %d, %d) ==> %s\\n",
327 (long) pid1, (long) pid2, fd_a, fd_b,
328 (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ?
329 "same" : "different");
333 main(int argc, char *argv[])
336 char pathname[] = "/tmp/kcmp.test";
338 fd1 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
342 printf("Parent PID is %ld\\n", (long) getpid());
343 printf("Parent opened file on FD %d\\n\\n", fd1);
350 printf("PID of child of fork() is %ld\\n", (long) getpid());
352 test_kcmp("Compare duplicate FDs from different processes:",
353 getpid(), getppid(), fd1, fd1);
355 fd2 = open(pathname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
358 printf("Child opened file on FD %d\\n", fd2);
360 test_kcmp("Compare FDs from distinct open()s in same process:",
361 getpid(), getpid(), fd1, fd2);
366 printf("Child duplicated FD %d to create FD %d\\n", fd1, fd3);
368 test_kcmp("Compare duplicated FDs in same process:",
369 getpid(), getpid(), fd1, fd3);