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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
kcmp(2) System Calls Manualkcmp(2)kcmp - compare two processes to determine if they share a kernel resource
Standard C library (libc,-lc)
#include <linux/kcmp.h>/* Definition ofKCMP_*constants */#include <sys/syscall.h>/* Definition ofSYS_*constants */#include <unistd.h>int syscall(SYS_kcmp, pid_tpid1, pid_tpid2, inttype,unsigned longidx1, unsigned longidx2);Note: glibc provides no wrapper forkcmp(), necessitating the use ofsyscall(2).
Thekcmp() system call can be used to check whether the two processes identified bypid1 andpid2 share a kernel resource such as virtual memory, file descriptors, and so on. Permission to employkcmp() is governed by ptrace access modePTRACE_MODE_READ_REALCREDSchecks against bothpid1 andpid2; seeptrace(2). Thetype 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 descriptoridx1 in the processpid1 refers to the same open file description (seeopen(2)) as file descriptoridx2 in the processpid2. The existence of two file descriptors that refer to the same open file description can occur as a result ofdup(2) (and similar)fork(2), or passing file descriptors via a domain socket (seeunix(7)).KCMP_FILES Check whether the processes share the same set of open file descriptors. The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_FILESflag inclone(2).KCMP_FS Check whether the processes share the same filesystem information (i.e., file mode creation mask, working directory, and filesystem root). The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_FSflag inclone(2).KCMP_IO Check whether the processes share I/O context. The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_IOflag inclone(2).KCMP_SIGHAND Check whether the processes share the same table of signal dispositions. The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_SIGHANDflag inclone(2).KCMP_SYSVSEM Check whether the processes share the same list of System V semaphore undo operations. The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_SYSVSEMflag inclone(2).KCMP_VM Check whether the processes share the same address space. The argumentsidx1 andidx2 are ignored. See the discussion of theCLONE_VMflag inclone(2).KCMP_EPOLL_TFD(since Linux 4.13) Check whether the file descriptoridx1 of the processpid1 is present in theepoll(7) instance described byidx2 of the processpid2. The argumentidx2 is a pointer to a structure where the target file is described. This structure has the form: struct kcmp_epoll_slot { __u32 efd; __u32 tfd; __u64 toff; }; Within this structure,efd is an epoll file descriptor returned fromepoll_create(2),tfd is a target file descriptor number, andtoff is a target file offset counted from zero. Several different targets may be registered with the same file descriptor number and setting a specific offset helps to investigate each of them. Note thekcmp() is not protected against false positives which may occur if the processes are currently running. One should stop the processes by sendingSIGSTOP(seesignal(7)) prior to inspection with this system call to obtain meaningful results.The return value of a successful call tokcmp() 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 thatv1 andv2 are the addresses of appropriate resources, then the return value is one of the following:0v1 is equal tov2; in other words, the two processes share the resource.1v1 is less thanv2.2v1 is greater thanv2.3v1 is not equal tov2, but ordering information is unavailable. On error, -1 is returned, anderrno is set to indicate the error.kcmp() was designed to return values suitable for sorting. This is particularly handy if one needs to compare a large number of file descriptors.
EBADFtype isKCMP_FILEandfd1 orfd2 is not an open file descriptor.EFAULTThe epoll slot addressed byidx2 is outside of the user's address space.EINVALtype is invalid.ENOENTThe target file is not present inepoll(7) instance.EPERMInsufficient permission to inspect process resources. TheCAP_SYS_PTRACEcapability is required to inspect processes that you do not own. Other ptrace limitations may also apply, such asCONFIG_SECURITY_YAMA, which, when/proc/sys/kernel/yama/ptrace_scope is 2, limitskcmp() to child processes; seeptrace(2).ESRCHProcesspid1 orpid2 does not exist.
Linux.
Linux 3.5. Before Linux 5.12, this system call is available only if the kernel is configured withCONFIG_CHECKPOINT_RESTORE, since the original purpose of the system call was for the checkpoint/restore in user space (CRIU) feature. (The alternative to this system call would have been to expose suitable process information via theproc(5) filesystem; this was deemed to be unsuitable for security reasons.) Since Linux 5.12, this system call is also available if the kernel is configured withCONFIG_KCMP.
Seeclone(2) for some background information on the shared resources referred to on this page.
The program below useskcmp() 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) ==> sameProgram source #define _GNU_SOURCE #include <err.h> #include <fcntl.h> #include <linux/kcmp.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <sys/syscall.h> #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> 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, pid_t pid1, pid_t pid2, int fd_a, int fd_b) { printf("\t%s\n", msg); printf("\t\tkcmp(%jd, %jd, KCMP_FILE, %d, %d) ==> %s\n", (intmax_t) pid1, (intmax_t) pid2, fd_a, fd_b, (kcmp(pid1, pid2, KCMP_FILE, fd_a, fd_b) == 0) ? "same" : "different"); } int main(void) { int fd1, fd2, fd3; static const char pathname[] = "/tmp/kcmp.test"; fd1 = open(pathname, O_CREAT | O_RDWR, 0600); if (fd1 == -1) err(EXIT_FAILURE, "open"); printf("Parent PID is %jd\n", (intmax_t) getpid()); printf("Parent opened file on FD %d\n\n", fd1); switch (fork()) { case -1: err(EXIT_FAILURE, "fork"); case 0: printf("PID of child of fork() is %jd\n", (intmax_t) getpid()); test_kcmp("Compare duplicate FDs from different processes:", getpid(), getppid(), fd1, fd1); fd2 = open(pathname, O_CREAT | O_RDWR, 0600); if (fd2 == -1) err(EXIT_FAILURE, "open"); 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) err(EXIT_FAILURE, "dup"); printf("Child duplicated FD %d to create FD %d\n", fd1, fd3); test_kcmp("Compare duplicated FDs in same process:", getpid(), getpid(), fd1, fd3); break; default: wait(NULL); } exit(EXIT_SUCCESS); }clone(2),unshare(2)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17kcmp(2)Pages that refer to this page:clone(2), open(2), pidfd_getfd(2), ptrace(2), syscalls(2), unshare(2), capabilities(7), epoll(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |