KCMP(2) Linux Programmer's Manual KCMP(2)
kcmp - compare two processes to determine if they share a kernel
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.
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 descrip-
tor 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 informa-
tion (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 dis-
positions. The arguments idx1 and idx2 are ignored.
Check whether the processes share the same list of System V sem-
aphore 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 tasks are running. One should stop tasks by sending SIGSTOP
(see signal(7)) prior to inspection with this system call to obtain
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.
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
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 secu-
See clone(2) for some background information on the shared resources
referred to on this page.
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latest version of this page, can be found at
Linux 2013-12-08 KCMP(2)
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