userfaultfd(2) System Calls Manual userfaultfd(2)
NAME
userfaultfd - create a file descriptor for handling page faults in user
space
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <fcntl.h> /* Definition of O_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <linux/userfaultfd.h> /* Definition of UFFD_* constants */
#include <unistd.h>
int syscall(SYS_userfaultfd, int flags);
Note: glibc provides no wrapper for userfaultfd(), necessitating the
use of syscall(2).
DESCRIPTION
userfaultfd() creates a new userfaultfd object that can be used for
delegation of page-fault handling to a user-space application, and re-
turns a file descriptor that refers to the new object. The new user-
faultfd object is configured using ioctl(2).
Once the userfaultfd object is configured, the application can use
read(2) to receive userfaultfd notifications. The reads from user-
faultfd may be blocking or non-blocking, depending on the value of
flags used for the creation of the userfaultfd or subsequent calls to
fcntl(2).
The following values may be bitwise ORed in flags to change the behav-
ior of userfaultfd():
O_CLOEXEC
Enable the close-on-exec flag for the new userfaultfd file de-
scriptor. See the description of the O_CLOEXEC flag in open(2).
O_NONBLOCK
Enables non-blocking operation for the userfaultfd object. See
the description of the O_NONBLOCK flag in open(2).
UFFD_USER_MODE_ONLY
This is an userfaultfd-specific flag that was introduced in
Linux 5.11. When set, the userfaultfd object will only be able
to handle page faults originated from the user space on the reg-
istered regions. When a kernel-originated fault was triggered
on the registered range with this userfaultfd, a SIGBUS signal
will be delivered.
When the last file descriptor referring to a userfaultfd object is
closed, all memory ranges that were registered with the object are un-
registered and unread events are flushed.
Userfaultfd supports three modes of registration:
UFFDIO_REGISTER_MODE_MISSING (since Linux 4.10)
When registered with UFFDIO_REGISTER_MODE_MISSING mode, user-
space will receive a page-fault notification when a missing page
is accessed. The faulted thread will be stopped from execution
until the page fault is resolved from user-space by either an
UFFDIO_COPY or an UFFDIO_ZEROPAGE ioctl.
UFFDIO_REGISTER_MODE_MINOR (since Linux 5.13)
When registered with UFFDIO_REGISTER_MODE_MINOR mode, user-space
will receive a page-fault notification when a minor page fault
occurs. That is, when a backing page is in the page cache, but
page table entries don't yet exist. The faulted thread will be
stopped from execution until the page fault is resolved from
user-space by an UFFDIO_CONTINUE ioctl.
UFFDIO_REGISTER_MODE_WP (since Linux 5.7)
When registered with UFFDIO_REGISTER_MODE_WP mode, user-space
will receive a page-fault notification when a write-protected
page is written. The faulted thread will be stopped from execu-
tion until user-space write-unprotects the page using an UFF-
DIO_WRITEPROTECT ioctl.
Multiple modes can be enabled at the same time for the same memory
range.
Since Linux 4.14, a userfaultfd page-fault notification can selectively
embed faulting thread ID information into the notification. One needs
to enable this feature explicitly using the UFFD_FEATURE_THREAD_ID fea-
ture bit when initializing the userfaultfd context. By default, thread
ID reporting is disabled.
Usage
The userfaultfd mechanism is designed to allow a thread in a multi-
threaded program to perform user-space paging for the other threads in
the process. When a page fault occurs for one of the regions regis-
tered to the userfaultfd object, the faulting thread is put to sleep
and an event is generated that can be read via the userfaultfd file de-
scriptor. The fault-handling thread reads events from this file de-
scriptor and services them using the operations described in
ioctl_userfaultfd(2). When servicing the page fault events, the fault-
handling thread can trigger a wake-up for the sleeping thread.
It is possible for the faulting threads and the fault-handling threads
to run in the context of different processes. In this case, these
threads may belong to different programs, and the program that executes
the faulting threads will not necessarily cooperate with the program
that handles the page faults. In such non-cooperative mode, the
process that monitors userfaultfd and handles page faults needs to be
aware of the changes in the virtual memory layout of the faulting
process to avoid memory corruption.
Since Linux 4.11, userfaultfd can also notify the fault-handling
threads about changes in the virtual memory layout of the faulting
process. In addition, if the faulting process invokes fork(2), the
userfaultfd objects associated with the parent may be duplicated into
the child process and the userfaultfd monitor will be notified (via the
UFFD_EVENT_FORK described below) about the file descriptor associated
with the userfault objects created for the child process, which allows
the userfaultfd monitor to perform user-space paging for the child
process. Unlike page faults which have to be synchronous and require
an explicit or implicit wakeup, all other events are delivered asyn-
chronously and the non-cooperative process resumes execution as soon as
the userfaultfd manager executes read(2). The userfaultfd manager
should carefully synchronize calls to UFFDIO_COPY with the processing
of events.
The current asynchronous model of the event delivery is optimal for
single threaded non-cooperative userfaultfd manager implementations.
Since Linux 5.7, userfaultfd is able to do synchronous page dirty
tracking using the new write-protect register mode. One should check
against the feature bit UFFD_FEATURE_PAGEFAULT_FLAG_WP before using
this feature. Similar to the original userfaultfd missing mode, the
write-protect mode will generate a userfaultfd notification when the
protected page is written. The user needs to resolve the page fault by
unprotecting the faulted page and kicking the faulted thread to con-
tinue. For more information, please refer to the "Userfaultfd write-
protect mode" section.
Userfaultfd operation
After the userfaultfd object is created with userfaultfd(), the appli-
cation must enable it using the UFFDIO_API ioctl(2) operation. This
operation allows a handshake between the kernel and user space to de-
termine the API version and supported features. This operation must be
performed before any of the other ioctl(2) operations described below
(or those operations fail with the EINVAL error).
After a successful UFFDIO_API operation, the application then registers
memory address ranges using the UFFDIO_REGISTER ioctl(2) operation.
After successful completion of a UFFDIO_REGISTER operation, a page
fault occurring in the requested memory range, and satisfying the mode
defined at the registration time, will be forwarded by the kernel to
the user-space application. The application can then use the UFF-
DIO_COPY , UFFDIO_ZEROPAGE , or UFFDIO_CONTINUE ioctl(2) operations to
resolve the page fault.
Since Linux 4.14, if the application sets the UFFD_FEATURE_SIGBUS fea-
ture bit using the UFFDIO_API ioctl(2), no page-fault notification will
be forwarded to user space. Instead a SIGBUS signal is delivered to
the faulting process. With this feature, userfaultfd can be used for
robustness purposes to simply catch any access to areas within the reg-
istered address range that do not have pages allocated, without having
to listen to userfaultfd events. No userfaultfd monitor will be re-
quired for dealing with such memory accesses. For example, this fea-
ture can be useful for applications that want to prevent the kernel
from automatically allocating pages and filling holes in sparse files
when the hole is accessed through a memory mapping.
The UFFD_FEATURE_SIGBUS feature is implicitly inherited through fork(2)
if used in combination with UFFD_FEATURE_FORK.
Details of the various ioctl(2) operations can be found in ioctl_user-
faultfd(2).
Since Linux 4.11, events other than page-fault may enabled during UFF-
DIO_API operation.
Up to Linux 4.11, userfaultfd can be used only with anonymous private
memory mappings. Since Linux 4.11, userfaultfd can be also used with
hugetlbfs and shared memory mappings.
Userfaultfd write-protect mode (since Linux 5.7)
Since Linux 5.7, userfaultfd supports write-protect mode for anonymous
memory. The user needs to first check availability of this feature us-
ing UFFDIO_API ioctl against the feature bit UFFD_FEATURE_PAGE-
FAULT_FLAG_WP before using this feature.
Since Linux 5.19, the write-protection mode was also supported on shmem
and hugetlbfs memory types. It can be detected with the feature bit
UFFD_FEATURE_WP_HUGETLBFS_SHMEM.
To register with userfaultfd write-protect mode, the user needs to ini-
tiate the UFFDIO_REGISTER ioctl with mode UFFDIO_REGISTER_MODE_WP set.
Note that it is legal to monitor the same memory range with multiple
modes. For example, the user can do UFFDIO_REGISTER with the mode set
to UFFDIO_REGISTER_MODE_MISSING | UFFDIO_REGISTER_MODE_WP. When there
is only UFFDIO_REGISTER_MODE_WP registered, user-space will not receive
any notification when a missing page is written. Instead, user-space
will receive a write-protect page-fault notification only when an ex-
isting but write-protected page got written.
After the UFFDIO_REGISTER ioctl completed with UFFDIO_REGISTER_MODE_WP
mode set, the user can write-protect any existing memory within the
range using the ioctl UFFDIO_WRITEPROTECT where uffdio_writepro-
tect.mode should be set to UFFDIO_WRITEPROTECT_MODE_WP.
When a write-protect event happens, user-space will receive a page-
fault notification whose uffd_msg.pagefault.flags will be with
UFFD_PAGEFAULT_FLAG_WP flag set. Note: since only writes can trigger
this kind of fault, write-protect notifications will always have the
UFFD_PAGEFAULT_FLAG_WRITE bit set along with the UFFD_PAGEFAULT_FLAG_WP
bit.
To resolve a write-protection page fault, the user should initiate an-
other UFFDIO_WRITEPROTECT ioctl, whose uffd_msg.pagefault.flags should
have the flag UFFDIO_WRITEPROTECT_MODE_WP cleared upon the faulted page
or range.
Userfaultfd minor fault mode (since Linux 5.13)
Since Linux 5.13, userfaultfd supports minor fault mode. In this mode,
fault messages are produced not for major faults (where the page was
missing), but rather for minor faults, where a page exists in the page
cache, but the page table entries are not yet present. The user needs
to first check availability of this feature using the UFFDIO_API ioctl
with the appropriate feature bits set before using this feature:
UFFD_FEATURE_MINOR_HUGETLBFS since Linux 5.13, or UFFD_FEATURE_MI-
NOR_SHMEM since Linux 5.14.
To register with userfaultfd minor fault mode, the user needs to initi-
ate the UFFDIO_REGISTER ioctl with mode UFFD_REGISTER_MODE_MINOR set.
When a minor fault occurs, user-space will receive a page-fault notifi-
cation whose uffd_msg.pagefault.flags will have the UFFD_PAGE-
FAULT_FLAG_MINOR flag set.
To resolve a minor page fault, the handler should decide whether or not
the existing page contents need to be modified first. If so, this
should be done in-place via a second, non-userfaultfd-registered map-
ping to the same backing page (e.g., by mapping the shmem or hugetlbfs
file twice). Once the page is considered "up to date", the fault can
be resolved by initiating an UFFDIO_CONTINUE ioctl, which installs the
page table entries and (by default) wakes up the faulting thread(s).
Minor fault mode supports only hugetlbfs-backed (since Linux 5.13) and
shmem-backed (since Linux 5.14) memory.
Reading from the userfaultfd structure
Each read(2) from the userfaultfd file descriptor returns one or more
uffd_msg structures, each of which describes a page-fault event or an
event required for the non-cooperative userfaultfd usage:
struct uffd_msg {
__u8 event; /* Type of event */
...
union {
struct {
__u64 flags; /* Flags describing fault */
__u64 address; /* Faulting address */
union {
__u32 ptid; /* Thread ID of the fault */
} feat;
} pagefault;
struct { /* Since Linux 4.11 */
__u32 ufd; /* Userfault file descriptor
of the child process */
} fork;
struct { /* Since Linux 4.11 */
__u64 from; /* Old address of remapped area */
__u64 to; /* New address of remapped area */
__u64 len; /* Original mapping length */
} remap;
struct { /* Since Linux 4.11 */
__u64 start; /* Start address of removed area */
__u64 end; /* End address of removed area */
} remove;
...
} arg;
/* Padding fields omitted */
} __packed;
If multiple events are available and the supplied buffer is large
enough, read(2) returns as many events as will fit in the supplied buf-
fer. If the buffer supplied to read(2) is smaller than the size of the
uffd_msg structure, the read(2) fails with the error EINVAL.
The fields set in the uffd_msg structure are as follows:
event The type of event. Depending of the event type, different
fields of the arg union represent details required for the event
processing. The non-page-fault events are generated only when
appropriate feature is enabled during API handshake with UFF-
DIO_API ioctl(2).
The following values can appear in the event field:
UFFD_EVENT_PAGEFAULT (since Linux 4.3)
A page-fault event. The page-fault details are available
in the pagefault field.
UFFD_EVENT_FORK (since Linux 4.11)
Generated when the faulting process invokes fork(2) (or
clone(2) without the CLONE_VM flag). The event details
are available in the fork field.
UFFD_EVENT_REMAP (since Linux 4.11)
Generated when the faulting process invokes mremap(2).
The event details are available in the remap field.
UFFD_EVENT_REMOVE (since Linux 4.11)
Generated when the faulting process invokes madvise(2)
with MADV_DONTNEED or MADV_REMOVE advice. The event de-
tails are available in the remove field.
UFFD_EVENT_UNMAP (since Linux 4.11)
Generated when the faulting process unmaps a memory
range, either explicitly using munmap(2) or implicitly
during mmap(2) or mremap(2). The event details are
available in the remove field.
pagefault.address
The address that triggered the page fault.
pagefault.flags
A bit mask of flags that describe the event. For
UFFD_EVENT_PAGEFAULT, the following flag may appear:
UFFD_PAGEFAULT_FLAG_WP
If this flag is set, then the fault was a write-protect
fault.
UFFD_PAGEFAULT_FLAG_MINOR
If this flag is set, then the fault was a minor fault.
UFFD_PAGEFAULT_FLAG_WRITE
If this flag is set, then the fault was a write fault.
If neither UFFD_PAGEFAULT_FLAG_WP nor UFFD_PAGEFAULT_FLAG_MINOR
are set, then the fault was a missing fault.
pagefault.feat.pid
The thread ID that triggered the page fault.
fork.ufd
The file descriptor associated with the userfault object created
for the child created by fork(2).
remap.from
The original address of the memory range that was remapped using
mremap(2).
remap.to
The new address of the memory range that was remapped using
mremap(2).
remap.len
The original length of the memory range that was remapped using
mremap(2).
remove.start
The start address of the memory range that was freed using mad-
vise(2) or unmapped
remove.end
The end address of the memory range that was freed using mad-
vise(2) or unmapped
A read(2) on a userfaultfd file descriptor can fail with the following
errors:
EINVAL The userfaultfd object has not yet been enabled using the UFF-
DIO_API ioctl(2) operation
If the O_NONBLOCK flag is enabled in the associated open file descrip-
tion, the userfaultfd file descriptor can be monitored with poll(2),
select(2), and epoll(7). When events are available, the file descrip-
tor indicates as readable. If the O_NONBLOCK flag is not enabled, then
poll(2) (always) indicates the file as having a POLLERR condition, and
select(2) indicates the file descriptor as both readable and writable.
RETURN VALUE
On success, userfaultfd() returns a new file descriptor that refers to
the userfaultfd object. On error, -1 is returned, and errno is set to
indicate the error.
ERRORS
EINVAL An unsupported value was specified in flags.
EMFILE The per-process limit on the number of open file descriptors has
been reached
ENFILE The system-wide limit on the total number of open files has been
reached.
ENOMEM Insufficient kernel memory was available.
EPERM (since Linux 5.2)
The caller is not privileged (does not have the CAP_SYS_PTRACE
capability in the initial user namespace), and /proc/sys/vm/un-
privileged_userfaultfd has the value 0.
VERSIONS
The userfaultfd() system call first appeared in Linux 4.3.
The support for hugetlbfs and shared memory areas and non-page-fault
events was added in Linux 4.11
STANDARDS
userfaultfd() is Linux-specific and should not be used in programs in-
tended to be portable.
NOTES
The userfaultfd mechanism can be used as an alternative to traditional
user-space paging techniques based on the use of the SIGSEGV signal and
mmap(2). It can also be used to implement lazy restore for check-
point/restore mechanisms, as well as post-copy migration to allow
(nearly) uninterrupted execution when transferring virtual machines and
Linux containers from one host to another.
BUGS
If the UFFD_FEATURE_EVENT_FORK is enabled and a system call from the
fork(2) family is interrupted by a signal or failed, a stale user-
faultfd descriptor might be created. In this case, a spurious
UFFD_EVENT_FORK will be delivered to the userfaultfd monitor.
EXAMPLES
The program below demonstrates the use of the userfaultfd mechanism.
The program creates two threads, one of which acts as the page-fault
handler for the process, for the pages in a demand-page zero region
created using mmap(2).
The program takes one command-line argument, which is the number of
pages that will be created in a mapping whose page faults will be han-
dled via userfaultfd. After creating a userfaultfd object, the program
then creates an anonymous private mapping of the specified size and
registers the address range of that mapping using the UFFDIO_REGISTER
ioctl(2) operation. The program then creates a second thread that will
perform the task of handling page faults.
The main thread then walks through the pages of the mapping fetching
bytes from successive pages. Because the pages have not yet been ac-
cessed, the first access of a byte in each page will trigger a page-
fault event on the userfaultfd file descriptor.
Each of the page-fault events is handled by the second thread, which
sits in a loop processing input from the userfaultfd file descriptor.
In each loop iteration, the second thread first calls poll(2) to check
the state of the file descriptor, and then reads an event from the file
descriptor. All such events should be UFFD_EVENT_PAGEFAULT events,
which the thread handles by copying a page of data into the faulting
region using the UFFDIO_COPY ioctl(2) operation.
The following is an example of what we see when running the program:
$ ./userfaultfd_demo 3
Address returned by mmap() = 0x7fd30106c000
fault_handler_thread():
poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106c00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106c00f in main(): A
Read address 0x7fd30106c40f in main(): A
Read address 0x7fd30106c80f in main(): A
Read address 0x7fd30106cc0f in main(): A
fault_handler_thread():
poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106d00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106d00f in main(): B
Read address 0x7fd30106d40f in main(): B
Read address 0x7fd30106d80f in main(): B
Read address 0x7fd30106dc0f in main(): B
fault_handler_thread():
poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fd30106e00f
(uffdio_copy.copy returned 4096)
Read address 0x7fd30106e00f in main(): C
Read address 0x7fd30106e40f in main(): C
Read address 0x7fd30106e80f in main(): C
Read address 0x7fd30106ec0f in main(): C
Program source
/* userfaultfd_demo.c
Licensed under the GNU General Public License version 2 or later.
*/
#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <linux/userfaultfd.h>
#include <poll.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
static int page_size;
static void *
fault_handler_thread(void *arg)
{
int nready;
long uffd; /* userfaultfd file descriptor */
ssize_t nread;
struct pollfd pollfd;
struct uffdio_copy uffdio_copy;
static int fault_cnt = 0; /* Number of faults so far handled */
static char *page = NULL;
static struct uffd_msg msg; /* Data read from userfaultfd */
uffd = (long) arg;
/* Create a page that will be copied into the faulting region. */
if (page == NULL) {
page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (page == MAP_FAILED)
err(EXIT_FAILURE, "mmap");
}
/* Loop, handling incoming events on the userfaultfd
file descriptor. */
for (;;) {
/* See what poll() tells us about the userfaultfd. */
pollfd.fd = uffd;
pollfd.events = POLLIN;
nready = poll(&pollfd, 1, -1);
if (nready == -1)
err(EXIT_FAILURE, "poll");
printf("\nfault_handler_thread():\n");
printf(" poll() returns: nready = %d; "
"POLLIN = %d; POLLERR = %d\n", nready,
(pollfd.revents & POLLIN) != 0,
(pollfd.revents & POLLERR) != 0);
/* Read an event from the userfaultfd. */
nread = read(uffd, &msg, sizeof(msg));
if (nread == 0) {
printf("EOF on userfaultfd!\n");
exit(EXIT_FAILURE);
}
if (nread == -1)
err(EXIT_FAILURE, "read");
/* We expect only one kind of event; verify that assumption. */
if (msg.event != UFFD_EVENT_PAGEFAULT) {
fprintf(stderr, "Unexpected event on userfaultfd\n");
exit(EXIT_FAILURE);
}
/* Display info about the page-fault event. */
printf(" UFFD_EVENT_PAGEFAULT event: ");
printf("flags = %"PRIx64"; ", msg.arg.pagefault.flags);
printf("address = %"PRIx64"\n", msg.arg.pagefault.address);
/* Copy the page pointed to by 'page' into the faulting
region. Vary the contents that are copied in, so that it
is more obvious that each fault is handled separately. */
memset(page, 'A' + fault_cnt % 20, page_size);
fault_cnt++;
uffdio_copy.src = (unsigned long) page;
/* We need to handle page faults in units of pages(!).
So, round faulting address down to page boundary. */
uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address &
~(page_size - 1);
uffdio_copy.len = page_size;
uffdio_copy.mode = 0;
uffdio_copy.copy = 0;
if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1)
err(EXIT_FAILURE, "ioctl-UFFDIO_COPY");
printf(" (uffdio_copy.copy returned %"PRId64")\n",
uffdio_copy.copy);
}
}
int
main(int argc, char *argv[])
{
int s;
char c;
char *addr; /* Start of region handled by userfaultfd */
long uffd; /* userfaultfd file descriptor */
size_t len, l; /* Length of region handled by userfaultfd */
pthread_t thr; /* ID of thread that handles page faults */
struct uffdio_api uffdio_api;
struct uffdio_register uffdio_register;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-pages\n", argv[0]);
exit(EXIT_FAILURE);
}
page_size = sysconf(_SC_PAGE_SIZE);
len = strtoull(argv[1], NULL, 0) * page_size;
/* Create and enable userfaultfd object. */
uffd = syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK);
if (uffd == -1)
err(EXIT_FAILURE, "userfaultfd");
uffdio_api.api = UFFD_API;
uffdio_api.features = 0;
if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1)
err(EXIT_FAILURE, "ioctl-UFFDIO_API");
/* Create a private anonymous mapping. The memory will be
demand-zero paged--that is, not yet allocated. When we
actually touch the memory, it will be allocated via
the userfaultfd. */
addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (addr == MAP_FAILED)
err(EXIT_FAILURE, "mmap");
printf("Address returned by mmap() = %p\n", addr);
/* Register the memory range of the mapping we just created for
handling by the userfaultfd object. In mode, we request to track
missing pages (i.e., pages that have not yet been faulted in). */
uffdio_register.range.start = (unsigned long) addr;
uffdio_register.range.len = len;
uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1)
err(EXIT_FAILURE, "ioctl-UFFDIO_REGISTER");
/* Create a thread that will process the userfaultfd events. */
s = pthread_create(&thr, NULL, fault_handler_thread, (void *) uffd);
if (s != 0) {
errc(EXIT_FAILURE, s, "pthread_create");
}
/* Main thread now touches memory in the mapping, touching
locations 1024 bytes apart. This will trigger userfaultfd
events for all pages in the region. */
l = 0xf; /* Ensure that faulting address is not on a page
boundary, in order to test that we correctly
handle that case in fault_handling_thread(). */
while (l < len) {
c = addr[l];
printf("Read address %p in %s(): ", addr + l, __func__);
printf("%c\n", c);
l += 1024;
usleep(100000); /* Slow things down a little */
}
exit(EXIT_SUCCESS);
}
SEE ALSO
fcntl(2), ioctl(2), ioctl_userfaultfd(2), madvise(2), mmap(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source
tree
Linux man-pages 6.03 2023-02-05 userfaultfd(2)
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