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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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UFFDIO_API(2const) UFFDIO_API(2const)
UFFDIO_API - enable operation of the userfaultfd and perform API
handshake
Standard C library (libc, -lc)
#include <linux/userfaultfd.h> /* Definition of UFFD* constants */
#include <sys/ioctl.h>
int ioctl(int fd, UFFDIO_API, struct uffdio_api *argp);
#include <linux/userfaultfd.h>
struct uffdio_api {
__u64 api; /* Requested API version (input) */
__u64 features; /* Requested features (input/output) */
__u64 ioctls; /* Available ioctl() operations (output) */
};
Enable operation of the userfaultfd and perform API handshake.
The api field denotes the API version requested by the
application. The kernel verifies that it can support the
requested API version, and sets the features and ioctls fields to
bit masks representing all the available features and the generic
ioctl(2) operations available.
Before Linux 4.11, the features field must be initialized to zero
before the call to UFFDIO_API, and zero (i.e., no feature bits) is
placed in the features field by the kernel upon return from
ioctl(2).
Since Linux 4.11, userfaultfd supports features that need to be
enabled explicitly. To enable any of the features, one needs to
set the corresponding feature bits in features when issuing the
UFFDIO_API ioctl.
For historical reasons, a temporary userfaultfd is needed to probe
what userfaultfd features the kernel supports. The application
needs to create a temporary userfaultfd, issue an UFFDIO_API ioctl
with features set to zero. After the UFFDIO_API ioctl returns
successfully, features should contain all the userfaultfd features
that the kernel supports. The temporary userfaultfd can be safely
closed after the probe.
If the application sets unsupported feature bits, the kernel will
zero out the returned uffdio_api structure and return EINVAL.
The following feature bits may be set:
UFFD_FEATURE_EVENT_FORK (since Linux 4.11)
When this feature is enabled, the userfaultfd objects
associated with a parent process are duplicated into the
child process during fork(2) and a UFFD_EVENT_FORK event is
delivered to the userfaultfd monitor
UFFD_FEATURE_EVENT_REMAP (since Linux 4.11)
If this feature is enabled, when the faulting process
invokes mremap(2), the userfaultfd monitor will receive an
event of type UFFD_EVENT_REMAP.
UFFD_FEATURE_EVENT_REMOVE (since Linux 4.11)
If this feature is enabled, when the faulting process calls
madvise(2) with the MADV_DONTNEED or MADV_REMOVE advice
value to free a virtual memory area the userfaultfd monitor
will receive an event of type UFFD_EVENT_REMOVE.
UFFD_FEATURE_EVENT_UNMAP (since Linux 4.11)
If this feature is enabled, when the faulting process
unmaps virtual memory either explicitly with munmap(2), or
implicitly during either mmap(2) or mremap(2), the
userfaultfd monitor will receive an event of type
UFFD_EVENT_UNMAP.
UFFD_FEATURE_MISSING_HUGETLBFS (since Linux 4.11)
If this feature bit is set, the kernel supports registering
userfaultfd ranges on hugetlbfs virtual memory areas
UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)
If this feature bit is set, the kernel supports registering
userfaultfd ranges on shared memory areas. This includes
all kernel shared memory APIs: System V shared memory,
tmpfs(5), shared mappings of /dev/zero, mmap(2) with the
MAP_SHARED flag set, memfd_create(2), and so on.
UFFD_FEATURE_SIGBUS (since Linux 4.14)
If this feature bit is set, no page-fault events
(UFFD_EVENT_PAGEFAULT) will be delivered. Instead, a
SIGBUS signal will be sent to the faulting process.
Applications using this feature will not require the use of
a userfaultfd monitor for processing memory accesses to the
regions registered with userfaultfd.
UFFD_FEATURE_THREAD_ID (since Linux 4.14)
If this feature bit is set, uffd_msg.pagefault.feat.ptid
will be set to the faulted thread ID for each page-fault
message.
UFFD_FEATURE_PAGEFAULT_FLAG_WP (since Linux 5.10)
If this feature bit is set, userfaultfd supports write-
protect faults for anonymous memory. (Note that shmem /
hugetlbfs support is indicated by a separate feature.)
UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)
If this feature bit is set, the kernel supports registering
userfaultfd ranges in minor mode on hugetlbfs-backed memory
areas.
UFFD_FEATURE_MINOR_SHMEM (since Linux 5.14)
If this feature bit is set, the kernel supports registering
userfaultfd ranges in minor mode on shmem-backed memory
areas.
UFFD_FEATURE_EXACT_ADDRESS (since Linux 5.18)
If this feature bit is set, uffd_msg.pagefault.address will
be set to the exact page-fault address that was reported by
the hardware, and will not mask the offset within the page.
Note that old Linux versions might indicate the exact
address as well, even though the feature bit is not set.
UFFD_FEATURE_WP_HUGETLBFS_SHMEM (since Linux 5.19)
If this feature bit is set, userfaultfd supports write-
protect faults for hugetlbfs and shmem / tmpfs memory.
UFFD_FEATURE_WP_UNPOPULATED (since Linux 6.4)
If this feature bit is set, the kernel will handle
anonymous memory the same way as file memory, by allowing
the user to write-protect unpopulated page table entries.
UFFD_FEATURE_POISON (since Linux 6.6)
If this feature bit is set, the kernel supports resolving
faults with the UFFDIO_POISON ioctl.
UFFD_FEATURE_WP_ASYNC (since Linux 6.7)
If this feature bit is set, the write protection faults
would be asynchronously resolved by the kernel.
UFFD_FEATURE_MOVE (since Linux 6.8)
If this feature bit is set, the kernel supports resolving
faults with the UFFDIO_MOVE ioctl.
The returned argp->ioctls field can contain the following bits:
1 << _UFFDIO_API
The UFFDIO_API operation is supported.
1 << _UFFDIO_REGISTER
The UFFDIO_REGISTER operation is supported.
1 << _UFFDIO_UNREGISTER
The UFFDIO_UNREGISTER operation is supported.
On success, 0 is returned.
On error, -1 is returned and errno is set to indicate the error.
EFAULT argp refers to an address that is outside the calling
process's accessible address space.
EINVAL The API version requested in the api field is not supported
by this kernel, or the features field passed to the kernel
includes feature bits that are not supported by the current
kernel version.
EINVAL A previous UFFDIO_API call already enabled one or more
features for this userfaultfd. Calling UFFDIO_API twice,
the first time with no features set, is explicitly allowed
as per the two-step feature detection handshake.
EPERM The UFFD_FEATURE_EVENT_FORK feature was enabled, but the
calling process doesn't have the CAP_SYS_PTRACE capability.
Linux.
Linux 4.3.
If an error occurs, the kernel may zero the provided uffdio_api
structure. The caller should treat its contents as unspecified,
and reinitialize it before re-attempting another UFFDIO_API call.
In order to detect available userfault features and enable some
subset of those features the userfaultfd file descriptor must be
closed after the first UFFDIO_API operation that queries features
availability and reopened before the second UFFDIO_API operation
that actually enables the desired features.
See userfaultfd(2).
ioctl(2), ioctl_userfaultfd(2), mmap(2), userfaultfd(2)
linux.git/Documentation/admin-guide/mm/userfaultfd.rst
This page is part of the man-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
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⟨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
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part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-18 UFFDIO_API(2const)
Pages that refer to this page: ioctl_userfaultfd(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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