pkey_alloc, pkey_free - allocate or free a protection key
int pkey_alloc(unsigned long flags, unsigned long access_rights);
int pkey_free(int pkey);
() allocates a protection key (pkey) and allows it to be passed
argument may contain zero or more disable
- Disable all data access to memory covered by the returned
- Disable write access to memory covered by the returned
() frees a protection key and makes it available for later
allocations. After a protection key has been freed, it may no longer be used
in any protection-key-related operations.
An application should not call pkey_free
() on any protection key which
has been assigned to an address range by pkey_mprotect
(2) and which is
still in use. The behavior in this case is undefined and may result in an
On success, pkey_alloc
() returns a positive protection key value.
() returns zero. On error, -1 is returned, and errno
- pkey, flags, or access_rights is
- (pkey_alloc()) All protection keys available for the
current process have been allocated. The number of keys available is
architecture-specific and implementation-specific and may be reduced by
kernel-internal use of certain keys. There are currently 15 keys available
to user programs on x86.
- This error will also be returned if the processor or
operating system does not support protection keys. Applications should
always be prepared to handle this error, since factors outside of the
application's control can reduce the number of available pkeys.
() and pkey_free
() were added to Linux in kernel 4.9.
Glibc support is not yet available.
() and pkey_free
() system calls are Linux-specific.
() is always safe to call regardless of whether or not the
operating system supports protection keys. It can be used in lieu of any other
mechanism for detecting pkey support and will simply fail with the error
if the operating system has no pkey support.
The kernel guarantees that the contents of the hardware rights register (PKRU)
will be preserved only for allocated protection keys. Any time a key is
unallocated (either before the first call returning that key from
() or after it is freed via pkey_free
()), the kernel
may make arbitrary changes to the parts of the rights register affecting
access to that key.
This page is part of release 4.13 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at