pkey_alloc, pkey_free - allocate or free a protection key
#define _GNU_SOURCE /* See feature_test_macros(7) */
int pkey_alloc(unsigned int flags, unsigned int access_rights);
int pkey_free(int pkey);
() allocates a protection key (pkey) and allows it to be passed
is reserved for future use and currently
must always be specified as 0.
argument may contain zero or more
- Disable all data access to memory covered by the returned protection
- Disable write access to memory covered by the returned protection
() 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. On
() returns zero. On error, -1 is returned, and
is set appropriately.
- pkey, flags, or access_rights is invalid.
- (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;
library support was added in glibc 2.27.
() 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.