pkey_alloc man page

pkey_alloc, pkey_free — allocate or free a protection key


#include <sys/mman.h>

int pkey_alloc(unsigned long flags, unsigned long access_rights);
int pkey_free(int pkey);


pkey_alloc() allocates a protection key (pkey) and allows it to be passed to pkey_mprotect(2).

The pkey_alloc() flags argument may contain zero or more disable operations:

Disable all data access to memory covered by the returned protection key.
Disable write access to memory covered by the returned protection key.

pkey_free() 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 error.

Return Value

On success, pkey_alloc() returns a positive protection key value. pkey_free() returns zero. On error, -1 is returned, and errno 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.


pkey_alloc() and pkey_free() were added to Linux in kernel 4.9. Glibc support is not yet available.

Conforming to

The pkey_alloc() and pkey_free() system calls are Linux-specific.


pkey_alloc() 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 ENOSPC 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 pkey_alloc() 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.


See pkeys(7).

See Also

pkey_mprotect(2), pkeys(7)