PKEY_ALLOC(2)              Linux Programmer's Manual             PKEY_ALLOC(2)

       pkey_alloc, pkey_free - allocate or free a protection key

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/mman.h>

       int pkey_alloc(unsigned int flags, unsigned int 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 is reserved for future use and currently must
       always be specified as 0.

       The pkey_alloc() access_rights 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.

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

       EINVAL pkey, flags, or access_rights is invalid.

       ENOSPC (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

       pkey_alloc() and pkey_free() were added to Linux in kernel 4.9; library
       support was added in glibc 2.27.

       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).

       pkey_mprotect(2), pkeys(7)

       This page is part of release 5.06 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2019-08-02                     PKEY_ALLOC(2)