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

       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 is reserved for future use  and  currently  must
       always be specified as 0.

       The  pkey_alloc() access_rights  argument may contain zero or more dis-
       able operations:

              Disable all data access to memory covered by the  returned  pro-
              tection key.

              Disable  write  access to memory covered by the returned protec-
              tion 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  cur-
              rently 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 4.15 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                             2018-02-02                     PKEY_ALLOC(2)
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