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

NAME

       pkey_alloc, pkey_free - allocate or free a protection key

SYNOPSIS

       #include <sys/mman.h>

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

DESCRIPTION

       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:

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

       PKEY_DISABLE_WRITE
	      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.  On success, pkey_free() returns zero.  On error, -1 is returned, and
       errno is set appropriately.

ERRORS

       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 architec-
	      ture-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.

VERSIONS

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

CONFORMING TO

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

NOTES

       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.

EXAMPLE

       See pkeys(7).

SEE ALSO

       pkey_mprotect(2), pkeys(7)

COLOPHON

       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 https://www.kernel.org/doc/man-pages/.

Linux								    2018-02-02							     PKEY_ALLOC(2)