# pkey_alloc

## NAME



           pkey_alloc, pkey_free - allocate or free a protection key
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#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_**);**
    


## 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 to indicate the error.
    


## 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 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.
    


## STANDARDS



           Linux.
    


## 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.
    


## EXAMPLES



           See pkeys(7).
    


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