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.\" Copyright (C) 2016 Intel Corporation
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pkey_alloc 2 2022-12-04 "Linux man-pages 6.03"
.SH NAME
pkey_alloc, pkey_free \- allocate or free a protection key
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #include <sys/mman.h>
.PP
.BI "int pkey_alloc(unsigned int " flags ", unsigned int " access_rights ");"
.BI "int pkey_free(int " pkey ");"
.fi
.SH DESCRIPTION
.BR pkey_alloc ()
allocates a protection key (pkey) and allows it to be passed to
.BR pkey_mprotect (2).
.PP
The
.BR pkey_alloc ()
.I flags
is reserved for future use and currently must always be specified as 0.
.PP
The
.BR pkey_alloc ()
.I access_rights
argument may contain zero or more disable operations:
.TP
.B PKEY_DISABLE_ACCESS
Disable all data access to memory covered by the returned protection key.
.TP
.B PKEY_DISABLE_WRITE
Disable write access to memory covered by the returned protection key.
.PP
.BR 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.
.PP
An application should not call
.BR pkey_free ()
on any protection key which has been assigned to an address
range by
.BR pkey_mprotect (2)
and which is still in use.
The behavior in this case is undefined and may result in an error.
.SH RETURN VALUE
On success,
.BR pkey_alloc ()
returns a positive protection key value.
On success,
.BR pkey_free ()
returns zero.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
.IR pkey ,
.IR flags ,
or
.I access_rights
is invalid.
.TP
.B ENOSPC
.RB ( 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.
.IP
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.
.SH VERSIONS
.BR pkey_alloc ()
and
.BR pkey_free ()
were added in Linux 4.9;
library support was added in glibc 2.27.
.SH STANDARDS
The
.BR pkey_alloc ()
and
.BR pkey_free ()
system calls are Linux-specific.
.SH NOTES
.BR 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
.B ENOSPC
if the operating system has no pkey support.
.PP
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
.BR pkey_alloc ()
or after it is freed via
.BR pkey_free ()),
the kernel may make arbitrary changes to the parts of the
rights register affecting access to that key.
.SH EXAMPLES
See
.BR pkeys (7).
.SH SEE ALSO
.BR pkey_mprotect (2),
.BR pkeys (7)
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