ARCH_PRCTL
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
arch_prctl - set architecture-specific thread state
SYNOPSIS
#include <asm/prctl.h>
#include <sys/prctl.h>
int arch_prctl(int code, unsigned long addr);
int arch_prctl(int code, unsigned long *addr);
DESCRIPTION
The
arch_prctl()
function sets architecture-specific process or thread state.
code
selects a subfunction
and passes argument
addr
to it;
addr
is interpreted as either an
unsigned long
for the "set" operations, or as an
unsigned long *,
for the "get" operations.
Subfunctions for x86-64 are:
- ARCH_SET_FS
-
Set the 64-bit base for the
FS
register to
addr.
- ARCH_GET_FS
-
Return the 64-bit base value for the
FS
register of the current thread in the
unsigned long
pointed to by
addr.
- ARCH_SET_GS
-
Set the 64-bit base for the
GS
register to
addr.
- ARCH_GET_GS
-
Return the 64-bit base value for the
GS
register of the current thread in the
unsigned long
pointed to by
addr.
RETURN VALUE
On success,
arch_prctl()
returns 0; on error, -1 is returned, and
errno
is set to indicate the error.
ERRORS
- EFAULT
-
addr
points to an unmapped address or is outside the process address space.
- EINVAL
-
code
is not a valid subcommand.
- EPERM
-
addr
is outside the process address space.
CONFORMING TO
arch_prctl()
is a Linux/x86-64 extension and should not be used in programs intended
to be portable.
NOTES
arch_prctl()
is supported only on Linux/x86-64 for 64-bit programs currently.
The 64-bit base changes when a new 32-bit segment selector is loaded.
ARCH_SET_GS
is disabled in some kernels.
Context switches for 64-bit segment bases are rather expensive.
As an optimization, if a 32-bit TLS base address is used,
arch_prctl()
may use a real TLS entry as if
set_thread_area(2)
had been called, instead of manipulating the segment base register directly.
Memory in the first 2 GB of address space can be allocated by using
mmap(2)
with the
MAP_32BIT
flag.
Because of the aforementioned optimization, using
arch_prctl()
and
set_thread_area(2)
in the same thread is dangerous, as they may overwrite each other's
TLS entries.
As of version 2.7, glibc provides no prototype for
arch_prctl().
You have to declare it yourself for now.
This may be fixed in future glibc versions.
FS
may be already used by the threading library.
Programs that use
ARCH_SET_FS
directly are very likely to crash.
SEE ALSO
mmap(2),
modify_ldt(2),
prctl(2),
set_thread_area(2)
AMD X86-64 Programmer's manual
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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