| .\" Copyright (C) 2003 Andi Kleen |
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| .TH ARCH_PRCTL 2 2017-09-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| arch_prctl \- set architecture-specific thread state |
| .SH SYNOPSIS |
| .nf |
| .B #include <asm/prctl.h> |
| .B #include <sys/prctl.h> |
| .PP |
| .BI "int arch_prctl(int " code ", unsigned long " addr ); |
| .BI "int arch_prctl(int " code ", unsigned long *" addr ); |
| .fi |
| .SH DESCRIPTION |
| .BR arch_prctl () |
| sets architecture-specific process or thread state. |
| .I code |
| selects a subfunction |
| and passes argument |
| .I addr |
| to it; |
| .I addr |
| is interpreted as either an |
| .I "unsigned long" |
| for the "set" operations, or as an |
| .IR "unsigned long\ *" , |
| for the "get" operations. |
| .PP |
| Subfunctions for x86-64 are: |
| .TP |
| .B ARCH_SET_FS |
| Set the 64-bit base for the |
| .I FS |
| register to |
| .IR addr . |
| .TP |
| .B ARCH_GET_FS |
| Return the 64-bit base value for the |
| .I FS |
| register of the current thread in the |
| .I unsigned long |
| pointed to by |
| .IR addr . |
| .TP |
| .B ARCH_SET_GS |
| Set the 64-bit base for the |
| .I GS |
| register to |
| .IR addr . |
| .TP |
| .B ARCH_GET_GS |
| Return the 64-bit base value for the |
| .I GS |
| register of the current thread in the |
| .I unsigned long |
| pointed to by |
| .IR addr . |
| .SH RETURN VALUE |
| On success, |
| .BR arch_prctl () |
| returns 0; on error, \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EFAULT |
| .I addr |
| points to an unmapped address or is outside the process address space. |
| .TP |
| .B EINVAL |
| .I code |
| is not a valid subcommand. |
| .TP |
| .B EPERM |
| .I addr |
| is outside the process address space. |
| .\" .SH AUTHOR |
| .\" Man page written by Andi Kleen. |
| .SH CONFORMING TO |
| .BR arch_prctl () |
| is a Linux/x86-64 extension and should not be used in programs intended |
| to be portable. |
| .SH NOTES |
| .BR arch_prctl () |
| is supported only on Linux/x86-64 for 64-bit programs currently. |
| .PP |
| The 64-bit base changes when a new 32-bit segment selector is loaded. |
| .PP |
| .B ARCH_SET_GS |
| is disabled in some kernels. |
| .PP |
| Context switches for 64-bit segment bases are rather expensive. |
| As an optimization, if a 32-bit TLS base address is used, |
| .BR arch_prctl () |
| may use a real TLS entry as if |
| .BR 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 |
| .BR mmap (2) |
| with the |
| .B MAP_32BIT |
| flag. |
| .PP |
| Because of the aforementioned optimization, using |
| .BR arch_prctl () |
| and |
| .BR set_thread_area (2) |
| in the same thread is dangerous, as they may overwrite each other's |
| TLS entries. |
| .PP |
| As of version 2.7, glibc provides no prototype for |
| .BR arch_prctl (). |
| You have to declare it yourself for now. |
| This may be fixed in future glibc versions. |
| .PP |
| .I FS |
| may be already used by the threading library. |
| Programs that use |
| .B ARCH_SET_FS |
| directly are very likely to crash. |
| .SH SEE ALSO |
| .BR mmap (2), |
| .BR modify_ldt (2), |
| .BR prctl (2), |
| .BR set_thread_area (2) |
| .PP |
| AMD X86-64 Programmer's manual |