arch_prctl

<html> <head> <title>arch_prctl</title> <META NAME="KEYWORDS" CONTENT="arch_prctl"> </head> <body BGCOLOR="#ffffff" LINK="#0000ff" VLINK="#0000ff" ALINK="#0000ff" TEXT="#000000"> <center> <h1>arch_prctl</h1></center> <PRE> <A HREF="/man2/ARCH_PRCTL">ARCH_PRCTL(2)</A> Linux Programmer's Manual <A HREF="/man2/ARCH_PRCTL">ARCH_PRCTL(2)</A> 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 arch_prctl() sets architecture-specific process or thread state. code selects a subfunction and passes argument addr to it; addr is inter- preted 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 ad- dress 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 pro- grams intended to be portable. NOTES arch_prctl() is supported only on Linux/x86-64 for 64-bit programs cur- rently. 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 <A HREF="/man2/set_thread_area">set_thread_area(2)</A> 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 <A HREF="/man2/mmap">mmap(2)</A> with the MAP_32BIT flag. Because of the aforementioned optimization, using arch_prctl() and <A HREF="/man2/set_thread_area">set_thread_area(2)</A> in the same thread is dangerous, as they may over- write 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 <A HREF="/man2/mmap">mmap(2)</A>, <A HREF="/man2/modify_ldt">modify_ldt(2)</A>, <A HREF="/man2/prctl">prctl(2)</A>, <A HREF="/man2/set_thread_area">set_thread_area(2)</A> AMD X86-64 Programmer's manual COLOPHON This page is part of release 5.05 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 2017-09-15 <A HREF="/man2/ARCH_PRCTL">ARCH_PRCTL(2)</A></PRE> <center> <h6>Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2025 <a href="http://www.he.net">Hurricane Electric</a>. All Rights Reserved.</h6></center> </body> </html>

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arch_prctl