arch_prctl(2)

1ARCH_PRCTL(2)              Linux Programmer's Manual             ARCH_PRCTL(2)
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NAME

6       arch_prctl - set architecture specific thread state
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SYNOPSIS

9       #include <asm/prctl.h>
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11       #include <sys/prctl.h>
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13       int arch_prctl(int code, unsigned long addr)
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DESCRIPTION

16       The  arch_prctl() function sets architecture specific process or thread
17       state.  code selects a subfunction and passes argument addr to it.
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19       Sub functions for x86-64 are:
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21       ARCH_SET_FS
22              Set the 64bit base for the FS register to addr.
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24       ARCH_GET_FS
25              Return the 64bit base value for the FS register of  the  current
26              thread in the unsigned long pointed to by the address parameter
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28       ARCH_SET_GS
29              Set the 64bit base for the GS register to addr.
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31       ARCH_GET_GS
32              Return  the  64bit base value for the GS register of the current
33              thread in the unsigned long pointed to by the address parameter.
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NOTES

36       arch_prctl() is only supported on Linux/x86-64 for 64bit programs  cur‐
37       rently.
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39       The 64bit base changes when a new 32bit segment selector is loaded.
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41       ARCH_SET_GS is disabled in some kernels.
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43       Context  switches  for 64bit segment bases are rather expensive. It may
44       be a faster alternative to set a 32bit base using a segment selector by
45       setting  up  an  LDT with modify_ldt(2) or using the set_thread_area(2)
46       system call in a 2.5 kernel.  arch_prctl() is only needed when you want
47       to  set  bases  that  are  larger than 4GB.  Memory in the first 2GB of
48       address space can be allocated by  using  mmap(2)  with  the  MAP_32BIT
49       flag.
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51       As of version 2.3.5, glibc provides no prototype for arch_prctl().  You
52       have to declare it yourself for now.  This may be fixed in future glibc
53       versions.
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55       FS may be already used by the threading library.
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ERRORS

58       EFAULT addr  points  to  an  unmapped address or is outside the process
59              address space.
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61       EINVAL code is not a valid subcommand.
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63       EPERM  addr is outside the process address space.
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AUTHOR

66       Man page written by Andi Kleen.
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CONFORMING TO

69       arch_prctl() is a Linux/x86-64 extension and should not be used in pro‐
70       grams intended to be portable.
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