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>
10       #include <sys/prctl.h>
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12       int arch_prctl(int code, unsigned long addr);
13       int arch_prctl(int code, unsigned long *addr);
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DESCRIPTION

16       arch_prctl()  sets architecture-specific process or thread state.  code
17       selects a subfunction and passes argument addr to it;  addr  is  inter‐
18       preted  as  either  an unsigned long for the "set" operations, or as an
19       unsigned long *, for the "get" operations.
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21       Subfunctions for x86-64 are:
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23       ARCH_SET_FS
24              Set the 64-bit base for the FS register to addr.
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26       ARCH_GET_FS
27              Return the 64-bit base value for the FS register of the  current
28              thread in the unsigned long pointed to by addr.
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30       ARCH_SET_GS
31              Set the 64-bit base for the GS register to addr.
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33       ARCH_GET_GS
34              Return  the 64-bit base value for the GS register of the current
35              thread in the unsigned long pointed to by addr.
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RETURN VALUE

38       On success, arch_prctl() returns 0; on error, -1 is returned, and errno
39       is set to indicate the error.
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ERRORS

42       EFAULT addr  points  to  an  unmapped address or is outside the process
43              address space.
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45       EINVAL code is not a valid subcommand.
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47       EPERM  addr is outside the process address space.
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CONFORMING TO

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

54       arch_prctl() is supported only on Linux/x86-64 for 64-bit programs cur‐
55       rently.
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57       The 64-bit base changes when a new 32-bit segment selector is loaded.
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59       ARCH_SET_GS is disabled in some kernels.
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61       Context switches for 64-bit segment bases are rather expensive.  As  an
62       optimization,  if  a  32-bit TLS base address is used, arch_prctl() may
63       use a real TLS entry as if set_thread_area(2) had been called,  instead
64       of  manipulating  the  segment  base  register directly.  Memory in the
65       first 2 GB of address space can be allocated by using mmap(2) with  the
66       MAP_32BIT flag.
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68       Because  of  the  aforementioned  optimization,  using arch_prctl() and
69       set_thread_area(2) in the same thread is dangerous, as they  may  over‐
70       write each other's TLS entries.
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72       As  of  version 2.7, glibc provides no prototype for arch_prctl().  You
73       have to declare it yourself for now.  This may be fixed in future glibc
74       versions.
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76       FS  may  be  already  used by the threading library.  Programs that use
77       ARCH_SET_FS directly are very likely to crash.
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COLOPHON

85       This page is part of release 4.16 of the Linux  man-pages  project.   A
86       description  of  the project, information about reporting bugs, and the
87       latest    version    of    this    page,    can     be     found     at
88       https://www.kernel.org/doc/man-pages/.
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92Linux                             2017-09-15                     ARCH_PRCTL(2)