mirror of https://github.com/mkerrisk/man-pages
138 lines
3.6 KiB
Groff
138 lines
3.6 KiB
Groff
.\" Copyright (c) 1995 Michael Chastain (mec@duracef.shout.net), 22 July 1995.
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.TH MODIFY_LDT 2 2012-07-13 "Linux" "Linux Programmer's Manual"
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.SH NAME
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modify_ldt \- get or set ldt
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.SH SYNOPSIS
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.nf
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.B #include <sys/types.h>
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.sp
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.BI "int modify_ldt(int " "func" ", void *" "ptr" ", unsigned long " "bytecount" );
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.fi
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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.BR modify_ldt ()
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reads or writes the local descriptor table (ldt) for a process.
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The ldt is a per-process memory management table used by the i386 processor.
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For more information on this table, see an Intel 386 processor handbook.
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.PP
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When
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.I func
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is 0,
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.BR modify_ldt ()
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reads the ldt into the memory pointed to by
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.IR ptr .
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The number of bytes read is the smaller of
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.I bytecount
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and the actual size of the ldt.
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.PP
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When
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.I func
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is 1,
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.BR modify_ldt ()
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modifies one ldt entry.
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.I ptr
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points to a
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.I user_desc
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structure
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and
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.I bytecount
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must equal the size of this structure.
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.\"
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.\" FIXME ? say something about func == 2 and func == 0x11?
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.\" In Linux 2.4, func == 2 returned "the default ldt"
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.\" In Linux 2.6, func == 2 is a nop, returning a zeroed out structure.
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.\" Linux 2.4 and 2.6 implement an operation for func == 0x11
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The
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.I user_desc
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structure is defined in \fI<asm/ldt.h>\fP as:
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.in +4n
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.nf
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struct user_desc {
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unsigned int entry_number;
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unsigned long base_addr;
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unsigned int limit;
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unsigned int seg_32bit:1;
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unsigned int contents:2;
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unsigned int read_exec_only:1;
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unsigned int limit_in_pages:1;
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unsigned int seg_not_present:1;
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unsigned int useable:1;
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};
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.fi
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.in
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.PP
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In Linux 2.4 and earlier, this structure was named
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.IR modify_ldt_ldt_s .
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.\" .PP
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.\" The ldt is specific for the calling process. Any attempts to change
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.\" the ldt to include the address space of another process or the kernel
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.\" will result in a segmentation violation when trying to access the memory
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.\" outside of the process address space. The memory protection is enforced
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.\" at the paging layer.
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.SH RETURN VALUE
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On success,
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.BR modify_ldt ()
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returns either the actual number of bytes read (for reading)
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or 0 (for writing).
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On failure,
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.BR modify_ldt ()
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returns \-1 and sets
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.I errno
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to indicate the error.
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.SH ERRORS
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.TP
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.B EFAULT
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.I ptr
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points outside the address space.
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.TP
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.B EINVAL
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.I ptr
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is 0,
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or
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.I func
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is 1 and
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.I bytecount
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is not equal to the size of the structure
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.IR modify_ldt_ldt_s ,
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or
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.I func
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is 1 and the new ldt entry has invalid values.
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.TP
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.B ENOSYS
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.I func
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is neither 0 nor 1.
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.SH CONFORMING TO
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This call is Linux-specific and should not be used in programs intended
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to be portable.
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.SH NOTES
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Glibc does not provide a wrapper for this system call; call it using
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.BR syscall (2).
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.SH SEE ALSO
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.BR vm86 (2)
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