597 lines
17 KiB
Groff
597 lines
17 KiB
Groff
.\" Copyright (c) 1993 Paul Kranenburg
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by Paul Kranenburg.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 23, 1993
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.Dt LINK 5
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.Os
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.Sh NAME
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.Nm link
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.Nd dynamic loader and link editor interface
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.Sh SYNOPSIS
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.In sys/types.h
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.In nlist.h
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.In link.h
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.Sh DESCRIPTION
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The include file
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.In link.h
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declares several structures that are present in dynamically linked
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programs and libraries.
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The structures define the interface between several components of the
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link-editor and loader mechanism.
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The layout of a number of these
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structures within the binaries resembles the a.out format in many places
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as it serves such similar functions as symbol definitions (including the
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accompanying string table) and relocation records needed to resolve
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references to external entities.
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It also records a number of data structures
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unique to the dynamic loading and linking process.
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These include references
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to other objects that are required to complete the link-editing process and
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indirection tables to facilitate
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.Em Position Independent Code
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(PIC for short) to improve sharing of code pages among different processes.
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The collection of data structures described here will be referred to as the
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.Em Run-time Relocation Section (RRS)
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and is embedded in the standard text and data segments of the dynamically
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linked program or shared object image as the existing
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.Xr a.out 5
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format offers no room for it elsewhere.
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.Pp
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Several utilities cooperate to ensure that the task of getting a program
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ready to run can complete successfully in a way that optimizes the use
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of system resources.
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The compiler emits PIC code from which shared libraries
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can be built by
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.Xr ld 1 .
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The compiler also includes size information of any initialized data items
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through the .size assembler directive.
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PIC code differs from conventional code
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in that it accesses data variables through an indirection table, the
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Global Offset Table, by convention accessible by the reserved name
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.Dv _GLOBAL_OFFSET_TABLE_ .
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The exact mechanism used for this is machine dependent, usually a machine
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register is reserved for the purpose.
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The rational behind this construct
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is to generate code that is independent of the actual load address.
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Only
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the values contained in the Global Offset Table may need updating at run-time
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depending on the load addresses of the various shared objects in the address
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space.
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.Pp
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Likewise, procedure calls to globally defined functions are redirected through
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the Procedure Linkage Table (PLT) residing in the data segment of the core
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image.
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Again, this is done to avoid run-time modifications to the text segment.
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.Pp
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The linker-editor allocates the Global Offset Table and Procedure Linkage Table
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when combining PIC object files into an image suitable for mapping into the
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process address space.
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It also collects all symbols that may be needed by the
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run-time link-editor and stores these along with the image's text and data bits.
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Another reserved symbol,
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.Em _DYNAMIC
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is used to indicate the presence of the run-time linker structures.
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Whenever
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_DYNAMIC is relocated to 0, there is no need to invoke the run-time
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link-editor.
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If this symbol is non-zero, it points at a data structure from
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which the location of the necessary relocation- and symbol information can
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be derived.
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This is most notably used by the start-up module,
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.Em crt0 .
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The _DYNAMIC structure is conventionally located at the start of the data
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segment of the image to which it pertains.
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.Sh DATA STRUCTURES
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The data structures supporting dynamic linking and run-time relocation
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reside both in the text and data segments of the image they apply to.
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The text segments contain read-only data such as symbols descriptions and
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names, while the data segments contain the tables that need to be modified by
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during the relocation process.
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.Pp
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The _DYNAMIC symbol references a
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.Fa _dynamic
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structure:
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.Bd -literal -offset indent
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struct _dynamic {
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int d_version;
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struct so_debug *d_debug;
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union {
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struct section_dispatch_table *d_sdt;
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} d_un;
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struct ld_entry *d_entry;
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};
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.Ed
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.Bl -tag -width d_version
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.It Fa d_version
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This field provides for different versions of the dynamic linking
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implementation.
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The current version numbers understood by
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.Xr ld 1
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and
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.Xr ld.so 1
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are
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.Em LD_VERSION_SUN (3) ,
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which is used by the
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.Tn SunOS
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4.x releases, and
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.Em LD_VERSION_BSD (8) ,
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which has been in use since
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.Fx 1.1 .
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.It Fa d_un
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Refers to a
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.Em d_version
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dependent data structure.
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.It Fa so_debug
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this field provides debuggers with a hook to access symbol tables of shared
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objects loaded as a result of the actions of the run-time link-editor.
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.El
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.Pp
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The
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.Fa section_dispatch_table
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structure is the main
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.Dq dispatcher
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table, containing offsets into the image's segments where various symbol
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and relocation information is located.
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.Bd -literal -offset indent
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struct section_dispatch_table {
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struct so_map *sdt_loaded;
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long sdt_sods;
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long sdt_filler1;
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long sdt_got;
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long sdt_plt;
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long sdt_rel;
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long sdt_hash;
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long sdt_nzlist;
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long sdt_filler2;
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long sdt_buckets;
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long sdt_strings;
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long sdt_str_sz;
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long sdt_text_sz;
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long sdt_plt_sz;
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};
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.Ed
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.Pp
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.Bl -tag -width sdt_filler1
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.It Fa sdt_loaded
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A pointer to the first link map loaded (see below).
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This field is set by
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.Nm ld.so
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.It Fa sdt_sods
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The start of a (linked) list of shared object descriptors needed by
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.Em this
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object.
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.It Fa sdt_filler1
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Deprecated (used by SunOS to specify library search rules).
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.It Fa sdt_got
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The location of the Global Offset Table within this image.
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.It Fa sdt_plt
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The location of the Procedure Linkage Table within this image.
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.It Fa sdt_rel
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The location of an array of
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.Fa relocation_info
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structures
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(see
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.Xr a.out 5 )
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specifying run-time relocations.
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.It Fa sdt_hash
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The location of the hash table for fast symbol lookup in this object's
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symbol table.
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.It Fa sdt_nzlist
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The location of the symbol table.
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.It Fa sdt_filler2
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Currently unused.
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.It Fa sdt_buckets
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The number of buckets in
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.Fa sdt_hash
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.It Fa sdt_strings
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The location of the symbol string table that goes with
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.Fa sdt_nzlist .
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.It Fa sdt_str_sz
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The size of the string table.
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.It Fa sdt_text_sz
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The size of the object's text segment.
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.It Fa sdt_plt_sz
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The size of the Procedure Linkage Table.
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.El
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.Pp
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A
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.Fa sod
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structure describes a shared object that is needed
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to complete the link edit process of the object containing it.
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A list of such objects
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(chained through
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.Fa sod_next )
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is pointed at
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by the
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.Fa sdt_sods
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in the section_dispatch_table structure.
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.Bd -literal -offset indent
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struct sod {
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long sod_name;
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u_int sod_library : 1,
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sod_reserved : 31;
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short sod_major;
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short sod_minor;
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long sod_next;
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};
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.Ed
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.Pp
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.Bl -tag -width sod_library
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.It Fa sod_name
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The offset in the text segment of a string describing this link object.
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.It Fa sod_library
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If set,
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.Fa sod_name
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specifies a library that is to be searched for by
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.Nm ld.so .
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The path name
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is obtained by searching a set of directories
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(see also
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.Xr ldconfig 8 )
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for a shared object matching
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.Em lib\&<sod_name>\&.so.n.m .
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If not set,
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.Fa sod_name
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should point at a full path name for the desired shared object.
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.It Fa sod_major
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Specifies the major version number of the shared object to load.
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.It Fa sod_minor
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Specifies the preferred minor version number of the shared object to load.
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.El
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.Pp
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The run-time link-editor maintains a list of structures called
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.Em link maps
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to keep track of all shared objects loaded into a process' address space.
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These structures are only used at run-time and do not occur within
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the text or data segment of an executable or shared library.
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.Bd -literal -offset indent
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struct so_map {
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caddr_t som_addr;
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char *som_path;
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struct so_map *som_next;
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struct sod *som_sod;
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caddr_t som_sodbase;
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u_int som_write : 1;
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struct _dynamic *som_dynamic;
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caddr_t som_spd;
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};
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.Ed
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.Bl -tag -width som_dynamic
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.It Fa som_addr
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The address at which the shared object associated with this link map has
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been loaded.
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.It Fa som_path
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The full path name of the loaded object.
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.It Fa som_next
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Pointer to the next link map.
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.It Fa som_sod
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The
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.Fa sod
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structure that was responsible for loading this shared object.
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.It Fa som_sodbase
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Tossed out in later versions of the run-time linker.
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.It Fa som_write
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Set if (some portion of) this object's text segment is currently writable.
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.It Fa som_dynamic
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Pointer to this object's
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.Fa _dynamic
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structure.
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.It Fa som_spd
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Hook for attaching private data maintained by the run-time link-editor.
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.El
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.Pp
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Symbol description with size.
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This is simply an
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.Fa nlist
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structure with one field
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.Pq Fa nz_size
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added.
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Used to convey size information on items in the data segment
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of shared objects.
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An array of these lives in the shared object's
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text segment and is addressed by the
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.Fa sdt_nzlist
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field of
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.Fa section_dispatch_table .
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.Bd -literal -offset indent
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struct nzlist {
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struct nlist nlist;
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u_long nz_size;
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#define nz_un nlist.n_un
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#define nz_strx nlist.n_un.n_strx
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#define nz_name nlist.n_un.n_name
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#define nz_type nlist.n_type
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#define nz_value nlist.n_value
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#define nz_desc nlist.n_desc
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#define nz_other nlist.n_other
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};
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.Ed
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.Bl -tag -width nz_size
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.It Fa nlist
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(see
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.Xr nlist 3 ) .
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.It Fa nz_size
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The size of the data represented by this symbol.
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.El
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.Pp
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A hash table is included within the text segment of shared object
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to facilitate quick lookup of symbols during run-time link-editing.
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The
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.Fa sdt_hash
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field of the
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.Fa section_dispatch_table
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structure points at an array of
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.Fa rrs_hash
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structures:
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.Bd -literal -offset indent
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struct rrs_hash {
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int rh_symbolnum; /* symbol number */
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int rh_next; /* next hash entry */
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};
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.Ed
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.Pp
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.Bl -tag -width rh_symbolnum
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.It Fa rh_symbolnum
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The index of the symbol in the shared object's symbol table (as given by the
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.Fa ld_symbols
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field).
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.It Fa rh_next
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In case of collisions, this field is the offset of the next entry in this
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hash table bucket.
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It is zero for the last bucket element.
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.El
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The
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.Fa rt_symbol
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structure is used to keep track of run-time allocated commons
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and data items copied from shared objects.
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These items are kept on linked list
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and is exported through the
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.Fa dd_cc
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field in the
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.Fa so_debug
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structure (see below) for use by debuggers.
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.Bd -literal -offset indent
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struct rt_symbol {
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struct nzlist *rt_sp;
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struct rt_symbol *rt_next;
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struct rt_symbol *rt_link;
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caddr_t rt_srcaddr;
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struct so_map *rt_smp;
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};
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.Ed
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.Pp
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.Bl -tag -width rt_scraddr
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.It Fa rt_sp
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The symbol description.
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.It Fa rt_next
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Virtual address of next rt_symbol.
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.It Fa rt_link
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Next in hash bucket.
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Used internally by
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.Nm ld.so .
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.It Fa rt_srcaddr
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Location of the source of initialized data within a shared object.
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.It Fa rt_smp
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The shared object which is the original source of the data that this
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run-time symbol describes.
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.El
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.Pp
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The
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.Fa so_debug
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structure is used by debuggers to gain knowledge of any shared objects
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that have been loaded in the process's address space as a result of run-time
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link-editing.
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Since the run-time link-editor runs as a part of process
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initialization, a debugger that wishes to access symbols from shared objects
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can only do so after the link-editor has been called from crt0.
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A dynamically linked binary contains a
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.Fa so_debug
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structure which can be located by means of the
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.Fa d_debug
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field in
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.Fa _dynamic .
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.Bd -literal -offset indent
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struct so_debug {
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int dd_version;
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int dd_in_debugger;
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int dd_sym_loaded;
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char *dd_bpt_addr;
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int dd_bpt_shadow;
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struct rt_symbol *dd_cc;
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};
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.Ed
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.Pp
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.Bl -tag -width dd_in_debugger
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.It Fa dd_version
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Version number of this interface.
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.It Fa dd_in_debugger
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Set by the debugger to indicate to the run-time linker that the program is
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run under control of a debugger.
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.It Fa dd_sym_loaded
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Set by the run-time linker whenever it adds symbols by loading shared objects.
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.It Fa dd_bpt_addr
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The address where a breakpoint will be set by the run-time linker to
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divert control to the debugger.
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This address is determined by the start-up
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module,
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.Pa crt0.o ,
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to be some convenient place before the call to _main.
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.It Fa dd_bpt_shadow
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Contains the original instruction that was at
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.Fa dd_bpt_addr .
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The debugger is expected to put this instruction back before continuing the
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program.
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.It Fa dd_cc
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A pointer to the linked list of run-time allocated symbols that the debugger
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may be interested in.
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.El
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.Pp
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The
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.Em ld_entry
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structure defines a set of service routines within
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.Nm ld.so .
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.\" See
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.\" .Xr libdl.a
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.\" for more information.
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.Bd -literal -offset indent
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struct ld_entry {
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void *(*dlopen)(char *, int);
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int (*dlclose)(void *);
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void *(*dlsym)(void *, char *);
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char *(*dlerror)(void);
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};
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.Ed
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.Pp
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The
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.Fa crt_ldso
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structure defines the interface between the start-up code in crt0 and
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.Nm ld.so .
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.Bd -literal -offset indent
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struct crt_ldso {
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int crt_ba;
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int crt_dzfd;
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int crt_ldfd;
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struct _dynamic *crt_dp;
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char **crt_ep;
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caddr_t crt_bp;
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char *crt_prog;
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char *crt_ldso;
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struct ld_entry *crt_ldentry;
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};
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#define CRT_VERSION_SUN 1
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#define CRT_VERSION_BSD_2 2
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#define CRT_VERSION_BSD_3 3
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#define CRT_VERSION_BSD_4 4
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.Ed
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.Bl -tag -width crt_dzfd
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.It Fa crt_ba
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The virtual address at which
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.Nm ld.so
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was loaded by crt0.
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.It Fa crt_dzfd
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On SunOS systems, this field contains an open file descriptor to
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.Dq Pa /dev/zero
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used to get demand paged zeroed pages.
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On
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.Fx
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systems it contains -1.
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.It Fa crt_ldfd
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Contains an open file descriptor that was used by crt0 to load
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.Nm ld.so .
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.It Fa crt_dp
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A pointer to main's
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.Fa _dynamic
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structure.
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.It Fa crt_ep
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A pointer to the environment strings.
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.It Fa crt_bp
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The address at which a breakpoint will be placed by the run-time linker
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if the main program is run by a debugger.
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See
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.Fa so_debug
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.It Fa crt_prog
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The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only).
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.It Fa crt_ldso
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The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only).
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.El
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.Pp
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The
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.Fa hints_header
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|
and
|
|
.Fa hints_bucket
|
|
structures define the layout of the library hints, normally found in
|
|
.Dq Pa /var/run/ld.so.hints ,
|
|
which is used by
|
|
.Nm ld.so
|
|
to quickly locate the shared object images in the
|
|
file system.
|
|
The organization of the hints file is not unlike that of an
|
|
.Dq a.out
|
|
object file, in that it contains a header determining the offset and size
|
|
of a table of fixed sized hash buckets and a common string pool.
|
|
.Bd -literal -offset indent
|
|
struct hints_header {
|
|
long hh_magic;
|
|
#define HH_MAGIC 011421044151
|
|
long hh_version;
|
|
#define LD_HINTS_VERSION_1 1
|
|
long hh_hashtab;
|
|
long hh_nbucket;
|
|
long hh_strtab;
|
|
long hh_strtab_sz;
|
|
long hh_ehints;
|
|
};
|
|
.Ed
|
|
.Bl -tag -width hh_strtab_sz
|
|
.It Fa hh_magic
|
|
Hints file magic number.
|
|
.It Fa hh_version
|
|
Interface version number.
|
|
.It Fa hh_hashtab
|
|
Offset of hash table.
|
|
.It Fa hh_strtab
|
|
Offset of string table.
|
|
.It Fa hh_strtab_sz
|
|
Size of strings.
|
|
.It Fa hh_ehints
|
|
Maximum usable offset in hints file.
|
|
.El
|
|
.Pp
|
|
.Bd -literal -offset indent
|
|
/*
|
|
* Hash table element in hints file.
|
|
*/
|
|
struct hints_bucket {
|
|
int hi_namex;
|
|
int hi_pathx;
|
|
int hi_dewey[MAXDEWEY];
|
|
int hi_ndewey;
|
|
#define hi_major hi_dewey[0]
|
|
#define hi_minor hi_dewey[1]
|
|
int hi_next;
|
|
};
|
|
.Ed
|
|
.Bl -tag -width hi_ndewey
|
|
.It Fa hi_namex
|
|
Index of the string identifying the library.
|
|
.It Fa hi_pathx
|
|
Index of the string representing the full path name of the library.
|
|
.It Fa hi_dewey
|
|
The version numbers of the shared library.
|
|
.It Fa hi_ndewey
|
|
The number of valid entries in
|
|
.Fa hi_dewey .
|
|
.It Fa hi_next
|
|
Next bucket in case of hashing collisions.
|
|
.El
|
|
.Sh CAVEATS
|
|
Only the (GNU) C compiler currently supports the creation of shared libraries.
|
|
Other programming languages cannot be used.
|