420 lines
13 KiB
Groff
420 lines
13 KiB
Groff
.\" $NetBSD: elf.5,v 1.13 2006/11/18 14:32:14 pooka Exp $
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.\"
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.\" Copyright (c) 2001, 2002 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This document is derived from work contributed to The NetBSD Foundation
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.\" by Antti Kantee.
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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 the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS BE
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.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd November 18, 2006
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.Dt ELF 5
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.Os
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.Sh NAME
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.Nm ELF
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.Nd executable and linking format
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.Sh SYNOPSIS
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.In elf.h
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.Sh DESCRIPTION
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Because of the flexible nature of ELF, the structures describing it are
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available both as 32bit and 64bit versions. This document uses the 32bit
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versions, refer to
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.Aq Pa elf.h
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for the corresponding 64bit versions.
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.Pp
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The four main types of an ELF object file are:
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.Bl -tag -width "relocatable"
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.It executable
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A file suitable for execution. It contains the information required for
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creating a new process image.
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.It relocatable
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Contains the necessary information to be run through the link editor
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.Xr ld 1
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to create an executable or a shared library.
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.It shared
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The shared object contains necessary information which can be used by
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either the link editor
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.Xr ld 1
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at link time or by the dynamic loader
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.Xr ld.elf_so 1
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at run time.
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.It core
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A file which describes the virtual address space and register state
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of a process. Core files are typically used in conjunction with
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debuggers such as
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.Xr gdb 1 .
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.El
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.Pp
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ELF files have a dual nature. The toolchain, including tools such as the
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.Xr as 1
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and linker
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.Xr ld 1 ,
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treats them as a set of sections described by their section headers. The system
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loader treats them as a set of segments described by the program headers.
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.Pp
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The general format of an ELF file is the following: The file starts with an
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ELF header. This is followed by a table of program headers (optional for
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relocatable and shared files). After this come the sections/segments.
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The file ends with a table of section headers (optional for executable
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files).
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.Pp
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A segment can be considered to consist of several sections. For example,
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all executable sections are typically packed into one loadable segment
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which is read-only and executable (see
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.Fa p_flags
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in the program header). This enables the system to map the entire file with
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just a few operations, one for each loadable segment, instead of doing
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numerous map operations for each section separately.
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.Pp
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Each file is described by the ELF header:
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.Bd -literal -offset indent
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typedef struct {
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unsigned char e_ident[ELF_NIDENT];
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Elf32_Half e_type;
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Elf32_Half e_machine;
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Elf32_Word e_version;
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Elf32_Addr e_entry;
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Elf32_Off e_phoff;
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Elf32_Off e_shoff;
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Elf32_Word e_flags;
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Elf32_Half e_ehsize;
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Elf32_Half e_phentsize;
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Elf32_Half e_phnum;
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Elf32_Half e_shentsize;
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Elf32_Half e_shnum;
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Elf32_Half e_shstrndx;
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} Elf32_Ehdr;
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.Ed
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.Pp
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.Bl -tag -width "e_phentsize"
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.It Fa e_ident[]
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The array contains the following information in the indicated locations:
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.Bl -tag -width EI_ABIVERSION
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.It Dv EI_MAG0
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The elements ranging from
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.Dv EI_MAG0
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to
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.Dv EI_MAG3
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contain the ELF magic number: \\0177ELF
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.It Dv EI_CLASS
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Contains the address size of the binary, either 32 or 64bit.
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.It Dv EI_DATA
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byte order
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.It Dv EI_VERSION
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Contains the ELF header version. This is currently always set to 1.
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.It Dv EI_OSABI
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Contains the operating system ABI identification. Note that even though the
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definition
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.Dv ELFOSABI_NETBSD
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exists,
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.Nx
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uses
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.Dv ELFOSABI_SYSV
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here, since the
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.Nx
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ABI does not deviate from the standard.
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.It Dv EI_ABIVERSION
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ABI version.
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.El
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.It Fa e_type
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Contains the file type identification. It can be either
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.Dv ET_REL ,
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.Dv ET_EXEC ,
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.Dv ET_DYN ,
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or
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.Dv ET_CORE
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for relocatable, executable, shared, or core, respectively.
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.It Fa e_machine
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Contains the machine type, e.g. SPARC, Alpha, MIPS, ...
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.It Fa e_entry
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The program entry point if the file is executable.
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.It Fa e_phoff
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The position of the program header table in the file or 0 if it doesn't exist.
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.It Fa e_shoff
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The position of the section header table in the file or 0 if it doesn't exist.
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.It Fa e_flags
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Contains processor-specific flags. For example, the SPARC port uses this
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space to specify what kind of memory store ordering is required.
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.It Fa e_ehsize
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The size of the ELF header.
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.It Fa e_phentsize
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The size of an entry in the program header table. All entries are the same
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size.
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.It Fa e_phnum
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The number of entries in the program header table, or 0 if none exists.
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.It Fa e_shentsize
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The size of an entry in the section header table. All entries are the same
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size.
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.It Fa e_shnum
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The number of entries in the section header table, or 0 if none exists.
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.It Fa e_shstrndx
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Contains the index number of the section which contains the section
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name strings.
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.El
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.Pp
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Each ELF section in turn is described by the section header:
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Word sh_name;
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Elf32_Word sh_type;
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Elf32_Word sh_flags;
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Elf32_Addr sh_addr;
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Elf32_Off sh_offset;
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Elf32_Word sh_size;
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Elf32_Word sh_link;
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Elf32_Word sh_info;
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Elf32_Word sh_addralign;
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Elf32_Word sh_entsize;
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} Elf32_Shdr;
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.Ed
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.Pp
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.Bl -tag -width "sh_addralign"
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.It Fa sh_name
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Contains an index to the position in the section header string section where
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the name of the current section can be found.
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.It Fa sh_type
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Contains the section type indicator. The more important possible values are:
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.Bl -tag -width "SHT_PROGBITS"
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.It Dv SHT_NULL
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Section is inactive. The other fields contain undefined values.
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.It Dv SHT_PROGBITS
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Section contains program information. It can be for example code, data,
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or debugger information.
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.It Dv SHT_SYMTAB
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Section contains a symbol table. This section usually contains all the
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symbols and is intended for the regular link editor
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.Xr ld 1 .
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.It Dv SHT_STRTAB
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Section contains a string table.
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.It Dv SHT_RELA
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Section contains relocation information with an explicit addend.
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.It Dv SHT_HASH
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Section contains a symbol hash table.
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.It Dv SHT_DYNAMIC
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Section contains dynamic linking information.
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.It Dv SHT_NOTE
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Section contains some special information. The format can be e.g.
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vendor-specific.
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.It Dv SHT_NOBITS
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Sections contains information similar to
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.Dv SHT_PROGBITS ,
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but takes up no space in the file. This can be used for e.g. bss.
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.It Dv SHT_REL
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Section contains relocation information without an explicit addend.
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.It Dv SHT_SHLIB
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This section type is reserved but has unspecified semantics.
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.It Dv SHT_DYNSYM
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Section contains a symbol table. This symbol table is intended for the
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dynamic linker, and is kept as small as possible to conserve space, since
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it must be loaded to memory at run time.
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.El
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.It Fa sh_flags
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Contains the section flags, which can have the following values or any
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combination of them:
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.Bl -tag -width SHF_EXECINSTR
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.It Dv SHF_WRITE
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Section is writable after it has been loaded.
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.It Dv SHF_ALLOC
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Section will occupy memory at run time.
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.It Dv SHF_EXECINSTR
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Section contains executable machine instructions.
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.El
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.It Fa sh_addr
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Address to where the section will be loaded, or 0 if this section does not
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reside in memory at run time.
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.It Fa sh_offset
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The byte offset from the beginning of the file to the beginning of this
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section. If the section is of type
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.Dv SHT_NOBITS ,
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this field specifies the conceptual placement in the file.
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.It Fa sh_size
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The size of the section in the file for all types except
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.Dv SHT_NOBITS .
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For that type the value may differ from zero, but the section will still
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always take up no space from the file.
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.It Fa sh_link
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Contains an index to the section header table. The interpretation depends
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on the section type as follows:
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.Pp
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.Bl -tag -compact -width SHT_DYNAMIC
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.It Dv SHT_REL
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.It Dv SHT_RELA
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Section index of the associated symbol table.
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.Pp
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.It Dv SHT_SYMTAB
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.It Dv SHT_DYNSYM
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Section index of the associated string table.
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.Pp
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.It Dv SHT_HASH
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Section index of the symbol table to which the hash table applies.
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.Pp
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.It Dv SHT_DYNAMIC
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Section index of of the string table by which entries in this section are used.
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.El
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.It Fa sh_info
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Contains extra information. The interpretation depends on the type as
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follows:
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.Pp
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.Bl -tag -compact -width SHT_DYNSYM
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.It Dv SHT_REL
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.It Dv SHT_RELA
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Section index of the section to which the relocation information applies.
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.Pp
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.It Dv SHT_SYMTAB
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.It Dv SHT_DYNSYM
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Contains a value one greater that the last local symbol table index.
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.El
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.It Fa sh_addralign
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Marks the section alignment requirement. If, for example, the section contains
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a doubleword, the entire section must be doubleword aligned to ensure proper
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alignment. Only 0 and integral powers of two are allowed. Values 0 and 1
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denote that the section has no alignment.
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.It Fa sh_entsize
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Contains the entry size of an element for sections which are constructed
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of a table of fixed-size entries. If the section does not hold a table of
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fixed-size entries, this value is 0.
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.El
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.Pp
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Every executable object must contain a program header. The program header
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contains information necessary in constructing a process image.
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Word p_type;
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Elf32_Off p_offset;
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Elf32_Addr p_vaddr;
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Elf32_Addr p_paddr;
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Elf32_Word p_filesz;
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Elf32_Word p_memsz;
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Elf32_Word p_flags;
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Elf32_Word p_align;
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} Elf32_Phdr;
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.Ed
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.Pp
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.Bl -tag -width p_offset
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.It Fa p_type
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Contains the segment type indicator. The possible values are:
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.Bl -tag -width PT_DYNAMIC
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.It Dv PT_NULL
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Segment is inactive. The other fields contain undefined values.
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.It Dv PT_LOAD
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Segment is loadable. It is loaded to the address described by
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.Fa p_vaddr .
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If
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.Fa p_memsz
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is greater than
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.Fa p_filesz ,
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the memory range from
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.Po Fa p_vaddr
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+
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.Fa p_filesz Pc
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to
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.Po Fa p_vaddr
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+
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.Fa p_memsz Pc
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is zero-filled when the segment is loaded.
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.Fa p_filesz
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can not be greater than
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.Fa p_memsz .
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Segments of this type are sorted in the header table by
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.Fa p_vaddr
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in ascending order.
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.It Dv PT_DYNAMIC
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Segment contains dynamic linking information.
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.It Dv PT_INTERP
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Segment contains a null-terminated path name to the interpreter. This segment
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may be present only once in a file, and it must appear before any loadable
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segments. This field will most likely contain the ELF dynamic loader:
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.Pa /libexec/ld.elf_so
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.It Dv PT_NOTE
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Segment contains some special information. Format can be e.g. vendor-specific.
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.It Dv PT_SHLIB
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This segment type is reserved but has unspecified semantics. Programs
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which contain a segment of this type do not conform to the ABI, and must
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indicate this by setting the appropriate ABI in the ELF header
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.Dv EI_OSABI
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field.
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.It Dv PT_PHDR
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The values in a program header of this type specify the characteristics
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of the program header table itself. For example, the
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.Fa p_vaddr
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field specifies the program header table location in memory once the
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program is loaded. This field may not occur more than once, may occur only
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if the program header table is part of the file memory image, and must
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come before any loadable segments.
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.El
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.It Fa p_offset
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Contains the byte offset from the beginning of the file to the beginning
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of this segment.
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.It Fa p_vaddr
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Contains the virtual memory address to which this segment is loaded.
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.It Fa p_paddr
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Contains the physical address to which this segment is loaded. This value
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is usually ignored, but may be used while bootstrapping or in embedded
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systems.
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.It Fa p_filesz
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Contains the number of bytes this segment occupies in the file image.
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.It Fa p_memsz
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Contains the number of bytes this segment occupies in the memory image.
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.It Fa p_flags
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Contains the segment flags, which specify the permissions for the segment
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after it has been loaded. The following values or any combination of them
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is acceptable:
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.Bl -tag -width PF_R
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.It Dv PF_R
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Segment can be read.
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.It Dv PF_W
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Segment can be written.
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.It Dv PF_X
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Segment is executable.
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.El
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.It Fa p_align
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Contains the segment alignment. Acceptable values are 0 and 1 for no alignment,
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and integral powers of two.
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.Fa p_vaddr
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should equal
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.Fa p_offset
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modulo
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.Fa p_align .
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.El
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.Sh SEE ALSO
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.Xr as 1 ,
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.Xr gdb 1 ,
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.Xr ld 1 ,
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.Xr ld.elf_so 1 ,
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.Xr execve 2 ,
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.Xr nlist 3 ,
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.Xr a.out 5 ,
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.Xr core 5 ,
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.Xr link 5 ,
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.Xr stab 5
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.Sh HISTORY
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The ELF object file format first appeared in
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.At V .
|