461 lines
14 KiB
Groff
461 lines
14 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This man page is derived from documentation contributed to Berkeley by
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.\" Donn Seeley at UUNET Technologies, Inc.
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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 University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)a.out.5 8.1 (Berkeley) 6/5/93
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.\" $FreeBSD$
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.\"
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.Dd June 5, 1993
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.Dt A.OUT 5
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.Os
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.Sh NAME
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.Nm a.out
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.Nd format of executable binary files
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.Sh SYNOPSIS
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.Fd #include <a.out.h>
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.Sh DESCRIPTION
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The include file
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.Aq Pa a.out.h
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declares three structures and several macros.
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The structures describe the format of
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executable machine code files
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.Pq Sq binaries
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on the system.
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.Pp
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A binary file consists of up to 7 sections.
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In order, these sections are:
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.Bl -tag -width "text relocations"
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.It exec header
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Contains parameters used by the kernel
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to load a binary file into memory and execute it,
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and by the link editor
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.Xr ld 1
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to combine a binary file with other binary files.
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This section is the only mandatory one.
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.It text segment
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Contains machine code and related data
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that are loaded into memory when a program executes.
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May be loaded read-only.
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.It data segment
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Contains initialized data; always loaded into writable memory.
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.It text relocations
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Contains records used by the link editor
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to update pointers in the text segment when combining binary files.
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.It data relocations
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Like the text relocation section, but for data segment pointers.
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.It symbol table
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Contains records used by the link editor
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to cross reference the addresses of named variables and functions
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.Pq Sq symbols
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between binary files.
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.It string table
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Contains the character strings corresponding to the symbol names.
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.El
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.Pp
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Every binary file begins with an
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.Fa exec
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structure:
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.Bd -literal -offset indent
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struct exec {
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unsigned long a_midmag;
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unsigned long a_text;
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unsigned long a_data;
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unsigned long a_bss;
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unsigned long a_syms;
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unsigned long a_entry;
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unsigned long a_trsize;
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unsigned long a_drsize;
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};
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.Ed
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.Pp
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The fields have the following functions:
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.Bl -tag -width a_trsize
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.It Fa a_midmag
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This field is stored in host byte-order.
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It has a number of sub-components accessed by the macros
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.Dv N_GETFLAG() ,
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.Dv N_GETMID() , and
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.Dv N_GETMAGIC() ,
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and set by the macro
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.Dv N_SETMAGIC().
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.Pp
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The macro
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.Dv N_GETFLAG()
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returns a few flags:
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.Bl -tag -width EX_DYNAMIC
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.It Dv EX_DYNAMIC
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indicates that the executable requires the services of the run-time link editor.
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.It Dv EX_PIC
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indicates that the object contains position independent code. This flag is
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set by
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.Xr as 1
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when given the
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.Sq -k
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flag and is preserved by
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.Xr ld 1
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if necessary.
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.El
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.Pp
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If both EX_DYNAMIC and EX_PIC are set, the object file is a position independent
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executable image (eg. a shared library), which is to be loaded into the
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process address space by the run-time link editor.
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.Pp
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The macro
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.Dv N_GETMID()
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returns the machine-id.
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This indicates which machine(s) the binary is intended to run on.
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.Pp
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.Dv N_GETMAGIC()
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specifies the magic number, which uniquely identifies binary files
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and distinguishes different loading conventions.
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The field must contain one of the following values:
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.Bl -tag -width ZMAGIC
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.It Dv OMAGIC
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The text and data segments immediately follow the header
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and are contiguous.
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The kernel loads both text and data segments into writable memory.
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.It Dv NMAGIC
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As with
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.Dv OMAGIC ,
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text and data segments immediately follow the header and are contiguous.
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However, the kernel loads the text into read-only memory
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and loads the data into writable memory at the next
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page boundary after the text.
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.It Dv ZMAGIC
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The kernel loads individual pages on demand from the binary.
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The header, text segment and data segment are all
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padded by the link editor to a multiple of the page size.
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Pages that the kernel loads from the text segment are read-only,
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while pages from the data segment are writable.
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.El
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.It Fa a_text
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Contains the size of the text segment in bytes.
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.It Fa a_data
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Contains the size of the data segment in bytes.
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.It Fa a_bss
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Contains the number of bytes in the
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.Sq bss segment
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and is used by the kernel to set the initial break
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.Pq Xr brk 2
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after the data segment.
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The kernel loads the program so that this amount of writable memory
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appears to follow the data segment and initially reads as zeroes.
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.It Fa a_syms
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Contains the size in bytes of the symbol table section.
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.It Fa a_entry
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Contains the address in memory of the entry point
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of the program after the kernel has loaded it;
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the kernel starts the execution of the program
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from the machine instruction at this address.
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.It Fa a_trsize
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Contains the size in bytes of the text relocation table.
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.It Fa a_drsize
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Contains the size in bytes of the data relocation table.
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.El
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.Pp
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The
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.Pa a.out.h
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include file defines several macros which use an
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.Fa exec
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structure to test consistency or to locate section offsets in the binary file.
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.Bl -tag -width N_BADMAG(exec)
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.It Fn N_BADMAG exec
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Nonzero if the
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.Fa a_magic
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field does not contain a recognized value.
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.It Fn N_TXTOFF exec
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The byte offset in the binary file of the beginning of the text segment.
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.It Fn N_SYMOFF exec
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The byte offset of the beginning of the symbol table.
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.It Fn N_STROFF exec
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The byte offset of the beginning of the string table.
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.El
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.Pp
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Relocation records have a standard format which
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is described by the
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.Fa relocation_info
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structure:
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.Bd -literal -offset indent
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struct relocation_info {
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int r_address;
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unsigned int r_symbolnum : 24,
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r_pcrel : 1,
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r_length : 2,
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r_extern : 1,
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r_baserel : 1,
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r_jmptable : 1,
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r_relative : 1,
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r_copy : 1;
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};
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.Ed
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.Pp
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The
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.Fa relocation_info
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fields are used as follows:
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.Bl -tag -width r_symbolnum
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.It Fa r_address
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Contains the byte offset of a pointer that needs to be link-edited.
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Text relocation offsets are reckoned from the start of the text segment,
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and data relocation offsets from the start of the data segment.
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The link editor adds the value that is already stored at this offset
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into the new value that it computes using this relocation record.
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.It Fa r_symbolnum
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Contains the ordinal number of a symbol structure
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in the symbol table (it is
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.Em not
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a byte offset).
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After the link editor resolves the absolute address for this symbol,
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it adds that address to the pointer that is undergoing relocation.
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(If the
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.Fa r_extern
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bit is clear, the situation is different; see below.)
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.It Fa r_pcrel
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If this is set,
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the link editor assumes that it is updating a pointer
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that is part of a machine code instruction using pc-relative addressing.
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The address of the relocated pointer is implicitly added
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to its value when the running program uses it.
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.It Fa r_length
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Contains the log base 2 of the length of the pointer in bytes;
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0 for 1-byte displacements, 1 for 2-byte displacements,
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2 for 4-byte displacements.
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.It Fa r_extern
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Set if this relocation requires an external reference;
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the link editor must use a symbol address to update the pointer.
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When the
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.Fa r_extern
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bit is clear, the relocation is
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.Sq local ;
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the link editor updates the pointer to reflect
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changes in the load addresses of the various segments,
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rather than changes in the value of a symbol (except when
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.Fa r_baserel
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is also set (see below).
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In this case, the content of the
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.Fa r_symbolnum
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field is an
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.Fa n_type
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value (see below);
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this type field tells the link editor
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what segment the relocated pointer points into.
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.It Fa r_baserel
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If set, the symbol, as identified by the
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.Fa r_symbolnum
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field, is to be relocated to an offset into the Global Offset Table.
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At run-time, the entry in the Global Offset Table at this offset is set to
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be the address of the symbol.
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.It Fa r_jmptable
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If set, the symbol, as identified by the
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.Fa r_symbolnum
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field, is to be relocated to an offset into the Procedure Linkage Table.
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.It Fa r_relative
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If set, this relocation is relative to the (run-time) load address of the
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image this object file is going to be a part of. This type of relocation
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only occurs in shared objects.
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.It Fa r_copy
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If set, this relocation record identifies a symbol whose contents should
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be copied to the location given in
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.Fa r_address.
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The copying is done by the run-time link-editor from a suitable data
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item in a shared object.
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.El
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.Pp
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Symbols map names to addresses (or more generally, strings to values).
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Since the link-editor adjusts addresses,
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a symbol's name must be used to stand for its address
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until an absolute value has been assigned.
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Symbols consist of a fixed-length record in the symbol table
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and a variable-length name in the string table.
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The symbol table is an array of
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.Fa nlist
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structures:
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.Bd -literal -offset indent
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struct nlist {
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union {
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char *n_name;
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long n_strx;
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} n_un;
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unsigned char n_type;
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char n_other;
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short n_desc;
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unsigned long n_value;
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};
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.Ed
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.Pp
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The fields are used as follows:
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.Bl -tag -width n_un.n_strx
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.It Fa n_un.n_strx
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Contains a byte offset into the string table
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for the name of this symbol.
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When a program accesses a symbol table with the
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.Xr nlist 3
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function,
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this field is replaced with the
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.Fa n_un.n_name
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field, which is a pointer to the string in memory.
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.It Fa n_type
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Used by the link editor to determine
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how to update the symbol's value.
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The
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.Fa n_type
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field is broken down into three sub-fields using bitmasks.
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The link editor treats symbols with the
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.Dv N_EXT
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type bit set as
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.Sq external
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symbols and permits references to them from other binary files.
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The
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.Dv N_TYPE
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mask selects bits of interest to the link editor:
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.Bl -tag -width N_TEXT
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.It Dv N_UNDF
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An undefined symbol.
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The link editor must locate an external symbol with the same name
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in another binary file to determine the absolute value of this symbol.
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As a special case, if the
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.Fa n_value
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field is nonzero and no binary file in the link-edit defines this symbol,
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the link-editor will resolve this symbol to an address
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in the bss segment,
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reserving an amount of bytes equal to
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.Fa n_value .
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If this symbol is undefined in more than one binary file
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and the binary files do not agree on the size,
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the link editor chooses the greatest size found across all binaries.
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.It Dv N_ABS
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An absolute symbol.
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The link editor does not update an absolute symbol.
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.It Dv N_TEXT
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A text symbol.
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This symbol's value is a text address and
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the link editor will update it when it merges binary files.
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.It Dv N_DATA
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A data symbol; similar to
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.Dv N_TEXT
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but for data addresses.
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The values for text and data symbols are not file offsets but
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addresses; to recover the file offsets, it is necessary
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to identify the loaded address of the beginning of the corresponding
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section and subtract it, then add the offset of the section.
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.It Dv N_BSS
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A bss symbol; like text or data symbols but
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has no corresponding offset in the binary file.
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.It Dv N_FN
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A filename symbol.
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The link editor inserts this symbol before
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the other symbols from a binary file when
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merging binary files.
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The name of the symbol is the filename given to the link editor,
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and its value is the first text address from that binary file.
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Filename symbols are not needed for link-editing or loading,
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but are useful for debuggers.
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.El
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.Pp
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The
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.Dv N_STAB
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mask selects bits of interest to symbolic debuggers
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such as
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.Xr gdb 1 ;
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the values are described in
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.Xr stab 5 .
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.It Fa n_other
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This field provides information on the nature of the symbol independent of
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the symbol's location in terms of segments as determined by the
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.Fa n_type
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field. Currently, the lower 4 bits of the
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.Fa n_other
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field hold one of two values:
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.Dv AUX_FUNC
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and
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.Dv AUX_OBJECT
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.Po
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see
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.Aq Pa link.h
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for their definitions
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.Pc .
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.Dv AUX_FUNC
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associates the symbol with a callable function, while
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.Dv AUX_OBJECT
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associates the symbol with data, irrespective of their locations in
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either the text or the data segment.
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This field is intended to be used by
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.Xr ld 1
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for the construction of dynamic executables.
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.It Fa n_desc
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Reserved for use by debuggers; passed untouched by the link editor.
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Different debuggers use this field for different purposes.
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.It Fa n_value
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Contains the value of the symbol.
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For text, data and bss symbols, this is an address;
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for other symbols (such as debugger symbols),
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the value may be arbitrary.
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.El
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.Pp
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The string table consists of an
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.Em unsigned long
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length followed by null-terminated symbol strings.
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The length represents the size of the entire table in bytes,
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so its minimum value (or the offset of the first string)
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is always 4 on 32-bit machines.
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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 brk 2 ,
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.Xr execve 2 ,
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.Xr nlist 3 ,
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.Xr core 5 ,
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.Xr elf 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
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.Pa a.out.h
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include file appeared in
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.At v7 .
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.Sh BUGS
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Since not all of the supported architectures use the
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.Fa a_midmag
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field,
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it can be difficult to determine what
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architecture a binary will execute on
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without examining its actual machine code.
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Even with a machine identifier,
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the byte order of the
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.Fa exec
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header is machine-dependent.
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.Pp
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Nobody seems to agree on what
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.Em bss
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stands for.
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