path: root/static/freebsd/man5/a.out.5 3.html

<table class="head">
  <tr>
    <td class="head-ltitle">A.OUT(5)</td>
    <td class="head-vol">File Formats Manual</td>
    <td class="head-rtitle">A.OUT(5)</td>
  </tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">a.out</code> &#x2014; <span class="Nd">format of
    executable binary files</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="In">#include
  &lt;<a class="In">a.out.h</a>&gt;</code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The include file
    <code class="In">&lt;<a class="In">a.out.h</a>&gt;</code> declares three
    structures and several macros. The structures describe the format of
    executable machine code files (&#x2018;binaries&#x2019;) on the system.</p>
<p class="Pp">A binary file consists of up to 7 sections. In order, these
    sections are:</p>
<dl class="Bl-tag">
  <dt>exec header</dt>
  <dd>Contains parameters used by the kernel to load a binary file into memory
      and execute it, and by the link editor <a class="Xr">ld(1)</a> to combine
      a binary file with other binary files. This section is the only mandatory
      one.</dd>
  <dt>text segment</dt>
  <dd>Contains machine code and related data that are loaded into memory when a
      program executes. May be loaded read-only.</dd>
  <dt>data segment</dt>
  <dd>Contains initialized data; always loaded into writable memory.</dd>
  <dt>text relocations</dt>
  <dd>Contains records used by the link editor to update pointers in the text
      segment when combining binary files.</dd>
  <dt>data relocations</dt>
  <dd>Like the text relocation section, but for data segment pointers.</dd>
  <dt>symbol table</dt>
  <dd>Contains records used by the link editor to cross reference the addresses
      of named variables and functions (&#x2018;symbols&#x2019;) between binary
      files.</dd>
  <dt>string table</dt>
  <dd>Contains the character strings corresponding to the symbol names.</dd>
</dl>
<p class="Pp">Every binary file begins with an <var class="Fa">exec</var>
    structure:</p>
<div class="Bd Pp Bd-indent Li">
<pre>struct exec {
	unsigned long	a_midmag;
	unsigned long	a_text;
	unsigned long	a_data;
	unsigned long	a_bss;
	unsigned long	a_syms;
	unsigned long	a_entry;
	unsigned long	a_trsize;
	unsigned long	a_drsize;
};</pre>
</div>
<p class="Pp">The fields have the following functions:</p>
<dl class="Bl-tag">
  <dt id="N_GETFLAG"><var class="Fa">a_midmag</var></dt>
  <dd>This field is stored in host byte-order. It has a number of sub-components
      accessed by the macros
      <a class="permalink" href="#N_GETFLAG"><code class="Fn">N_GETFLAG</code></a>(),
      <code class="Fn">N_GETMID</code>(), and
      <code class="Fn">N_GETMAGIC</code>(), and set by the macro
      <a class="permalink" href="#N_SETMAGIC"><code class="Fn" id="N_SETMAGIC">N_SETMAGIC</code></a>().
    <p class="Pp" id="N_GETFLAG~2">The macro
        <a class="permalink" href="#N_GETFLAG~2"><code class="Fn">N_GETFLAG</code></a>()
        returns a few flags:</p>
    <dl class="Bl-tag">
      <dt id="EX_DYNAMIC"><a class="permalink" href="#EX_DYNAMIC"><code class="Dv">EX_DYNAMIC</code></a></dt>
      <dd>indicates that the executable requires the services of the run-time
          link editor.</dd>
      <dt id="EX_PIC"><a class="permalink" href="#EX_PIC"><code class="Dv">EX_PIC</code></a></dt>
      <dd>indicates that the object contains position independent code. This
          flag is set by <a class="Xr">as(1)</a> when given the
          &#x2018;-k&#x2019; flag and is preserved by <a class="Xr">ld(1)</a> if
          necessary.</dd>
    </dl>
    <p class="Pp">If both EX_DYNAMIC and EX_PIC are set, the object file is a
        position independent executable image (e.g. a shared library), which is
        to be loaded into the process address space by the run-time link
      editor.</p>
    <p class="Pp" id="N_GETMID">The macro
        <a class="permalink" href="#N_GETMID"><code class="Fn">N_GETMID</code></a>()
        returns the machine-id. This indicates which machine(s) the binary is
        intended to run on.</p>
    <p class="Pp" id="N_GETMAGIC"><a class="permalink" href="#N_GETMAGIC"><code class="Fn">N_GETMAGIC</code></a>()
        specifies the magic number, which uniquely identifies binary files and
        distinguishes different loading conventions. The field must contain one
        of the following values:</p>
    <dl class="Bl-tag">
      <dt id="OMAGIC"><a class="permalink" href="#OMAGIC"><code class="Dv">OMAGIC</code></a></dt>
      <dd>The text and data segments immediately follow the header and are
          contiguous. The kernel loads both text and data segments into writable
          memory.</dd>
      <dt id="NMAGIC"><a class="permalink" href="#NMAGIC"><code class="Dv">NMAGIC</code></a></dt>
      <dd>As with <code class="Dv">OMAGIC</code>, text and data segments
          immediately follow the header and are contiguous. However, the kernel
          loads the text into read-only memory and loads the data into writable
          memory at the next page boundary after the text.</dd>
      <dt id="ZMAGIC"><a class="permalink" href="#ZMAGIC"><code class="Dv">ZMAGIC</code></a></dt>
      <dd>The kernel loads individual pages on demand from the binary. The
          header, text segment and data segment are all padded by the link
          editor to a multiple of the page size. Pages that the kernel loads
          from the text segment are read-only, while pages from the data segment
          are writable.</dd>
    </dl>
  </dd>
  <dt><var class="Fa">a_text</var></dt>
  <dd>Contains the size of the text segment in bytes.</dd>
  <dt><var class="Fa">a_data</var></dt>
  <dd>Contains the size of the data segment in bytes.</dd>
  <dt id="bss"><var class="Fa">a_bss</var></dt>
  <dd>Contains the number of bytes in the &#x2018;bss segment&#x2019; and is
      used by the kernel to set the initial break (<a class="Xr">brk(2)</a>)
      after the data segment. The kernel loads the program so that this amount
      of writable memory appears to follow the data segment and initially reads
      as zeroes. (<a class="permalink" href="#bss"><i class="Em">bss</i></a> =
      block started by symbol)</dd>
  <dt><var class="Fa">a_syms</var></dt>
  <dd>Contains the size in bytes of the symbol table section.</dd>
  <dt><var class="Fa">a_entry</var></dt>
  <dd>Contains the address in memory of the entry point of the program after the
      kernel has loaded it; the kernel starts the execution of the program from
      the machine instruction at this address.</dd>
  <dt><var class="Fa">a_trsize</var></dt>
  <dd>Contains the size in bytes of the text relocation table.</dd>
  <dt><var class="Fa">a_drsize</var></dt>
  <dd>Contains the size in bytes of the data relocation table.</dd>
</dl>
<p class="Pp">The <code class="In">&lt;<a class="In">a.out.h</a>&gt;</code>
    include file defines several macros which use an <var class="Fa">exec</var>
    structure to test consistency or to locate section offsets in the binary
    file.</p>
<dl class="Bl-tag">
  <dt id="N_BADMAG"><a class="permalink" href="#N_BADMAG"><code class="Fn">N_BADMAG</code></a>(<var class="Fa">exec</var>)</dt>
  <dd>Nonzero if the <var class="Fa">a_magic</var> field does not contain a
      recognized value.</dd>
  <dt id="N_TXTOFF"><a class="permalink" href="#N_TXTOFF"><code class="Fn">N_TXTOFF</code></a>(<var class="Fa">exec</var>)</dt>
  <dd>The byte offset in the binary file of the beginning of the text
    segment.</dd>
  <dt id="N_SYMOFF"><a class="permalink" href="#N_SYMOFF"><code class="Fn">N_SYMOFF</code></a>(<var class="Fa">exec</var>)</dt>
  <dd>The byte offset of the beginning of the symbol table.</dd>
  <dt id="N_STROFF"><a class="permalink" href="#N_STROFF"><code class="Fn">N_STROFF</code></a>(<var class="Fa">exec</var>)</dt>
  <dd>The byte offset of the beginning of the string table.</dd>
</dl>
<p class="Pp">Relocation records have a standard format which is described by
    the <var class="Fa">relocation_info</var> structure:</p>
<div class="Bd Pp Bd-indent Li">
<pre>struct relocation_info {
	int		r_address;
	unsigned int	r_symbolnum : 24,
			r_pcrel : 1,
			r_length : 2,
			r_extern : 1,
			r_baserel : 1,
			r_jmptable : 1,
			r_relative : 1,
			r_copy : 1;
};</pre>
</div>
<p class="Pp">The <var class="Fa">relocation_info</var> fields are used as
    follows:</p>
<dl class="Bl-tag">
  <dt><var class="Fa">r_address</var></dt>
  <dd>Contains the byte offset of a pointer that needs to be link-edited. Text
      relocation offsets are reckoned from the start of the text segment, and
      data relocation offsets from the start of the data segment. The link
      editor adds the value that is already stored at this offset into the new
      value that it computes using this relocation record.</dd>
  <dt id="not"><var class="Fa">r_symbolnum</var></dt>
  <dd>Contains the ordinal number of a symbol structure in the symbol table (it
      is <a class="permalink" href="#not"><i class="Em">not</i></a> a byte
      offset). After the link editor resolves the absolute address for this
      symbol, it adds that address to the pointer that is undergoing relocation.
      (If the <var class="Fa">r_extern</var> bit is clear, the situation is
      different; see below.)</dd>
  <dt><var class="Fa">r_pcrel</var></dt>
  <dd>If this is set, the link editor assumes that it is updating a pointer that
      is part of a machine code instruction using pc-relative addressing. The
      address of the relocated pointer is implicitly added to its value when the
      running program uses it.</dd>
  <dt><var class="Fa">r_length</var></dt>
  <dd>Contains the log base 2 of the length of the pointer in bytes; 0 for
      1-byte displacements, 1 for 2-byte displacements, 2 for 4-byte
      displacements.</dd>
  <dt><var class="Fa">r_extern</var></dt>
  <dd>Set if this relocation requires an external reference; the link editor
      must use a symbol address to update the pointer. When the
      <var class="Fa">r_extern</var> bit is clear, the relocation is
      &#x2018;local&#x2019;; the link editor updates the pointer to reflect
      changes in the load addresses of the various segments, rather than changes
      in the value of a symbol (except when <var class="Fa">r_baserel</var> is
      also set (see below). In this case, the content of the
      <var class="Fa">r_symbolnum</var> field is an <var class="Fa">n_type</var>
      value (see below); this type field tells the link editor what segment the
      relocated pointer points into.</dd>
  <dt><var class="Fa">r_baserel</var></dt>
  <dd>If set, the symbol, as identified by the <var class="Fa">r_symbolnum</var>
      field, is to be relocated to an offset into the Global Offset Table. At
      run-time, the entry in the Global Offset Table at this offset is set to be
      the address of the symbol.</dd>
  <dt><var class="Fa">r_jmptable</var></dt>
  <dd>If set, the symbol, as identified by the <var class="Fa">r_symbolnum</var>
      field, is to be relocated to an offset into the Procedure Linkage
    Table.</dd>
  <dt><var class="Fa">r_relative</var></dt>
  <dd>If set, this relocation is relative to the (run-time) load address of the
      image this object file is going to be a part of. This type of relocation
      only occurs in shared objects.</dd>
  <dt><var class="Fa">r_copy</var></dt>
  <dd>If set, this relocation record identifies a symbol whose contents should
      be copied to the location given in <var class="Fa">r_address</var>. The
      copying is done by the run-time link-editor from a suitable data item in a
      shared object.</dd>
</dl>
<p class="Pp">Symbols map names to addresses (or more generally, strings to
    values). Since the link-editor adjusts addresses, a symbol's name must be
    used to stand for its address until an absolute value has been assigned.
    Symbols consist of a fixed-length record in the symbol table and a
    variable-length name in the string table. The symbol table is an array of
    <var class="Fa">nlist</var> structures:</p>
<div class="Bd Pp Bd-indent Li">
<pre>struct nlist {
	union {
		const char	*n_name;
		long		n_strx;
	} n_un;
	unsigned char		n_type;
	char			n_other;
	short			n_desc;
	unsigned long		n_value;
};</pre>
</div>
<p class="Pp">The fields are used as follows:</p>
<dl class="Bl-tag">
  <dt><var class="Fa">n_un.n_strx</var></dt>
  <dd>Contains a byte offset into the string table for the name of this symbol.
      When a program accesses a symbol table with the <a class="Xr">nlist(3)</a>
      function, this field is replaced with the
      <var class="Fa">n_un.n_name</var> field, which is a pointer to the string
      in memory.</dd>
  <dt><var class="Fa">n_type</var></dt>
  <dd>Used by the link editor to determine how to update the symbol's value. The
      <var class="Fa">n_type</var> field is broken down into three sub-fields
      using bitmasks. The link editor treats symbols with the
      <code class="Dv">N_EXT</code> type bit set as &#x2018;external&#x2019;
      symbols and permits references to them from other binary files. The
      <code class="Dv">N_TYPE</code> mask selects bits of interest to the link
      editor:
    <dl class="Bl-tag">
      <dt id="N_UNDF"><a class="permalink" href="#N_UNDF"><code class="Dv">N_UNDF</code></a></dt>
      <dd>An undefined symbol. The link editor must locate an external symbol
          with the same name in another binary file to determine the absolute
          value of this symbol. As a special case, if the
          <var class="Fa">n_value</var> field is nonzero and no binary file in
          the link-edit defines this symbol, the link-editor will resolve this
          symbol to an address in the bss segment, reserving an amount of bytes
          equal to <var class="Fa">n_value</var>. If this symbol is undefined in
          more than one binary file and the binary files do not agree on the
          size, the link editor chooses the greatest size found across all
          binaries.</dd>
      <dt id="N_ABS"><a class="permalink" href="#N_ABS"><code class="Dv">N_ABS</code></a></dt>
      <dd>An absolute symbol. The link editor does not update an absolute
          symbol.</dd>
      <dt id="N_TEXT"><a class="permalink" href="#N_TEXT"><code class="Dv">N_TEXT</code></a></dt>
      <dd>A text symbol. This symbol's value is a text address and the link
          editor will update it when it merges binary files.</dd>
      <dt id="N_DATA"><a class="permalink" href="#N_DATA"><code class="Dv">N_DATA</code></a></dt>
      <dd>A data symbol; similar to <code class="Dv">N_TEXT</code> but for data
          addresses. The values for text and data symbols are not file offsets
          but addresses; to recover the file offsets, it is necessary to
          identify the loaded address of the beginning of the corresponding
          section and subtract it, then add the offset of the section.</dd>
      <dt id="N_BSS"><a class="permalink" href="#N_BSS"><code class="Dv">N_BSS</code></a></dt>
      <dd>A bss symbol; like text or data symbols but has no corresponding
          offset in the binary file.</dd>
      <dt id="N_FN"><a class="permalink" href="#N_FN"><code class="Dv">N_FN</code></a></dt>
      <dd>A filename symbol. The link editor inserts this symbol before the
          other symbols from a binary file when merging binary files. The name
          of the symbol is the filename given to the link editor, and its value
          is the first text address from that binary file. Filename symbols are
          not needed for link-editing or loading, but are useful for
        debuggers.</dd>
    </dl>
    <p class="Pp">The <code class="Dv">N_STAB</code> mask selects bits of
        interest to symbolic debuggers such as <a class="Xr">gdb(1)</a>
        (<span class="Pa">ports/devel/gdb</span>); the values are described in
        <a class="Xr">stab(5)</a>.</p>
  </dd>
  <dt><var class="Fa">n_other</var></dt>
  <dd>This field provides information on the nature of the symbol independent of
      the symbol's location in terms of segments as determined by the
      <var class="Fa">n_type</var> field. Currently, the lower 4 bits of the
      <var class="Fa">n_other</var> field hold one of two values:
      <code class="Dv">AUX_FUNC</code> and <code class="Dv">AUX_OBJECT</code>
      (see <code class="In">&lt;<a class="In">link.h</a>&gt;</code> for their
      definitions). <code class="Dv">AUX_FUNC</code> associates the symbol with
      a callable function, while <code class="Dv">AUX_OBJECT</code> associates
      the symbol with data, irrespective of their locations in either the text
      or the data segment. This field is intended to be used by
      <a class="Xr">ld(1)</a> for the construction of dynamic executables.</dd>
  <dt><var class="Fa">n_desc</var></dt>
  <dd>Reserved for use by debuggers; passed untouched by the link editor.
      Different debuggers use this field for different purposes.</dd>
  <dt><var class="Fa">n_value</var></dt>
  <dd>Contains the value of the symbol. For text, data and bss symbols, this is
      an address; for other symbols (such as debugger symbols), the value may be
      arbitrary.</dd>
</dl>
<p class="Pp" id="unsigned">The string table consists of an
    <a class="permalink" href="#unsigned"><i class="Em">unsigned long</i></a>
    length followed by null-terminated symbol strings. The length represents the
    size of the entire table in bytes, so its minimum value (or the offset of
    the first string) is always 4 on 32-bit machines.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
  ALSO</a></h1>
<p class="Pp"><a class="Xr">as(1)</a>, <a class="Xr">gdb(1)</a>
    (<span class="Pa">ports/devel/gdb</span>), <a class="Xr">ld(1)</a>,
    <a class="Xr">brk(2)</a>, <a class="Xr">execve(2)</a>,
    <a class="Xr">nlist(3)</a>, <a class="Xr">core(5)</a>,
    <a class="Xr">elf(5)</a>, <a class="Xr">link(5)</a>,
    <a class="Xr">stab(5)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="In">&lt;<a class="In">a.out.h</a>&gt;</code>
    include file appeared in <span class="Ux">Version&#x00A0;7 AT&amp;T
    UNIX</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="BUGS"><a class="permalink" href="#BUGS">BUGS</a></h1>
<p class="Pp">Since not all of the supported architectures use the
    <var class="Fa">a_midmag</var> field, it can be difficult to determine what
    architecture a binary will execute on without examining its actual machine
    code. Even with a machine identifier, the byte order of the
    <var class="Fa">exec</var> header is machine-dependent.</p>
</section>
</div>
<table class="foot">
  <tr>
    <td class="foot-date">June 10, 2010</td>
    <td class="foot-os">FreeBSD 15.0</td>
  </tr>
</table>