a.out —
format of executable binary files
The include file <a.out.h>
  declares three structures and several macros. The structures describe the
  format of executable machine code files (‘binaries’) on the
  system.
A binary file consists of up to 7 sections. In order, these
    sections are:
  - exec header
- Contains parameters used by the kernel to load a binary file into memory
      and execute it, and by the link editor
      ld(1) to combine a binary file
      with other binary files. This section is the only mandatory one.
- text segment
- Contains machine code and related data that are loaded into memory when a
      program executes. May be loaded read-only.
- data segment
- Contains initialized data; always loaded into writable memory.
- text relocations
- Contains records used by the link editor to update pointers in the text
      segment when combining binary files.
- data relocations
- Like the text relocation section, but for data segment pointers.
- symbol table
- Contains records used by the link editor to cross reference the addresses
      of named variables and functions (‘symbols’) between binary
      files.
- string table
- Contains the character strings corresponding to the symbol names.
Every binary file begins with an exec
    structure:
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;
};
 
The fields have the following functions:
  - a_midmag
- This field is stored in network byte-order so that binaries for machines
      with alternative byte orders can be distinguished. It has a number of
      sub-components accessed by the macros N_GETFLAG(),N_GETMID(),andN_GETMAGIC(), and set by the macroN_SETMAGIC().The macro N_GETFLAG()returns a few
        flags:
 
      - EX_DYNAMIC
- indicates that the executable requires the services of the run-time
          link editor.
- EX_PIC
- indicates that the object contains position independent code. This
          flag is set by as(1) when
          given the ‘-k’ flag and is preserved by
          ld(1) if necessary.
 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. The macro N_GETMID()returns the
        machine-id. This indicates which machine(s) the binary is intended to
        run on.
 N_GETMAGIC()specifies the magic
        number, which uniquely identifies binary files and distinguishes
        different loading conventions. The field must contain one of the
        following values:
 
      - OMAGIC
- The text and data segments immediately follow the header and are
          contiguous. The kernel loads both text and data segments into writable
          memory.
- NMAGIC
- As with OMAGIC, 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.
- ZMAGIC
- 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.
 
- a_text
- Contains the size of the text segment in bytes.
- a_data
- Contains the size of the data segment in bytes.
- a_bss
- Contains the number of bytes in the ‘bss segment’ and is
      used by the kernel to set the initial break
      (brk(2)) 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_syms
- Contains the size in bytes of the symbol table section.
- a_entry
- 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.
- a_trsize
- Contains the size in bytes of the text relocation table.
- a_drsize
- Contains the size in bytes of the data relocation table.
The a.out.h include file defines several
    macros which use an exec structure to test consistency
    or to locate section offsets in the binary file.
  - N_BADMAG(exec)
- Nonzero if the a_magic field does not contain a
      recognized value.
- N_TXTOFF(exec)
- The byte offset in the binary file of the beginning of the text
    segment.
- N_SYMOFF(exec)
- The byte offset of the beginning of the symbol table.
- N_STROFF(exec)
- The byte offset of the beginning of the string table.
Relocation records have a standard format which is described by
    the relocation_info structure:
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;
};
 
The relocation_info fields are used as
    follows:
  - r_address
- 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.
- r_symbolnum
- Contains the ordinal number of a symbol structure in the symbol table (it
      is not 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 r_extern bit
      is clear, the situation is different; see below.)
- r_pcrel
- 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.
- r_length
- 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.
- r_extern
- Set if this relocation requires an external reference; the link editor
      must use a symbol address to update the pointer. When the
      r_extern bit is clear, the relocation is
      ‘local’; 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 r_baserel is
      also set, see below). In this case, the content of the
      r_symbolnum field is an n_type
      value (see below); this type field tells the link editor what segment the
      relocated pointer points into.
- r_baserel
- If set, the symbol, as identified by the r_symbolnum
      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.
- r_jmptable
- If set, the symbol, as identified by the r_symbolnum
      field, is to be relocated to an offset into the Procedure Linkage
    Table.
- r_relative
- 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.
- r_copy
- If set, this relocation record identifies a symbol whose contents should
      be copied to the location given in r_address. The
      copying is done by the run-time link-editor from a suitable data item in a
      shared object.
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
    nlist structures:
struct nlist {
	union {
		char	*n_name;
		long	n_strx;
	} n_un;
	unsigned char	n_type;
	char		n_other;
	short		n_desc;
	unsigned long	n_value;
};
 
The fields are used as follows:
  - n_un.n_strx
- Contains a byte offset into the string table for the name of this symbol.
      When a program accesses a symbol table with the
      nlist(3) function, this field
      is replaced with the n_un.n_name field, which is a
      pointer to the string in memory.
- n_type
- Used by the link editor to determine how to update the symbol's value. The
      n_type field is broken down into three sub-fields
      using bitmasks. The link editor treats symbols with the
      N_EXTtype bit set as ‘external’
      symbols and permits references to them from other binary files. TheN_TYPEmask selects bits of interest to the link
      editor:
      - N_UNDF
- 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
          n_value 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 n_value. 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.
- N_ABS
- An absolute symbol. The link editor does not update an absolute
          symbol.
- N_TEXT
- A text symbol. This symbol's value is a text address and the link
          editor will update it when it merges binary files.
- N_DATA
- A data symbol; similar to N_TEXTbut 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.
- N_BSS
- A bss symbol; like text or data symbols but has no corresponding
          offset in the binary file.
- N_FN
- 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.
 The N_STABmask selects bits of
        interest to symbolic debuggers such as
        gdb(1); the values are
        described in stab(5).
 
- n_other
- This field provides information on the nature of the symbol independent of
      the symbol's location in terms of segments as determined by the
      n_type field. Currently, the lower 4 bits of the
      n_other field hold one of two values:
      AUX_FUNCandAUX_OBJECT(see<link.h>for their
      definitions).AUX_FUNCassociates the symbol with
      a callable function, whileAUX_OBJECTassociates
      the symbol with data, irrespective of their locations in either the text
      or the data segment. This field is intended to be used by
      ld(1) for the construction of
      dynamic executables.
- n_desc
- Reserved for use by debuggers; passed untouched by the link editor.
      Different debuggers use this field for different purposes.
- n_value
- 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.
The string table consists of an unsigned long
    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.
The a.out.h include file appeared in
  Version 7 AT&T UNIX.
Nobody seems to agree on what bss stands for.
New binary file formats may be supported in the future, and they
    probably will not be compatible at any level with this ancient format.