A.OUT(5)

NAME
a.out - format of executable binary files

SYNOPSIS
#include <a.out.h>

DESCRIPTION
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 host byte-order. It has a number of sub-components accessed by the macros N_GETFLAG(), N_GETMID(), and N_GETMAGIC(), and set by the macro N_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 (eg. 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_EXT type bit set as ‘external’ symbols and permits references to them from other binary files. The N_TYPE mask 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_TEXT 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.

    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_STAB mask 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_FUNC and AUX_OBJECT ( see <link.h> for their definitions). AUX_FUNC associates the symbol with a callable function, while AUX_OBJECT 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 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.

SEE ALSO
as(1), gdb(1), ld(1), brk(2), execve(2), nlist(3), core(5), link(5), stab(5)

HISTORY
The a.out.h include file appeared in AT&T UNIX.

BUGS
Since not all of the supported architectures use the a_midmag 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 exec header is machine-dependent.

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.