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a.out - format of executable binary files

 

NAME

      a.out - format of executable binary files
 

SYNOPSIS

      #include <a.out.h>
 

DESCRIPTION

      The include file #include <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 ref‐
                        erence 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 inde‐
                            pendent code.  This flag is set by as(1) when given
                            the ‘-k’ flag and is preserved by ld(1) if neces‐
                            sary.
 
                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 identi‐
                fies binary files and distinguishes different loading conven‐
                tions.  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 fol‐
                        low 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 seg‐
                        ment 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 ini‐
                tially reads as zeroes.  (bss = block started by symbol)
 
      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 pro‐
                gram after the kernel has loaded it; the kernel starts the exe‐
                cution 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 #include <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 recog‐
                      nized 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 edi‐
                   tor 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 displace‐
                   ments, 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 Ta‐
                   ble.  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 mem‐
                   ory.
 
      n_type       Used by the link editor to determine how to update the sym‐
                   bol’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 per‐
                   mits 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 unde‐
                           fined 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 sec‐
                           tion 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 debug‐
                   gers 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 #include <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.
      as(1), gdb(1), ld(1), brk(2), execve(2), nlist(3), core(5), elf(5),
      link(5), stab(5)
 

HISTORY

      The #include <a.out.h>
      include file appeared in Version 7 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 identi‐
      fier, the byte order of the exec header is machine-dependent.
 

Sections

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