MirBSD manpage: a.out(5)

A.OUT(5)                     BSD Reference Manual                     A.OUT(5)


     a.out - format of executable binary files


     #include <sys/types.h>
     #include <a.out.h>


     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

     data segment      Contains initialized data; always loaded into writable

     text relocations  Contains records used by the link editor to update
                       pointers in the text segment when combining binary

     data relocations  Like the text relocation section, but for data segment

     symbol table      Contains records used by the link editor to cross
                       reference the addresses of named variables and func-
                       tions ("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 {
                   u_int32_t       a_midmag;
                   u_int32_t       a_text;
                   u_int32_t       a_data;
                   u_int32_t       a_bss;
                   u_int32_t       a_syms;
                   u_int32_t       a_entry;
                   u_int32_t       a_trsize;
                   u_int32_t       a_drsize;

     The fields have the following functions:

     a_midmag  This field is stored in network byte-order so that binaries for
               machines with alternate byte orders can be distinguished. It
               has a number of sub-components accessed by the macros
               N_GETFLAG(), N_GETMID(), and N_GETMAGIC(), and set by the macro

               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 in-
                           dependent code. This flag is set by as(1) when
                           given the -k flag and is preserved by ld(1) if

               If both EX_DYNAMIC and EX_PIC are set, the object file is a po-
               sition 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 segment
                       are read-only, while pages from the data segment are

     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 initial-
               ly 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 pro-
               gram after the kernel has loaded it; the kernel starts the exe-
               cution of the program from the machine instruction at this ad-

     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 struc-
     ture to test consistency or to locate section offsets in the binary file.

     N_BADMAG(exec)   Non-zero if the a_magic field does not contain a recog-
                      nized value.

     N_TXTOFF(exec)   The byte offset of the beginning of the text segment.

     N_DATOFF(exec)   The byte offset of the beginning of the data segment.

     N_DRELOFF(exec)  The byte offset of the beginning of the data relocation

     N_TRELOFF(exec)  The byte offset of the beginning of the text relocation

     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
                  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

     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

           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

     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 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 non-zero
                          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 ad-
                          dresses. 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 ad-
                          dress 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 sym-
                          bol 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 de-
                  buggers 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

     n_value      Contains the value of the symbol. For text, data and BSS
                  symbols, this is an address; for other symbols (such as de-
                  bugger symbols), the value may be arbitrary.

     The string table consists of an u_int32_t 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), link(5),


     The a.out.h include file appeared in Version 3 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.

MirBSD #10-current               June 5, 1993                                4

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