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     A.OUT(5)                                                 A.OUT(5)

          a.out - object file format

          #include <a.out.h>

          A.out is the default name of the output file of the assem-
          bler as(1) or the link editor ld(1). Both programs make
          a.out executable if there were no errors and no unresolved
          external references.  An object file has five sections: a
          header, the program text and data, relocation information, a
          symbol table and a string table (in that order).  The last
          three may be absent; see strip(1) and option -s of ld(1).
          The header format, given in <a.out.h>, is

          struct exec {
                   long      a_magic;   /* magic number */
                   unsigned  a_text;    /* size of text segment */
                   unsigned  a_data;    /* size of initialized data */
                   unsigned  a_bss;     /* size of uninitialized data */
                   unsigned  a_syms;    /* size of symbol table */
                   unsigned  a_entry;   /* entry point */
                   unsigned  a_trsize;  /* size of text relocation */
                   unsigned  a_drsize;  /* size of data relocation */
          #define  OMAGIC    0407       /* old impure format */
          #define  NMAGIC    0410       /* read-only text */
          #define  ZMAGIC    0413       /* demand load format */

          Macros which take `exec' structures as arguments and tell
          whether the file has a reasonable magic number or return
          #define  N_BADMAG(x) (((x).a_magic)!=OMAGIC && \
                   ((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
          #define  N_TXTOFF(x) \
                   ((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
          #define N_SYMOFF(x) (N_TXTOFF(x) + (x).a_text+(x).a_data + \
          #define  N_STROFF(x) (N_SYMOFF(x) + (x).a_syms)

          Sizes are expressed in bytes.  The size of the header is not
          included in any of the other sizes.

          When an file is executed, a memory image of three segments
          is set up: the text segment, the data segment, and a stack.
          The text segment begins at virtual address 0.  Following the
          text segment is the data segment, in which explicitly ini-
          tialized data come first, then other data, initialized to 0.
          The stack occupies the highest possible locations in the

     A.OUT(5)                                                 A.OUT(5)

          core image, automatically growing downwards from 0x7ffff400
          as needed.  The data segment may be extended by brk(2).

          If the magic number in the header is OMAGIC, the text seg-
          ment is neither write-protected nor shared and the data seg-
          ment is immediately contiguous with the text segment.  This
          kind of executable program is rarely used.  If the magic
          number is NMAGIC or ZMAGIC, the data segment is loaded at
          the first 0 mod 1024 address following the text segment, and
          the text segment is write-protected and shared among all
          processes that are executing the same file.  ZMAGIC format,
          which ld(1) produces by default, supports demand paging: the
          text and data segments are multiples of 1024 bytes long and
          begin at byte offsets of 0 mod 1024 in the file.

          Macros are provided to compute the absolute offset of vari-
          ous parts of the file:

          N_TXTOFF  Text segment.
          N_SYMOFF  Symbol table.
          N_STROFF  String table.

          The offsets of the data segment, text relocation informa-
          tion, and data relocation information are obtained by suc-
          cessively adding to N_TXTOFF the size fields a_text, a_data,
          and a_trsize.  The first 4 bytes of the string table contain
          it size, including the 4 bytes.

          The layout of a symbol table entry, as given in <a.out.h>,

          struct nlist {
                   union {
                       char      *n_name;/* for use when in-core */
                       long      n_strx; /* index into file string table */
                   } n_un;
                   unsigned char n_type; /* type flag; see below */
                   char          n_other;
                   short         n_desc; /* see stab(5) */
                   unsigned      n_value;/* value of this symbol (or struct offset) */

          Basic values for n_type:

          #define  N_UNDF        0x0     /* undefined */
          #define  N_ABS         0x2     /* absolute */
          #define  N_TEXT        0x4     /* text */
          #define  N_DATA        0x6     /* data */
          #define  N_BSS         0x8     /* bss */
          #define  N_COMM        0x12    /* common (internal to ld) */
          #define  N_FN          0x1f    /* file name symbol */
          #define  N_EXT         01      /* external bit, or'ed in */

     A.OUT(5)                                                 A.OUT(5)

          #define  N_TYPE        0x1e    /* mask for all the type bits */

          Other permanent symbol table entries have some N_STAB bits
          set.  These are given in `<stab.h>':
          #define  N_STAB        0xe0    /* if any of these bits set, keep */

          The field n_un.n_strx gives an index into the string table;
          0 indicates that no name is associated with the entry.  The
          field n_un.n_name can be used to refer to the symbol name
          only if the program sets this up using n_strx and appropri-
          ate data from the string table.

          A symbol of type undefined external with nonzero value field
          names a common region; the value specifies its size.

          Relocation information occupies eight bytes per relocatable

          struct relocation_info {
                   int       r_address;       /* address of datum to be relocated */
                   unsigned  r_symbolnum:24,  /* local symbol ordinal */
                             r_pcrel:1,       /* is referenced relative to pc */
                             r_length:2,      /* 0=byte, 1=word, 2=long */
                             r_extern:1,      /* symbol value unknown */
                             :4;              /* nothing, yet */

          If r_extern is 1, the datum designated by r_address and
          r_length will be relocated by adding to it the value of the
          associated external symbol.  If r_extern is 0, r_symbolnum
          is encoded in the style of n_type and the value will be
          relocated by adding the relocated base of the designated
          area (text, initialized data, or common data).

          adb(1), as(1), ld(1), nm(1), stab(5), strip(1)