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A.OUT(5)                      File Formats Manual                     A.OUT(5)

       a.out - assembler and link editor output

       #include <&lt;a.out.h>&gt;

       A.out  is  the  output  file of the assembler as(1) and the link editor
       ld(1).  Both programs make a.out executable if there were no errors and
       no  unresolved external references.  Layout information as given in the
       include file for the VAX-11 is:

        * Header prepended to each a.out file.
       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 offsets to text|symbols|strings.
       #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 + (x).a_trsize+(x).a_drsize)
       #define  N_STROFF(x) \
                (N_SYMOFF(x) + (x).a_syms)

       The file has five sections: a header, the program text and data,  relo-
       cation  information, a symbol table and a string table (in that order).
       The last three may be omitted if the program was loaded with  the  `-s'
       option  of  ld  or  if  the symbols and relocation have been removed by

       In the header the sizes of each section are given in bytes.   The  size
       of the header is not included in any of the other sizes.

       When  an a.out file is executed, three logical segments are set up: the
       text segment, the data segment (with uninitialized data,  which  starts
       off  as  all  0, following initialized), and a stack.  The text segment
       begins at 0 in the core image; the header is not loaded.  If the  magic
       number  in the header is OMAGIC (0407), it indicates that the text seg-
       ment is not to be write-protected and shared, so the  data  segment  is
       immediately  contiguous with the text segment.  This is the oldest kind
       of executable program and is rarely  used.   If  the  magic  number  is
       NMAGIC  (0410) or ZMAGIC (0413), the data segment begins at the first 0
       mod 1024 byte boundary following the text segment, and the text segment
       is  not  writable  by the program; if other processes are executing the
       same file, they will share the text segment.  For  ZMAGIC  format,  the
       text  segment  begins  at a 0 mod 1024 byte boundary in the a.out file,
       the remaining bytes after the header in the first  block  are  reserved
       and  should be zero.  In this case the text and data sizes must both be
       multiples of 1024 bytes, and the pages of the file will be brought into
       the  running image as needed, and not pre-loaded as with the other for-
       mats.  This is especially suitable for very large programs and  is  the
       default format produced by ld(1).

       The stack will occupy the highest possible locations in the core image:
       growing downwards from 0x7ffff000.  The stack is automatically extended
       as required.  The data segment is only extended as requested by brk(2).

       After  the  header  in  the file follow the text, data, text relocation
       data relocation, symbol table and string table in that order.  The text
       begins at the byte 1024 in the file for ZMAGIC format or just after the
       header for the other formats.  The N_TXTOFF macro returns this absolute
       file  position  when  given  the name of an exec structure as argument.
       The data segment is contiguous with the text and  immediately  followed
       by  the  text relocation and then the data relocation information.  The
       symbol table follows all this; its position is computed by the N_SYMOFF
       macro.   Finally, the string table immediately follows the symbol table
       at a position which can be gotten easily using N_STROFF.  The  first  4
       bytes  of  the string table are not used for string storage, but rather
       contain the size of the string table; this size INCLUDES the  4  bytes,
       the minimum string table size is thus 4.

       The  layout  of a symbol table entry and the principal flag values that
       distinguish symbol types are given in the include file as follows:

        * Format of a symbol table entry.
       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, i.e. N_TEXT etc; see below */
                char          n_other;
                short         n_desc;  /* see <stab.h> */
                unsigned      n_value; /* value of this symbol (or offset) */
       #define  n_hash        n_desc   /* used internally by ld */

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

        * Other permanent symbol table entries have some of the N_STAB bits set.
        * These are given in <stab.h>
       #define  N_STAB        0xe0     /* if any of these bits set, don't discard */

        * Format for namelist values.
       #define  N_FORMAT      "%08x"

       In the a.out file a symbol's n_un.n_strx field gives an index into  the
       string table.  A n_strx value of 0 indicates that no name is associated
       with a particular symbol table 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 appropriate data from the string table.

       If a symbol's type is undefined external, and the value field  is  non-
       zero,  the symbol is interpreted by the loader ld as the name of a com-
       mon region whose size is indicated by the value of the symbol.

       The value of a byte in the text or data which is not  a  portion  of  a
       reference  to  an undefined external symbol is exactly that value which
       will appear in memory when the file is executed.  If a byte in the text
       or  data involves a reference to an undefined external symbol, as indi-
       cated by the relocation information, then the value stored in the  file
       is  an  offset  from  the associated external symbol.  When the file is
       processed by the link editor and the external symbol  becomes  defined,
       the value of the symbol will be added to the bytes in the file.

       If  relocation  information  is  present, it amounts to eight bytes per
       relocatable datum as in the following structure:

        * Format of a relocation datum.
       struct relocation_info {
                int       r_address;       /* address which is relocated */
                unsigned  r_symbolnum:24,  /* local symbol ordinal */
                          r_pcrel:1,       /* was relocated pc relative already */
                          r_length:2,      /* 0=byte, 1=word, 2=long */
                          r_extern:1,      /* does not include value of sym referenced */
                          :4;              /* nothing, yet */

       There  is  no  relocation  information  if  a_trsize+a_drsize==0.    If
       r_extern is 0, then r_symbolnum is actually a n_type for the relocation
       (i.e. N_TEXT meaning relative to segment text origin.)

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

       Not having the size of the string table in the header is  a  loss,  but
       expanding  the  header  size  would have meant stripped executable file
       incompatibility, and we couldn't hack this just now.

4th Berkeley Distribution      25 February 1983                       A.OUT(5)