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A.OUT(5)		      1986			 A.OUT(5)



N\bNA\bAM\bME\bE
     a.out - assembler and link editor output

S\bSY\bYN\bNO\bOP\bPS\bSI\bIS\bS
     #\b#i\bin\bnc\bcl\blu\bud\bde\be <\b<a\ba.\b.o\bou\but\bt.\b.h\bh>\b>

D\bDE\bES\bSC\bCR\bRI\bIP\bPT\bTI\bIO\bON\bN
     _\bA._\bo_\bu_\bt is the output file of the assembler _\ba_\bs(1) and the link
     editor _\bl_\bd(1).  Both programs make _\ba._\bo_\bu_\bt executable if there
     were no errors and no unresolved external references.  Lay-
     out 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, relocation 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 _\bl_\bd or if the sym-
     bols and relocation have been removed by _\bs_\bt_\br_\bi_\bp(1).





Printed 7/27/90                May				1






A.OUT(5)		      1986			 A.OUT(5)



     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 _\ba._\bo_\bu_\bt file is executed, three logical segments are
     set up: the text segment, the data segment (with uninitial-
     ized data, which starts off as all 0, following initial-
     ized), 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
     segment 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 execut-
     ing 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 _\ba._\bo_\bu_\bt 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 formats.  This is especially suitable for very
     large programs and is the default format produced by _\bl_\bd(1).

     The stack will occupy the highest possible locations in the
     core image, growing downwards from USRSTACK (from
     <_\bm_\ba_\bc_\bh_\bi_\bn_\be/_\bv_\bm_\bp_\ba_\br_\ba_\bm._\bh>).  The stack is automatically extended
     as required.  The data segment is only extended as requested
     by _\bb_\br_\bk(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 argu-
     ment.  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



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A.OUT(5)		      1986			 A.OUT(5)



     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 _\ba._\bo_\bu_\bt 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
     _\bl_\bd as the name of a common region whose size is indicated by



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A.OUT(5)		      1986			 A.OUT(5)



     the value of the symbol.

     The value of a byte in the text or data which is not a por-
     tion 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 indicated 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.)

S\bSE\bEE\bE A\bAL\bLS\bSO\bO
     adb(1), as(1), ld(1), nm(1), dbx(1), stab(5), strip(1)

B\bBU\bUG\bGS\bS
     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.














Printed 7/27/90                May				4