NAME

a.out − assembler and link editor output

SYNOPSIS

#include <a.out.h>

DESCRIPTION

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 is:

/*

* Header prepended to each a.out file.

*/
struct exec {
longa_magic;/* magic number */
unsigneda_text;/* size of text segment */
unsigneda_data;/* size of initialized data */
unsigneda_bss;/* size of uninitialized data */
unsigneda_syms;/* size of symbol table */
unsigneda_trsize;/* size of text relocation */
unsigneda_drsize;/* size of data relocation */
unsigneda_entry;/* entry point */
};
 #defineOMAGIC0407/* old impure format */
#defineNMAGIC0410/* read-only text */
 /*

* Macros which take exec structures as arguments and tell whether

* the file has a reasonable magic number or offsets to

* text|symbols|strings.

*/
#defineN_BADMAG(x) \
(((x).a_magic)!=OMAGIC && ((x).a_magic)!=NMAGIC )
 #defineN_TXTOFF(x) \
(sizeof (struct exec))
#define N_SYMOFF(x) \
(N_TXTOFF(x) + (x).a_text+(x).a_data \
+ (x).a_trsize+(x).a_drsize)
#defineN_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 ld or if the symbols and relocation have been removed by strip(1).

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 0x1000 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) the data segment begins at the first 4096 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.  This is the default format produced by ld.

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 immediately following the header, as given by the N_TXTOFF macro.  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 */
longn_strx;/* index into file string table */
} n_un;
unsigned charn_type;/* type flag, i.e. N_TEXT etc */
charn_other;
shortn_desc;/* see <stab.h> */
unsignedn_value;/* value of this symbol */
};
#definen_hashn_desc/* used internally by ld */
 /*

* Simple values for n_type.

*/
#defineN_UNDF0x0/* undefined */
#defineN_ABS0x2/* absolute */
#defineN_TEXT0x4/* text */
#defineN_DATA0x6/* data */
#defineN_BSS0x8/* bss */
#defineN_COMM0x12/* common (internal to ld) */
#defineN_FN0x1f/* file name symbol */
 #defineN_EXT01/* external bit, or’ed in */
#defineN_TYPE0x1e/* 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>

*/
#defineN_STAB0xe0/* if any of these set, keep */
 /*

* Format for namelist values.

*/
#defineN_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 common 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 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 {
intr_address;/* address which is relocated */
unsignedr_symbolnum:24,/* local symbol ordinal */
r_pcrel:1, /* was relocated pc relative */
r_length:2,/* 0=byte, 1=word, 2=long */
r_extern:1,/* doesn’t include sym’s value */
: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.) 

SEE ALSO

as(1), ld(1), strip(1), nm(1), adb(1), dbx(1), stab(4)

Version 2.3  —  July 04, 1985