HUNK_EXT — Exports and Imports

Overview

HUNK_EXT ($000003EF) is the external symbol record used in object files (HUNK_UNIT format). It carries both exported symbols (definitions visible to the linker) and imported symbols (references to symbols defined in other object files or libraries).

HUNK_EXT does not appear in final executables — the linker resolves all externals during the link step and emits HUNK_RELOC32 instead.

Record Format

HUNK_EXT ($000003EF)

[Repeat until terminator:]
  <type_and_namelen>    Longword: bits[31:24] = EXT type, bits[23:0] = name length in longs
  <name...>             Symbol name, padded to longword boundary
  [type-specific data]

<0x00000000>            Terminator (name length = 0)
HUNK_END

EXT Type Codes

Value	Name	Direction	Description
0	`EXT_SYMB`	export	Absolute symbol (no relocation)
1	`EXT_DEF`	export	Defined symbol at offset in current hunk
2	`EXT_ABS`	export	Absolute value symbol
3	`EXT_RES`	export	Resident library symbol
129	`EXT_REF32`	import	32-bit reference (absolute)
130	`EXT_COMMON`	import/def	Common BSS block
131	`EXT_REF16`	import	16-bit reference
132	`EXT_REF8`	import	8-bit reference
133	`EXT_DEXT32`	import	32-bit data-relative reference
134	`EXT_DEXT16`	import	16-bit data-relative reference
135	`EXT_DEXT8`	import	8-bit data-relative reference

Export Records

EXT_DEF — Defined Symbol

The most common export — a function or data label at a fixed offset within the current hunk:

bits[31:24] = 0x01        EXT_DEF
bits[23:0]  = name_longs  name length in longwords
<name...>                 symbol name (padded)
<offset>                  offset within current hunk (longword)

Example: exporting _init at offset 0x10 in the code hunk:

$01000005   ; EXT_DEF, name = 5 longs (20 chars)
"_ini"
"t   "
"    "      ; (padded to 5 longs)
$00000010   ; offset $10

EXT_ABS — Absolute Symbol

Symbol has an absolute value (not relative to any hunk):

bits[31:24] = 0x02
<name...>
<absolute_value>

Import Records

EXT_REF32 — 32-bit Reference

Used when the current object references an external symbol. The linker patches these after symbol resolution.

bits[31:24] = 0x81        EXT_REF32
bits[23:0]  = name_longs
<name...>                 symbol name being referenced
<num_refs>                number of reference sites in current hunk
<ref_offset_0>            byte offset within current hunk
<ref_offset_1>
...

Example: _start calls _DOSBase (external):

$81000004   ; EXT_REF32, name = 4 longs
"_DOS"
"Base"
$00000003   ; 3 reference sites
$0000001C   ; offset $1C
$00000034   ; offset $34
$00000048   ; offset $48

EXT_COMMON — Common Block (BSS)

Uninitialized data shared across multiple object files (like C extern int x;). The linker allocates one block, all references point to it:

bits[31:24] = 0x82        EXT_COMMON
<name...>
<size>                    size in bytes (the common block size)
<num_refs>                reference sites
<offsets...>

Worked Binary Example

Object file exporting _foo (at offset 0) and importing _puts:

Offset  Hex                         Meaning
$00:    00 00 03 E7                 HUNK_UNIT
$04:    00 00 00 01                 name length = 1 long
$08:    "foo\0"                     unit name = "foo"
$0C:    00 00 03 E9                 HUNK_CODE
$10:    00 00 00 08                 8 longwords = 32 bytes of code
$14:    [32 bytes of code...]
$34:    00 00 03 EF                 HUNK_EXT
$38:    01 00 00 01                 EXT_DEF, name = 1 long
$3C:    "_foo"                      symbol name "_foo"
$40:    00 00 00 00                 at offset 0 in code hunk
$44:    81 00 00 01                 EXT_REF32, name = 1 long
$48:    "_put"  (truncated)
$4C:    "s\0\0\0"
$50:    00 00 00 01                 1 reference
$54:    00 00 00 08                 at offset $08 in code
$58:    00 00 00 00                 HUNK_EXT terminator
$5C:    00 00 03 F2                 HUNK_END

Linker Resolution

When the linker processes multiple object files:

Build a global symbol table from all EXT_DEF records
For each EXT_REF32, find the defining object and record the target hunk + offset
Emit HUNK_RELOC32 in the output executable to patch the reference sites at load time
EXT_COMMON blocks are allocated once; all references redirected to that allocation

References

NDK39: dos/doshunks.h — EXT_ type constants
vlink documentation: http://sun.hasenbraten.de/vlink/release/vlink.pdf
ADE (Amiga Developer Environment) linker source code
SAS/C 6.x reference manual — object file format appendix