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Object File Format (HUNK_UNIT)

Overview

Compiler output (.o files) uses HUNK_UNIT format — a variation of the HUNK executable format for relocatable, unlinked code. Multiple object files are combined by the linker to produce a final executable.

Object File vs Executable

Feature Object file Executable
Magic word HUNK_UNIT ($3E7) HUNK_HEADER ($3F3)
Unit name Yes (HUNK_NAME) No
External refs HUNK_EXT (imports+exports) None (resolved by linker)
Relocation HUNK_RELOC32 (partial) HUNK_RELOC32 (complete)
BSS HUNK_BSS HUNK_BSS
Loaded directly No Yes

Object File Structure

HUNK_UNIT ($000003E7)     — identifies this as an object file
HUNK_NAME ($000003E8)     — optional: source/module name
  <num_longs>
  <name string>

--- For each code/data/bss section: ---

HUNK_CODE / HUNK_DATA / HUNK_BSS
  <data>

HUNK_RELOC32              — intra-object relocations (if any)

HUNK_EXT                  — external symbol definitions + references
  EXT_DEF  _myfunc  = offset X    (export)
  EXT_REF32  _printf  [offsets]   (import)

HUNK_SYMBOL               — optional local symbols

HUNK_END

--- Repeat for additional sections ---

Multi-Section Object Files

A single .o file can contain multiple code/data sections. Each section is a separate hunk terminated by HUNK_END:

HUNK_UNIT
HUNK_NAME  "mymodule.c"
HUNK_CODE              ; section 0: main code
  [code...]
HUNK_EXT               ; exports/imports for section 0
HUNK_END
HUNK_CODE              ; section 1: __initdata (constructor table)
  [init code...]
HUNK_END
HUNK_DATA              ; section 2: initialized data
  [data...]
HUNK_RELOC32           ; internal relocs for section 2
HUNK_END
HUNK_BSS               ; section 3: uninitialized data
HUNK_END

Compiler Section Naming Convention

Different compilers use different conventions for code/data section organization:

SAS/C 6.x

Section Type Contents
.text HUNK_CODE Compiled functions
.data HUNK_DATA Initialized globals
.bss HUNK_BSS Uninitialized globals
__CSEG HUNK_CODE (alternate) code
__DSEG HUNK_DATA (alternate) data

SAS/C 6.x uses HUNK_NAME to embed section names (matching HUNK_NAME format).

GCC (m68k-amigaos)

GCC emits more sections:

.text           — code
.data           — initialized data
.bss            — BSS
.rodata         — read-only data (string literals, const)
.ctors          — C++ constructor table
.dtors          — C++ destructor table

VBCC follows a similar scheme to GCC.

Library Archives (.lib)

A .lib file is an archive of object files, using HUNK_LIB ($3FA) and HUNK_INDEX ($3FB):

HUNK_LIB ($000003FA)
  <total_size>        size of all following library data in longs
  [HUNK_UNIT blocks for each member ...]

HUNK_INDEX ($000003FB)
  <size_in_longs>     size of index data
  [index entries...]

The index maps symbol names to their containing HUNK_UNIT within the library, allowing the linker to extract only the needed object files.

Libraries shipped with SAS/C, GCC, and VBCC use this format.

Linker Operation (Overview)

The linker (slink, blink, ld) processes object files:

  1. Collect all HUNK_EXT exports from every .o into a global symbol table
  2. Resolve HUNK_EXT imports — for each EXT_REF32, find the defining object
  3. Pull in library members — if an imported symbol lives in a .lib, add that object file
  4. Merge sections — combine all .text hunks into one code hunk, all .data into one data hunk, etc.
  5. Emit HUNK_RELOC32 — for each resolved external reference, emit a relocation entry
  6. Write HUNK_HEADER — calculate final hunk sizes and write executable header

Inspecting Object Files

hexdump

xxd myfile.o | head -40   # look for $000003E7 (HUNK_UNIT) at start

hunkinfo (community tool)

hunkinfo myfile.o   # lists all hunks, sizes, symbols, externals

IDA Pro

IDA can load .o files directly using the Amiga HUNK loader plugin — useful for reversing library object files without full executable context.

References

  • NDK39: dos/doshunks.h
  • SAS/C 6.x Programmer's Guide — object file chapter
  • VBCC documentation — object file format
  • vlink linker manual (covers HUNK_LIB/HUNK_INDEX): http://sun.hasenbraten.de/vlink/
  • GCC m68k-amigaos port documentation