REL file format: header

Introduction

The header starts at offset 0 and every number is a big-endian 32-bit value except the four one-byte section indexes. Later versions add fields at the end, so the header is 0x40 bytes in version 1, 0x48 in version 2 and 0x4C in version 3. The layout follows the Mario Kart Wii wiki “REL (File Format)”, read through a page summary (our words). The page gives the three end offsets and the “version 2 and later” and “version 3 only” notes; the mapping of end offset to version is read from those notes.

Fields

Offset	Size	Field	Meaning
`0x00`	4	Module id	Must be nonzero and unique among a game’s modules. 0 names the main program in the import table.
`0x04`	4	Next	Filled in by the loader at run time (the link of the module list).
`0x08`	4	Previous	Filled in by the loader at run time.
`0x0C`	4	Section count	The number of entries in the section table.
`0x10`	4	Section table offset	Where the section table is.
`0x14`	4	Name offset	The offset of the module’s path string, relative to a separate name file (the `.str` file), not to this one.
`0x18`	4	Name size	The length of that path.
`0x1C`	4	Version	The format version: 1, 2 or 3.
`0x20`	4	BSS size	The size of the zero-filled area.
`0x24`	4	Relocation offset	Where the first relocation list is; the same as the first import entry’s offset.
`0x28`	4	Import table offset	Where the import table is.
`0x2C`	4	Import table size	Its size in bytes (8 per entry).
`0x30`	1	Prolog section	The index of the section holding `_prolog`; 0 means skip.
`0x31`	1	Epilog section	The section holding `_epilog`; 0 means skip.
`0x32`	1	Unresolved section	The section holding `_unresolved`; 0 means skip.
`0x33`	1	BSS section	Filled in at run time.
`0x34`	4	Prolog offset	The offset of `_prolog` inside its section.
`0x38`	4	Epilog offset	The offset of `_epilog` inside its section.
`0x3C`	4	Unresolved offset	The offset of `_unresolved` inside its section.
`0x40`	4	Alignment	Version 2 and later: the alignment of the load address. The page says “as a power of 2”.
`0x44`	4	BSS alignment	Version 2 and later: the alignment of the BSS.
`0x48`	4	Fixed size	Version 3 only: space usable for other purposes when the module is linked with `OSLinkFixed`.

The loader calls the three entry points by name: _prolog when the module is linked, _epilog when it is unlinked, and _unresolved when the module calls a function that was not resolved.

What the viewer checks

These rules are the viewer’s own test for “is this a REL”. A file named .rel that breaks one is shown as not a valid REL.

The version is 1, 2 or 3, and the file holds that version’s header.
The module id is not 0, and the section count is from 1 to 255.
The section table lies after the header, inside the file; its first entry is empty (the page says so); every section with an offset lies inside the file; no two overlap.
The import table size is a multiple of 8 and the table is inside the file; the relocation offset is inside the file.
The prolog, epilog and unresolved sections, when not 0, exist, are code, and their offsets are inside them.
In version 2 and later the alignments are 0 or a power of two.
Every import points inside the file, and its relocation list ends with the end marker (type 203) before the end of the file.

Not checked: that the next and previous links are zero (they are run-time fields, and a file saved from memory may hold values).

Not described

Known, not yet covered

Reference: whether the alignment is a byte count or an exponent (the page says “as a power of 2”; the viewer accepts a power-of-two byte count and 0), the name file’s layout, and how OSLinkFixed uses the fixed size.
Viewer: only built modules were checked.

Unknown

Whether any module has a section count above 255 or a version other than 1 to 3: none is accepted.

REL File Format – Header