LIB file format: short import object

Introduction

An import library tells the linker which functions a DLL exports. Older tools wrote full COFF objects with the import tables already built. Newer ones write a short form instead: a library member with a 20-byte header and two strings, from which the linker builds the rest.

Structure

0import header20 bytes
20symbol name, zero-terminatedvaries
…DLL name, zero-terminatedvaries

A short import object.

The header, IMPORT_OBJECT_HEADER:

Offset	Size	Field	Meaning
0	2	Sig1	0: IMAGE_FILE_MACHINE_UNKNOWN
2	2	Sig2	`0xFFFF`
4	2	Version	The structure version, 0
6	2	Machine	The processor the DLL is for
8	4	TimeDateStamp	When the file was made
12	4	SizeOfData	The size of the strings after the header
16	2	OrdinalOrHint	The ordinal when the name type says so; otherwise a hint for looking up the name
18	2	Flags	Bits 0 and 1: the import type. Bits 2 to 4: the name type. The rest is reserved.

Import type

Value	Constant	Meaning
0	IMPORT_OBJECT_CODE	Executable code
1	IMPORT_OBJECT_DATA	Data
2	IMPORT_OBJECT_CONST	Specified as CONST in the .def file

Name type

Value	Constant	Meaning
0	IMPORT_OBJECT_ORDINAL	Import by ordinal: OrdinalOrHint is the ordinal
1	IMPORT_OBJECT_NAME	The import name is the symbol name exactly
2	IMPORT_OBJECT_NAME_NOPREFIX	The symbol name without its leading `?`, `@` or (on x86) `_`
3	IMPORT_OBJECT_NAME_UNDECORATE	As above, and cut at the first `@`
4	IMPORT_OBJECT_NAME_EXPORTAS	The exported name is a third string after the DLL’s name

Strings

The symbol’s public name comes first and the DLL’s name second, each ending in a zero byte. SizeOfData counts both. With name type 4 a third string gives the name to import.

Variants

Windows COFF library – a Windows static or import library in the ar archive format
Short import object – one member of an import library: a header and two strings
OMF library – a library of OMF object modules from 16-bit compilers

.LIB File Format