llvm/docs/PDB/index.md

# The PDB File Format

(pdb-intro)=

## Introduction

PDB (Program Database) is a file format invented by Microsoft and which contains
debug information that can be consumed by debuggers and other tools. Since
officially supported APIs exist on Windows for querying debug information from
PDBs even without the user understanding the internals of the file format, a
large ecosystem of tools has been built for Windows to consume this format. In
order for Clang to be able to generate programs that can interoperate with these
tools, it is necessary for us to generate PDB files ourselves.

At the same time, LLVM has a long history of being able to cross-compile from
any platform to any platform, and we wish for the same to be true here. So it
is necessary for us to understand the PDB file format at the byte-level so that
we can generate PDB files entirely on our own.

This manual describes what we know about the PDB file format today. The layout
of the file, the various streams contained within, the format of individual
records within, and more.

We would like to extend our heartfelt gratitude to Microsoft, without whom we
would not be where we are today. Much of the knowledge contained within this
manual was learned through reading code published by Microsoft on their [GitHub
repo](https://github.com/Microsoft/microsoft-pdb).

For more documentation, see Microsoft's [pdb-rs GitHub repo](https://github.com/microsoft/pdb-rs/blob/main/docs/index.md).

(pdb-layout)=

## File Layout

:::{important}
Unless otherwise specified, all numeric values are encoded in little endian.
If you see a type such as `uint16_t` or `uint64_t` going forward, always
assume it is little endian!
:::

```{toctree}
:hidden:

MsfFile
PdbStream
TpiStream
DbiStream
ModiStream
PublicStream
GlobalStream
HashTable
CodeViewSymbols
CodeViewTypes
```

(msf)=

### The MSF Container

A PDB file is an MSF (Multi-Stream Format) file. An MSF file is a "file system
within a file". It contains multiple streams (aka files) which can represent
arbitrary data, and these streams are divided into blocks which may not
necessarily be contiguously laid out within the MSF container file.
Additionally, the MSF contains a stream directory (aka MFT) which describes how
the streams (files) are laid out within the MSF.

For more information about the MSF container format, stream directory, and
block layout, see {doc}`MsfFile`.

(streams)=

### Streams

The PDB format contains a number of streams which describe various information
such as the types, symbols, source files, and compilands (e.g. object files)
of a program, as well as some additional streams containing hash tables that are
used by debuggers and other tools to provide fast lookup of records and types
by name, and various other information about how the program was compiled such
as the specific toolchain used, and more. A summary of streams contained in a
PDB file is as follows:

:::{list-table}
:header-rows: 1

* - Name
  - Stream Index
  - Contents
* - Old Directory
  - Fixed Stream Index 0
  - Previous MSF Stream Directory
* - PDB Stream
  - Fixed Stream Index 1
  - Basic File Information

    Fields to match EXE to this PDB

    Map of named streams to stream indices
* - TPI Stream
  - Fixed Stream Index 2
  - CodeView Type Records

    Index of TPI Hash Stream
* - DBI Stream
  - Fixed Stream Index 3
  - Module/Compiland Information

    Indices of individual module streams

    Indices of public / global streams

    Section Contribution Information

    Source File Information

    References to streams containing FPO / PGO Data
* - IPI Stream
  - Fixed Stream Index 4
  - CodeView Type Records

    Index of IPI Hash Stream
* - /LinkInfo
  - Contained in PDB Stream Named Stream map
  - Unknown
* - /src/headerblock
  - Contained in PDB Stream Named Stream map
  - Summary of embedded source file content (e.g. natvis files)
* - /names
  - Contained in PDB Stream Named Stream map
  - PDB-wide global string table used for string de-duplication
* - Module Info Stream
  - Contained in DBI Stream

    One for each compiland
  - CodeView Symbol Records for this module

    Line Number Information
* - Public Stream
  - Contained in DBI Stream
  - Public (Exported) Symbol Records

    Index of Public Hash Stream
* - Global Stream
  - Contained in DBI Stream
  - Single combined symbol-table

    Index of Global Hash Stream
* - TPI Hash Stream
  - Contained in TPI Stream
  - Hash table for looking up TPI records by name
* - IPI Hash Stream
  - Contained in IPI Stream
  - Hash table for looking up IPI records by name
:::

More information about the structure of each of these can be found on the
following pages:

{doc}`PdbStream`
: Information about the PDB Info Stream and how it is used to match PDBs to EXEs.

{doc}`TpiStream`
: Information about the TPI stream and the CodeView records contained within.

{doc}`DbiStream`
: Information about the DBI stream and relevant substreams including the
  Module Substreams, source file information, and CodeView symbol records
  contained within.

{doc}`ModiStream`
: Information about the Module Information Stream, of which there is one for
  each compilation unit and the format of symbols contained within.

{doc}`PublicStream`
: Information about the Public Symbol Stream.

{doc}`GlobalStream`
: Information about the Global Symbol Stream.

{doc}`HashTable`
: Information about the serialized hash table format used internally to
  represent things such as the Named Stream Map and the Hash Adjusters in the
  {doc}`TPI/IPI Stream <TpiStream>`.

## CodeView

CodeView is another format which comes into the picture. While MSF defines
the structure of the overall file, and PDB defines the set of streams that
appear within the MSF file and the format of those streams, CodeView defines
the format of **symbol and type records** that appear within specific streams.
Refer to the pages on {doc}`CodeViewSymbols` and {doc}`CodeViewTypes` for
more information about the CodeView format.