docs/PDB/index.rst

=====================================
The PDB File Format
=====================================

.. contents::
   :local:

.. _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>`__.

.. _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
   HashStream
   CodeViewSymbols
   CodeViewTypes

.. _msf:

The MSF Container
-----------------
A PDB file is really just a special case of an MSF (Multi-Stream Format) file.
An MSF file is actually a miniature "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 file (aka fragmented).  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:

+--------------------+------------------------------+-------------------------------------------+
| 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                 |
|                    |                              | - FPO / PGO Data                          |
+--------------------+------------------------------+-------------------------------------------+
| IPI Stream         | - Fixed Stream Index 4       | - CodeView Type Records                   |
|                    |                              | - Index of IPI Hash Stream                |
+--------------------+------------------------------+-------------------------------------------+
| /LinkInfo          | - Contained in PDB Stream    | - Unknown                                 |
|                    |   Named Stream map           |                                           |
+--------------------+------------------------------+-------------------------------------------+
| /src/headerblock   | - Contained in PDB Stream    | - Unknown                                 |
|                    |   Named Stream map           |                                           |
+--------------------+------------------------------+-------------------------------------------+
| /names             | - Contained in PDB Stream    | - PDB-wide global string table used for   |
|                    |   Named Stream map           |   string de-duplication                   |
+--------------------+------------------------------+-------------------------------------------+
| Module Info Stream | - Contained in DBI Stream    | - CodeView Symbol Records for this module |
|                    | - One for each compiland     | - Line Number Information                 |
+--------------------+------------------------------+-------------------------------------------+
| Public Stream      | - Contained in DBI Stream    | - Public (Exported) Symbol Records        |
|                    |                              | - Index of Public Hash Stream             |
+--------------------+------------------------------+-------------------------------------------+
| Global Stream      | - Contained in DBI Stream    | - Global Symbol Records                   |
|                    |                              | - 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:`HashStream`
   Information about the Hash Table stream, and how it can be used to quickly look up records
   by name.

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.