Linkers & Loaders
by John R. Levine
Table of Contents 1
Table of Contents
Chapter 0: Front Matter ........................................................ 1
Dedication .............................................................................................. 1
Introduction ............................................................................................ 1
Who is this book for? ......................................................................... 2
Chapter summaries ............................................................................. 3
The project ......................................................................................... 4
Acknowledgements ............................................................................ 5
Contact us ........................................................................................... 6
Chapter 1: Linking and Loading ........................................... 7
What do linkers and loaders do? ............................................................ 7
Address binding: a historical perspective .............................................. 7
Linking vs. loading .............................................................................. 10
Tw o -pass linking .............................................................................. 12
Object code libraries ........................................................................ 15
Relocation and code modification .................................................... 17
Compiler Drivers .................................................................................. 18
Linker command languages ............................................................. 19
Linking: a true-life example ................................................................. 20
Exercises .............................................................................................. 25
Chapter 2: Architectural Issues ........................................... 27
Application Binary Interfaces .............................................................. 27
Memory Addresses .............................................................................. 28
Byte Order and Alignment ............................................................... 28
Address formation ................................................................................ 30
Instruction formats ............................................................................... 31
Procedure Calls and Addressability ..................................................... 32
Procedure calls ................................................................................. 33
2Table of Contents
Data and instruction references ............................................................ 36
IBM 370 ........................................................................................... 37
SPARC ............................................................................................. 40
SPARC V8 ................................................................................... 40
SPARC V9 ................................................................................... 42
Intel x86 ........................................................................................... 43
Paging and Virtual Memory ................................................................. 45
The program address space .............................................................. 48
Mapped files ..................................................................................... 49
Shared libraries and programs ......................................................... 51
Position-independent code ............................................................... 51
Intel 386 Segmentation ........................................................................ 53
Embedded architectures ....................................................................... 55
Address space quirks ........................................................................ 56
Non-uniform memory ...................................................................... 56
Memory alignment ........................................................................... 57
Exercises .............................................................................................. 57
Chapter 3: Object Files ........................................................ 59
What goes into an object file? .............................................................. 59
Designing an object format .............................................................. 60
The null object format: MS-DOS .COM files ...................................... 61
Code sections: Unix a.out files ............................................................. 61
a.out headers .................................................................................... 64
Interactions with virtual memory ..................................................... 65
Relocation: MS-DOS EXE files ........................................................... 72
Symbols and relocation ........................................................................ 74
Relocatable a.out .................................................................................. 75
Relocation entries ............................................................................. 78
Symbols and strings ......................................................................... 80
a.out summary .................................................................................. 82
Unix ELF ............................................................................................. 82
Relocatable files ............................................................................... 85
ELF executable files ......................................................................... 92
ELF summary ................................................................................... 94
Table of Contents 3
IBM 360 object format ......................................................................... 94
ESD records ..................................................................................... 95
TXT records ..................................................................................... 97
RLD records ..................................................................................... 97
END records ..................................................................................... 98
Summary .......................................................................................... 98
Microsoft Portable Executable format ................................................. 99
PE special sections ......................................................................... 105
Running a PE executable ............................................................... 107
PE and COFF ................................................................................. 107
PE summary ................................................................................... 108
Intel/Microsoft OMF files .................................................................. 108
OMF records .................................................................................. 110
Details of an OMF file ................................................................... 111
Summary of OMF .......................................................................... 114
Comparison of object formats ............................................................ 114
Project ................................................................................................ 115
Exercises ............................................................................................ 117
Chapter 4: Storage allocation ............................................ 119
Segments and addresses ..................................................................... 119
Simple storage layout ......................................................................... 120
Multiple segment types ...................................................................... 121
Segment and page alignment ............................................................. 124
Common blocks and other special segments ..................................... 125
Common ......................................................................................... 125
C++ duplicate removal................................................................... 127
Initializers and finalizers ................................................................ 130
IBM pseudo-registers ..................................................................... 131
Special tables ................................................................................. 134
X86 segmented storage allocation ................................................. 134
Linker control scripts ......................................................................... 136
Embedded system storage allocation ................................................. 138
Storage allocation in practice ............................................................. 138
Storage allocation in Unix a.out linkers ......................................... 139
4Table of Contents
Storage allocation in ELF .............................................................. 141
Storage allocation in Windows linkers .......................................... 144
Exercises ............................................................................................ 146
Project ................................................................................................ 147
Chapter 5: Symbol management ....................................... 149
Binding and name resolution ............................................................. 149
Symbol table formats ......................................................................... 150
Module tables ................................................................................. 153
Global symbol table ....................................................................... 154
Symbol resolution .......................................................................... 157
Special symbols ............................................................................. 158
Name mangling .................................................................................. 158
Simple C and Fortran name mangling ........................................... 158
C++ type encoding: types and scopes ............................................ 160
Link-time type checking ................................................................ 163
Weak external and other kinds of symbols ........................................ 164
Maintaining debugging information .................................................. 164
Line number information ............................................................... 164
Symbol and variable information ................................................... 165
Practical issues ............................................................................... 166
Exercises ............................................................................................ 167
Project ................................................................................................ 167
Chapter 6: Libraries ........................................................... 169
Purpose of libraries ............................................................................ 169
Library formats .................................................................................. 169
Using the operating system ............................................................ 169
Unix and Windows Archive files ................................................... 170
Unix archives............................................................................. 170
Extension to 64 bits ........................................................................ 174
Intel OMF libraries ........................................................................ 174
Creating libraries ................................................................................ 176
Searching libraries ............................................................................. 177
