DWARF
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DWARF is a widely used, standardized debugging data format. DWARF was originally designed along with Executable and Linkable Format (ELF), although it is independent of object file formats.[1] The name is a medieval fantasy complement to "ELF" that had no official meaning, although the name "Debugging With Arbitrary Record Formats" has since been proposed as a backronym.[1]
DWARF originated with the C compiler and sdb debugger in Unix System V Release 4 (SVR4).[1]
History
[edit]The first version of DWARF proved to use excessive amounts of storage, and an incompatible successor, DWARF-2, superseded it and added various encoding schemes to reduce data size. DWARF did not immediately gain universal acceptance; for instance, when Sun Microsystems adopted ELF as part of their move to Solaris, they opted to continue using stabs, in an embedding known as "stabs-in-elf". Linux followed suit, and DWARF-2 did not become the default until the late 1990s.
The DWARF Workgroup of the Free Standards Group released DWARF version 3 in January 2006,[2] adding (among other things) support for C++ namespaces, Fortran 90 allocatable data and additional compiler optimization techniques.
The DWARF committee published version 4 of DWARF, which offers "improved data compression, better description of optimized code, and support for new language features in C++", in 2010.[3]
Version 5 of the DWARF format was published in February 2017.[4][5] It "incorporates improvements in many areas: better data compression, separation of debugging data from executable files, improved description of macros and source files, faster searching for symbols, improved debugging of optimized code, as well as numerous improvements in functionality and performance."
Structure
[edit]DWARF uses a data structure called a Debugging Information Entry (DIE) to represent each variable, type, procedure, etc. A DIE has a tag (e.g., DW_TAG_variable, DW_TAG_pointer_type, DW_TAG_subprogram) and attributes (key-value pairs). A DIE can have nested (child) DIEs, forming a tree structure. A DIE attribute can refer to another DIE anywhere in the tree—for instance, a DIE representing a variable would have a DW_AT_type entry pointing to the DIE describing the variable's type.
To save space, two large tables needed by symbolic debuggers are represented as byte-coded instructions for simple, special-purpose finite-state machines. The Line Number Table, which maps code locations to source code locations and vice versa, also specifies which instructions are part of function prologues and epilogues. The Call Frame Information table allows debuggers to locate frames on the call stack.
DWARF has been divided into different sections such as .debug_info,[6] .debug_frame, etc.
| Section Name | Description |
|---|---|
| .debug_types | Contains the non-trivial types. Stored as a hash structure so that duplicate type definition can be removed easily. |
| .debug_str | Contains strings that are not placed in the .debug_info and .debug_types sections. The linker merges the string tables to eliminate duplicates. |
| .debug_aranges | Contain range lists to define what pieces of a program's text belong to which subprograms and compilation units. |
| .debug_loc | Contain lists of expressions that describe to the debugger the location of a variable based on the PC value. |
| .debug_line | Contain a description of the mapping from PC values to source locations. |
| .debug_abbrev | Provide the definitions for abbreviation codes used in describing the debug info in the .debug_info and .debug_types sections. |
| .debug_pubnames | Is an index to help fetching of symbol debug information without having to scan object files. |
| .debug_pubtypes | Is an index to help fetching of type information. |
.debug_info
[edit]The .debug_info section of an ELF contains information generated by compilers to describe the source code while debugging by keeping symbols and its type, scope, file, line number, etc. The .debug_info[7] section is one of the main components of DWARF[8][9] debug info files.[10] This is generated by a compiler when -g switch or its variants are used.
Tools
[edit]Libdwarf is a library that provides access to the DWARF debugging information in executable files and object files.[11]
Further reading
[edit]Michael Eager, chair of the DWARF Standards Committee, has written an introduction to debugging formats and DWARF 3, Introduction to the DWARF Debugging Format.[1]
References
[edit]- ^ a b c d Michael J. Eager (April 2012). "Introduction to the DWARF Debugging Format" (PDF). Retrieved 2015-01-08.
- ^ "DWARF Version 3 Standard Released" (Press release). Free Standards Group. January 4, 2006. Archived from the original on 2011-07-25. Retrieved 2007-06-25.
- ^ "DWARF Version 4 Released". The DWARF committee. June 16, 2010. Archived from the original on 2020-07-30. Retrieved 2010-06-24.
- ^ "DWARF Version 5 Standard Released". The DWARF committee. February 15, 2017. Retrieved 2017-08-07.
- ^ "DWARF 5 Standard". The DWARF committee. February 15, 2017. Retrieved 2017-08-07.
- ^ "Exploring the DWARF debug format information".
- ^ "Debugging Options (Using the GNU Compiler Collection (GCC))". gcc.gnu.org.
- ^ "Dwarf Extensions". gcc.gnu.org.
- ^ You, Ilsun; Youn, Taek-Young (2023-02-03). Information Security Applications: 23rd International Conference, WISA 2022, Jeju Island, South Korea, August 24–26, 2022, Revised Selected Papers. Springer Nature. ISBN 978-3-031-25659-2.
- ^ "ELF section retrieval with debuginfod". www.redhat.com. Retrieved 2023-09-25.
- ^ "libdwarf: A Consumer Library Interface to DWARF". www.prevanders.net. Retrieved 2023-12-06.
External links
[edit]- Official website
- Libdwarf, a C library intended to simplify reading (and writing) applications using DWARF2, DWARF3.
- elfutils, another C library for ELF/DWARF file processing.
- How debuggers work: Part 3 - Debugging information
- Debugging formats DWARF and STAB