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What is DWARF Introduction to the DWARF Format 2012.05.03.

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Presentation on theme: "What is DWARF Introduction to the DWARF Format 2012.05.03."— Presentation transcript:

1 What is DWARF Introduction to the DWARF Format

2 Reference Introduction to the DWARF Debugging Format, Michael J. Eager, Eager Consulting DWARF Debugging Information Format, Version 4, DWARF Debugging Information Format Committee; Gnu gold linker, GNU Binutils Gnu BFD, GNU Binutils

3 Introduction DWARF is a format for debugging. DWARF data are several sections in the object file The purpose is to figure out the mapping between object code and source code. DWARF is a tree structure with nodes called Debugging Information Entry (DIE). The tree describes the whole source program readelf --dump-debug to dump the debug data

4 Debug Sections NamesFunctions.debug_abbrevAbbreviations used in the.debug_info section.debug_infoThe core DWARF data containing DIEs.debug_frameCall Frame Information  similar to eh_frame.debug_lineLine Number Program.debug_macinfoMacro descriptions.debug_arangesA mapping between memory address and compilation.debug_locA lookup table for global objects and functions.debug_pubnamesA lookup table for global objects and functions.debug_pubtypesA lookup table for global types.debug_rangesAddress ranges referenced by DIEs.debug_strString table used by.debug_info

5 DIE DIEs are used to represent types, variables, or functions. DIE contains a Tag and a set of Attributes – TAG Specify what this DIE describes DW_TAG_XXX : DW_TAG_base_type, DW_TAG_variable,... – Attribute A detail description such as name, size, address,… DW_AT_XXX: DW_AT_name, DW_AT_byte_size,…

6 Base Types and Variables Describes 32-bit and 16-bit int base type DW_TAG_base_type DW_AT_name = int DW_AT_byte_size = 4 DW_AT_encoding = signed DW_TAG_base_type DW_AT_name = int DW_AT_byte_size = 2 DW_AT_encoding = signed : DW_TAG_variable DW_AT_name = px DW_AT_type = : DW_TAG_pointer_type DW_AT_byte_size = 4 DW_AT_type = : DW_TAG_base_type DW_AT_name = word DW_AT_byte_size = 4 DW_AT_encoding = signed Describes int *px

7 Describe Executable Code Compilation Unit DIE – Each separately compiled source file is a compilation unit – Each compilation unit has a DIE to describe its name, directory, address, … – The parent of all other DIEs for this compilation unit Functions – Subprograms DIE Describes name, low/high pc Return value : DW_AT_TYPE – Input parameter DIE Owned by subprogram Described parameter name, type, …

8 Executable Code, an Example 1 DW_TAG_compile_unit [has children] DW_AT_producer DW_FORM_strp DW_AT_language DW_FORM_data1 DW_AT_name DW_FORM_strp DW_AT_comp_dir DW_FORM_strp DW_AT_low_pc DW_FORM_addr DW_AT_high_pc DW_FORM_addr DW_AT_stmt_list DW_FORM_data4 Compilation Unit DIE : Abbrev Number: 1 (DW_TAG_compile_unit) DW_AT_producer : (indirect string, offset: 0x0): GNU C DW_AT_language : 1 (ANSI C) DW_AT_name : (indirect string, offset: 0xc): test.c DW_AT_comp_dir : (indirect string, offset: 0x13): /proj/mtk02422/test/test2 DW_AT_low_pc : 0x0 DW_AT_high_pc : 0x30 DW_AT_stmt_list : 0x0. debug_abbrev. debug_info int foo(int i) { return i++; } int foo(int i) { return i++; } Describe the data types Describe the value

9 Executable Code, an Example Subprograms DIE : Abbrev Number: 2 (DW_TAG_subprogram) DW_AT_external : 1 DW_AT_name : foo DW_AT_decl_file : 1 DW_AT_decl_line : 1 DW_AT_prototyped : 1 DW_AT_type : DW_AT_low_pc : 0x0 DW_AT_high_pc : 0x30 DW_AT_frame_base : 0x0 (location list) DW_AT_sibling : … : Abbrev Number: 4 (DW_TAG_base_type) DW_AT_byte_size : 4 DW_AT_encoding : 5 (signed) DW_AT_name : int. debug_info

10 Other DWARF Data Line Number Table –.debug_line – Mapping between the source lines and the memory address – Line number program The byte-coded instruction stream – State machine Expand the line number program and recreate the line number table Macro Information –.debug_macinfo Call Frame Information –.debug_frame – Virtually unwind and find out the desired data in the call stack – CIE (Common Information Entry) – FDE (Frame Description Entry)

11 What linkers should do? Related options – --strip-debug Strip the debugging information – --strip-debug-non-line Emit only debug line number information (gold only) – --strip-debug-gdb Strip debug symbols that are unused by gdb (at least version 6.7) (gold only) Section merge, applying relocation (Normal passes) Post processing – Compress the sections – Reduce the debug output data.debug_abbrev.debug_info DWARF reader – For error message

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