Itanium C++ ABI

Revised March 14, 2017

Introduction

The Itanium C++ ABI is an ABI for C++. As an ABI, it gives precise rules for implementing the language, ensuring that separately-compiled parts of a program can successfully interoperate. Although it was initially developed for the Itanium architecture, it is not platform-specific and can be layered portably on top of an arbitrary C ABI. Accordingly, it is used as the standard C++ ABI for many major operating systems on all major architectures, and is implemented in many major C++ compilers, including GCC and Clang.

Contact

This website may be mirrored in many places, some of which may become stale. The current canonical location is:

http://itanium-cxx-abi.github.io/cxx-abi/

The primary discussion forum for the ABI is the GitHub repository. Please open a new issue for any new topic you want to discuss. Previously, the ABI was discussed on a mailing list, cxx-abi-dev, whose archives are still accessible.

Drafts

Draft C++ ABI for Itanium
Draft psABI exception handling specification
ABI Examples code samples and tests
Sample GCC code:
- typeinfo: class type_info declaration
- cxxabi.h: ABI support type_info classes and prototypes
- vec.cc: ABI vector new and delete helpers
- tinfo.h: runtime header for tinfo.cc & tinfo2.cc
- tinfo.cc: ABI dynamic cast and catch matching routines
- tinfo2.cc: ABI non-class type_info definitions

Proposals and auxiliary information:

Proposals to the base ABI group for COMDAT sections and removing limitations on section indices.
SGI Interface Section proposal.
Vtable layout examples.
WG21 argument destruction writeup (PDF)

Legacy open issues list

Legacy closed issues list

Objectives

Interoperable C++ compilation on Itanium: we want users to be able to build relocatable objects with different compilers and link them together, and if possible even to ship common DSOs. This objective implies agreement on:
- Data representation
- Object file representation
- Library API
ISO Standard C++: highest priority is functionality and performance of standard-compliant code. It should not be sacrificed for the benefit of language extensions or legacy implementations (though considering them as tie-breakers is fine).
Some areas will be easier to agree on than others. Our priorities should be based on achieving as much interoperability as possible if we can't attain perfection. That is, it is better to end up with a few restrictions being required for interoperable code, than to have no interoperability at all. This suggests priorities as follows:
1. Items requiring base ABI changes that might affect other languages, and will therefore become impossible soon. Examples include exception handling / stack unwind, or ELF changes (not extensions).
2. Core features where differences will prevent virtually any C++ object code from porting. Examples include data layout and calling conventions.
3. Limited usage features, where users can achieve portability by avoiding the feature. An example might be multi-threading.
4. Peripheral features, where the requirements on users to achieve portability are clear and easy to implement. An example is non-explicit inlining, where compilers would presumably allow it to just be suppressed.
5. Tool interfaces, which affect how users build code, rather than what they build. An example is the compilation command line.
Mechanisms/methods which allow coexistence of incompatible implementations may be suitable in some cases. For instance, packaging vendor-specific compiler support runtimes in DSOs occupying distinct namespaces might allow multiple such DSOs to be loaded for mixed objects and avoid requiring that all vendors have the same support runtimes.

Action Item Status

# Action Who Status Opened Closed

1 Distribute Sun C++ ABI Mike Ball closed 990603 990930

2 Distribute Sun C++ ABI Rationale Mike Ball closed 990603 990930

3 Distribute Taligent C++ ABI Cary Coutant moot 990603 991101

4 Expedite Itanium RT Arch doc release Cary Coutant closed 990603 990720

5 Set up n-way NDA for eligible members Priti Shrivastav open 990603

6 Organize/summarize object layout issues and alternatives Matt Austern closed 990603 990624

7 Write-up of Vfunc call protocol Christophe de Dinechin closed 990610 990805

8 Write-up of object layout strawman Matt Austern closed 990610 990624

9 Check with c++-core about empty base placement Jason Merrill closed 990610 990618

10 Describe dynamic cast / inaccessible base issue Daveed Vandevoorde closed 990617 990701

11 Summarize ctor/dtor issues Michael Lam closed 990617 990729

12 Describe Intel exception model Priti Shrivastav closed 990624 990818

13 Propose RTTI representation Daveed Vandevoorde closed 990701 990819

14 Open EDG exception stack? Daveed Vandevoorde moot 990715 990930

15 Priority scheme descriptor for C-2 Jim Dehnert closed 990715 990804

16 Covariant return scheme Jason Merrill closed 990715 990729

17 Validate Christophe's B-6 Vtable layout Jim Dehnert closed 990729 990811

18 Check Sun dynamic cast algorithm Michael Lam closed 990805 990812

19 Write up C-3 destructor proposal Jim Dehnert closed 990805 990811

20 Look at implications of discussion on B-1, B-6 Christophe de Dinechin closed 990812 990830

21 Look at alternative implementations of C-2 Jim Dehnert closed 990812 991110

22 Describe HP hash processing for RTTI Christophe de Dinechin moot 990826 990930

23 Complete RTTI proposal Daveed Vandevoorde closed 990826 991006

24 Summarize Vtable issues Jason Merrill closed 990909 990930

25 Update traceback personality API Christophe de Dinechin closed 990923 000105

26 Push COMDAT to base ABI group Jim Dehnert closed 991014 000317

27 Writeup Vtable Changes Jason Merrill closed 991014 991021

28 Write Virtual destructor proposal Christophe de Dinechin closed 991014 000217

29 dynamic_cast pseudocode Jason Merrill closed 991028 000712

30 Mangling proposal Daveed Vandevoorde closed 991028 000118

31 Construction vtable writeup Coleen Phillimore closed 000127 000216

32 Raise sorting issue C-2 with base ABI group Jim Dehnert closed 991202 000717

33 long double in psABI Christophe de Dinechin closed 000210 0003xx

34 incomplete_type_info proposal Christophe de Dinechin closed 000210 000316

35 new array cookie writeup Matt Austern closed 000210 000217

36 Mangling document update Daveed Vandevoorde closed 000210 000217

37 Validate utility of has-public-base Nathan Sidwell closed 000218 000705

#	Action	Who	Status	Opened	Closed
1	Distribute Sun C++ ABI	Mike Ball	closed	990603	990930
2	Distribute Sun C++ ABI Rationale	Mike Ball	closed	990603	990930
3	Distribute Taligent C++ ABI	Cary Coutant	moot	990603	991101
4	Expedite Itanium RT Arch doc release	Cary Coutant	closed	990603	990720
5	Set up n-way NDA for eligible members	Priti Shrivastav	open	990603
6	Organize/summarize object layout issues and alternatives	Matt Austern	closed	990603	990624
7	Write-up of Vfunc call protocol	Christophe de Dinechin	closed	990610	990805
8	Write-up of object layout strawman	Matt Austern	closed	990610	990624
9	Check with c++-core about empty base placement	Jason Merrill	closed	990610	990618
10	Describe dynamic cast / inaccessible base issue	Daveed Vandevoorde	closed	990617	990701
11	Summarize ctor/dtor issues	Michael Lam	closed	990617	990729
12	Describe Intel exception model	Priti Shrivastav	closed	990624	990818
13	Propose RTTI representation	Daveed Vandevoorde	closed	990701	990819
14	Open EDG exception stack?	Daveed Vandevoorde	moot	990715	990930
15	Priority scheme descriptor for C-2	Jim Dehnert	closed	990715	990804
16	Covariant return scheme	Jason Merrill	closed	990715	990729
17	Validate Christophe's B-6 Vtable layout	Jim Dehnert	closed	990729	990811
18	Check Sun dynamic cast algorithm	Michael Lam	closed	990805	990812
19	Write up C-3 destructor proposal	Jim Dehnert	closed	990805	990811
20	Look at implications of discussion on B-1, B-6	Christophe de Dinechin	closed	990812	990830
21	Look at alternative implementations of C-2	Jim Dehnert	closed	990812	991110
22	Describe HP hash processing for RTTI	Christophe de Dinechin	moot	990826	990930
23	Complete RTTI proposal	Daveed Vandevoorde	closed	990826	991006
24	Summarize Vtable issues	Jason Merrill	closed	990909	990930
25	Update traceback personality API	Christophe de Dinechin	closed	990923	000105
26	Push COMDAT to base ABI group	Jim Dehnert	closed	991014	000317
27	Writeup Vtable Changes	Jason Merrill	closed	991014	991021
28	Write Virtual destructor proposal	Christophe de Dinechin	closed	991014	000217
29	dynamic_cast pseudocode	Jason Merrill	closed	991028	000712
30	Mangling proposal	Daveed Vandevoorde	closed	991028	000118
31	Construction vtable writeup	Coleen Phillimore	closed	000127	000216
32	Raise sorting issue C-2 with base ABI group	Jim Dehnert	closed	991202	000717
33	long double in psABI	Christophe de Dinechin	closed	000210	0003xx
34	incomplete_type_info proposal	Christophe de Dinechin	closed	000210	000316
35	new array cookie writeup	Matt Austern	closed	000210	000217
36	Mangling document update	Daveed Vandevoorde	closed	000210	000217
37	Validate utility of has-public-base	Nathan Sidwell	closed	000218	000705

Issue Status

In the following table, the class column attempts to classify the issue on the basis of what it likely affects. The identifiers used are:

call	Function call interface, i.e. call linkage
data	Data layout
lib	Runtime library support
lif	Library interface, i.e. API
g	Potential gABI impact
ps	Potential psABI impact
source	Source code conventions (i.e. API, not ABI)
tools	May affect how program construction tools interact

# Issue Class Status Source Opened Closed

A Object Layout

A-1 Vptr location data closed SGI 990520 990624

A-2 Virtual base classes data closed SGI 990520 990624

A-3 Multiple inheritance data closed SGI 990520 990701

A-4 Empty base classes data closed SGI 990520 990624

A-5 Empty parameters data closed SGI 990520 001117

A-6 RTTI (type_info) .o representation data call ps closed SGI 990520 991028

A-7 Vptr sharing with primary base class data closed HP 990603 990729

A-8 (Virtual) base class alignment data closed HP 990603 990624

A-9 Sorting fields as allowed by [class.mem]/12 data closed HP 990603 990624

A-10 Class parameters in registers call closed HP 990603 990701

A-11 Pointers to member functions data closed Cygnus 990603 990812

A-12 Merging secondary vtables data closed Sun 990610 990805

A-13 Parameter struct field promotion call closed SGI 990603 990701

A-14 Pointers to data members data closed SGI 990729 990805

A-15 Empty bit-fields data closed CodeSourcery 991214 000106

A-16 Nearly empty virtual bases data closed SGI 991228 000106

A-17 Primary indirect virtual base allocation data closed SGI 991228 000113

A-18 Virtual base alignment data closed SGI 991228 000113

A-19 Primary indirect virtual base choice data closed All 000106 000120

A-20 Operator new array cookies data closed All 000113 000120

A-21 Placement new array cookies data closed All 000113 000217

A-22 RTTI for reference types data closed CodeSourcery 000119 000203

A-23 RTTI class descriptors data closed CodeSourcery 000124 000302

A-24 RTTI for incomplete types data closed CodeSourcery 000126 000330

A-25 Excess-width bitfields data closed IBM 000204 000217

A-26 NULL pointers to member functions data closed CodeSourcery 000221 000302

A-27 NULL pointers to data members data closed CodeSourcery 000222 000302

A-28 RTTI equality testing data closed CodeSourcery 000406 000504

A-29 RTTI pointer-to-member data closed CodeSourcery 000407 000504

A-30 RTTI portability data closed HUB 001012 001109

A-31 Overlaying tail padding data closed CodeSourcery 001019 001109

B Virtual Function Handling

B-1 Adjustment of "this" pointer (e.g. thunks) data call closed SGI 990520 991202

B-2 Covariant return types call closed SGI 990520 990722

B-3 Allowed caching of vtable contents call closed HP 990603 990805

B-4 Function descriptors in vtable data closed HP 990603 990805

B-5 Where are vtables emitted? data closed HP 990603 991118

B-6 Virtual function table layout data closed SGI 990520 991028

B-7 Objects and Vtables in shared memory data closed HP 990624 990805

B-8 dynamic_cast data closed SGI 990628 991014

B-9 Primary base vtable embedding data closed Cygnus 000217 000302

B-10 Pure virtual runtime call closed CodeSourcery 000629 000706

C Object Construction/Destruction

C-1 Interaction with .init/.fini lif ps closed SGI 990520 991202

C-2 Order of ctors/dtors w.r.t. link lif ps closed HP 990603 000817

C-3 Order of ctors/dtors w.r.t. DSOs ps closed HP 990603 000504

C-4 Construction vtables call closed Cygnus 990603 000504

C-5 Calling virtual destructors call closed Sun 990603 991104

C-6 Extra parameters to ctors/dtors call closed Cygnus 990603 991104

C-7 Passing value parameters by reference call closed All 990625 990805

C-8 Returning classes with non-trival copy constructors call closed All 990625 990722

C-9 Passing parameters with ctors/dtors call closed All 991028 991104

C-10 Synthesized copy assignments call closed All 991028 991028

C-11 Array constructors/destructors call closed Cygnus 000130 000309

C-12 Constructor return values call closed Cygnus 000130 000309

C-13 Allocating constructors call closed HP 000309 000803

C-14 Local-scope dynamic constructors data closed all 000309 000511

C-15 Alternate array allocators call closed CodeSourcery 000628 000720

C-16 Copy constructor runtime call closed CodeSourcery 000628 000720

C-17 Uncatch exception runtime call table all 000706 000720

C-18 Result buffers call closed SGI 000724 000817

C-19 NULL ctor/dtor API parameters call closed CodeSourcery 000806 000831

D Exception Handling

D-0 Exception handling framework lib ps closed SGI 990520 991216

D-1 Language-specific data area format lib ps open SGI 990520

D-2 Unwind personality routines lib ps closed SGI 990520 000106

D-3 Unwind process clarification lib ps closed SGI 990520 000106

D-4 Unwind routines nested? lib ps closed SGI 990520 991209

D-5 Interaction with other languages (e.g. Java) lib ps closed HP 990603 991007

D-6 Allow resumption in other languages? lib ps closed HP 990603 991007

D-7 Interaction with signals or asynch events lib ps closed HP 990603 991209

D-8 Interaction with threads packages lib ps closed SGI 990603 000106

D-9 longjmp interaction lib ps closed HP 990908 000113

D-10 psABI proposal lib ps closed all 991216 000120

D-11 pthreads interface lib ps closed all 991216 000203

D-12 Table location lib ps closed all 991216 000504

D-13 _Unwind_ForcedUnwind lib ps closed all 991216 000120

D-14 __cxa_begin/end_catch lib closed all 991216 001109

D-15 Terminate handler and threads lib closed all 991216 000106

D-16 Exception specifications lib closed all 991216 000113

D-17 bad_cast, bad_typeid runtime call closed CodeSourcery 000629 000706

D-18 __cxa_throw_type_info lib closed all 001012 001109

E Template Instantiation Model

E-1 When does instantiation occur? tools closed SGI 990520 000511

E-2 Export template model tools tabled SGI 990520

E-3 Template repository tools closed HP 990603 000511

F Name Mangling

F-1 Mangling convention call closed SGI 990520 000330

F-2 Mangled name size call g closed SGI 990520 000511

F-3 Consistency checks call g tabled SGI 990520

F-4 Empty throw specs call g tabled HP 990930

F-5 ILP32 vs. LP64 call closed HP 000210 000824

F-6 Demangling lib closed Cygnus 000210 000504

F-7 Mangling statics call closed HP 000223 000504

F-8 Identifiers with unicode letters call closed HU-Berlin 000323 000413

F-9 Strings with unicode letters call closed HU-Berlin 000323 000413

F-10 Mangling function return types call closed all 000330 000413

F-11 Hash for local strings call closed all 000330 000504

G Miscellaneous

G-1 Basic command line options tools closed HP 990603 000824

G-2 Detection of 1-def rule violations call closed Sun 990603 000504

G-3 Inlined routine linkage call closed Sun 990603 991202

G-4 Dynamic init of local static objects and multithreading call closed SCO 990607 001109

G-5 Varargs routine interface call closed HU-B 990810 991014

G-6 bool parameters call closed all 991104 991202

H Runtime Library Interface

H-1 Runtime library DSO name tools closed SGI 990616 000817

H-2 Runtime library API lif tabled SGI 990616

#	Issue	Class	Status	Source	Opened	Closed
A	Object Layout
A-1	Vptr location	data	closed	SGI	990520	990624
A-2	Virtual base classes	data	closed	SGI	990520	990624
A-3	Multiple inheritance	data	closed	SGI	990520	990701
A-4	Empty base classes	data	closed	SGI	990520	990624
A-5	Empty parameters	data	closed	SGI	990520	001117
A-6	RTTI (`type_info`) .o representation	data call ps	closed	SGI	990520	991028
A-7	Vptr sharing with primary base class	data	closed	HP	990603	990729
A-8	(Virtual) base class alignment	data	closed	HP	990603	990624
A-9	Sorting fields as allowed by [class.mem]/12	data	closed	HP	990603	990624
A-10	Class parameters in registers	call	closed	HP	990603	990701
A-11	Pointers to member functions	data	closed	Cygnus	990603	990812
A-12	Merging secondary vtables	data	closed	Sun	990610	990805
A-13	Parameter struct field promotion	call	closed	SGI	990603	990701
A-14	Pointers to data members	data	closed	SGI	990729	990805
A-15	Empty bit-fields	data	closed	CodeSourcery	991214	000106
A-16	Nearly empty virtual bases	data	closed	SGI	991228	000106
A-17	Primary indirect virtual base allocation	data	closed	SGI	991228	000113
A-18	Virtual base alignment	data	closed	SGI	991228	000113
A-19	Primary indirect virtual base choice	data	closed	All	000106	000120
A-20	Operator new array cookies	data	closed	All	000113	000120
A-21	Placement new array cookies	data	closed	All	000113	000217
A-22	RTTI for reference types	data	closed	CodeSourcery	000119	000203
A-23	RTTI class descriptors	data	closed	CodeSourcery	000124	000302
A-24	RTTI for incomplete types	data	closed	CodeSourcery	000126	000330
A-25	Excess-width bitfields	data	closed	IBM	000204	000217
A-26	NULL pointers to member functions	data	closed	CodeSourcery	000221	000302
A-27	NULL pointers to data members	data	closed	CodeSourcery	000222	000302
A-28	RTTI equality testing	data	closed	CodeSourcery	000406	000504
A-29	RTTI pointer-to-member	data	closed	CodeSourcery	000407	000504
A-30	RTTI portability	data	closed	HUB	001012	001109
A-31	Overlaying tail padding	data	closed	CodeSourcery	001019	001109
B	Virtual Function Handling
B-1	Adjustment of "this" pointer (e.g. thunks)	data call	closed	SGI	990520	991202
B-2	Covariant return types	call	closed	SGI	990520	990722
B-3	Allowed caching of vtable contents	call	closed	HP	990603	990805
B-4	Function descriptors in vtable	data	closed	HP	990603	990805
B-5	Where are vtables emitted?	data	closed	HP	990603	991118
B-6	Virtual function table layout	data	closed	SGI	990520	991028
B-7	Objects and Vtables in shared memory	data	closed	HP	990624	990805
B-8	dynamic_cast	data	closed	SGI	990628	991014
B-9	Primary base vtable embedding	data	closed	Cygnus	000217	000302
B-10	Pure virtual runtime	call	closed	CodeSourcery	000629	000706
C	Object Construction/Destruction
C-1	Interaction with .init/.fini	lif ps	closed	SGI	990520	991202
C-2	Order of ctors/dtors w.r.t. link	lif ps	closed	HP	990603	000817
C-3	Order of ctors/dtors w.r.t. DSOs	ps	closed	HP	990603	000504
C-4	Construction vtables	call	closed	Cygnus	990603	000504
C-5	Calling virtual destructors	call	closed	Sun	990603	991104
C-6	Extra parameters to ctors/dtors	call	closed	Cygnus	990603	991104
C-7	Passing value parameters by reference	call	closed	All	990625	990805
C-8	Returning classes with non-trival copy constructors	call	closed	All	990625	990722
C-9	Passing parameters with ctors/dtors	call	closed	All	991028	991104
C-10	Synthesized copy assignments	call	closed	All	991028	991028
C-11	Array constructors/destructors	call	closed	Cygnus	000130	000309
C-12	Constructor return values	call	closed	Cygnus	000130	000309
C-13	Allocating constructors	call	closed	HP	000309	000803
C-14	Local-scope dynamic constructors	data	closed	all	000309	000511
C-15	Alternate array allocators	call	closed	CodeSourcery	000628	000720
C-16	Copy constructor runtime	call	closed	CodeSourcery	000628	000720
C-17	Uncatch exception runtime	call	table	all	000706	000720
C-18	Result buffers	call	closed	SGI	000724	000817
C-19	NULL ctor/dtor API parameters	call	closed	CodeSourcery	000806	000831
D	Exception Handling
D-0	Exception handling framework	lib ps	closed	SGI	990520	991216
D-1	Language-specific data area format	lib ps	open	SGI	990520
D-2	Unwind personality routines	lib ps	closed	SGI	990520	000106
D-3	Unwind process clarification	lib ps	closed	SGI	990520	000106
D-4	Unwind routines nested?	lib ps	closed	SGI	990520	991209
D-5	Interaction with other languages (e.g. Java)	lib ps	closed	HP	990603	991007
D-6	Allow resumption in other languages?	lib ps	closed	HP	990603	991007
D-7	Interaction with signals or asynch events	lib ps	closed	HP	990603	991209
D-8	Interaction with threads packages	lib ps	closed	SGI	990603	000106
D-9	longjmp interaction	lib ps	closed	HP	990908	000113
D-10	psABI proposal	lib ps	closed	all	991216	000120
D-11	pthreads interface	lib ps	closed	all	991216	000203
D-12	Table location	lib ps	closed	all	991216	000504
D-13	_Unwind_ForcedUnwind	lib ps	closed	all	991216	000120
D-14	__cxa_begin/end_catch	lib	closed	all	991216	001109
D-15	Terminate handler and threads	lib	closed	all	991216	000106
D-16	Exception specifications	lib	closed	all	991216	000113
D-17	bad_cast, bad_typeid runtime	call	closed	CodeSourcery	000629	000706
D-18	__cxa_throw_type_info	lib	closed	all	001012	001109
E	Template Instantiation Model
E-1	When does instantiation occur?	tools	closed	SGI	990520	000511
E-2	Export template model	tools	tabled	SGI	990520
E-3	Template repository	tools	closed	HP	990603	000511
F	Name Mangling
F-1	Mangling convention	call	closed	SGI	990520	000330
F-2	Mangled name size	call g	closed	SGI	990520	000511
F-3	Consistency checks	call g	tabled	SGI	990520
F-4	Empty throw specs	call g	tabled	HP	990930
F-5	ILP32 vs. LP64	call	closed	HP	000210	000824
F-6	Demangling	lib	closed	Cygnus	000210	000504
F-7	Mangling statics	call	closed	HP	000223	000504
F-8	Identifiers with unicode letters	call	closed	HU-Berlin	000323	000413
F-9	Strings with unicode letters	call	closed	HU-Berlin	000323	000413
F-10	Mangling function return types	call	closed	all	000330	000413
F-11	Hash for local strings	call	closed	all	000330	000504
G	Miscellaneous
G-1	Basic command line options	tools	closed	HP	990603	000824
G-2	Detection of 1-def rule violations	call	closed	Sun	990603	000504
G-3	Inlined routine linkage	call	closed	Sun	990603	991202
G-4	Dynamic init of local static objects and multithreading	call	closed	SCO	990607	001109
G-5	Varargs routine interface	call	closed	HU-B	990810	991014
G-6	bool parameters	call	closed	all	991104	991202
H	Runtime Library Interface
H-1	Runtime library DSO name	tools	closed	SGI	990616	000817
H-2	Runtime library API	lif	tabled	SGI	990616