@c Copyright (C) 2002-2021 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Compatibility
@chapter Binary Compatibility
@cindex binary compatibility
@cindex ABI
@cindex application binary interface

Binary compatibility encompasses several related concepts:

@table @dfn
@item application binary interface (ABI)
The set of runtime conventions followed by all of the tools that deal
with binary representations of a program, including compilers, assemblers,
linkers, and language runtime support.
Some ABIs are formal with a written specification, possibly designed
by multiple interested parties.  Others are simply the way things are
actually done by a particular set of tools.

@item ABI conformance
A compiler conforms to an ABI if it generates code that follows all of
the specifications enumerated by that ABI@.
A library conforms to an ABI if it is implemented according to that ABI@.
An application conforms to an ABI if it is built using tools that conform
to that ABI and does not contain source code that specifically changes
behavior specified by the ABI@.

@item calling conventions
Calling conventions are a subset of an ABI that specify of how arguments
are passed and function results are returned.

@item interoperability
Different sets of tools are interoperable if they generate files that
can be used in the same program.  The set of tools includes compilers,
assemblers, linkers, libraries, header files, startup files, and debuggers.
Binaries produced by different sets of tools are not interoperable unless
they implement the same ABI@.  This applies to different versions of the
same tools as well as tools from different vendors.

@item intercallability
Whether a function in a binary built by one set of tools can call a
function in a binary built by a different set of tools is a subset
of interoperability.

@item implementation-defined features
Language standards include lists of implementation-defined features whose
behavior can vary from one implementation to another.  Some of these
features are normally covered by a platform's ABI and others are not.
The features that are not covered by an ABI generally affect how a
program behaves, but not intercallability.

@item compatibility
Conformance to the same ABI and the same behavior of implementation-defined
features are both relevant for compatibility.
@end table

The application binary interface implemented by a C or C++ compiler
affects code generation and runtime support for:

@itemize @bullet
@item
size and alignment of data types
@item
layout of structured types
@item
calling conventions
@item
register usage conventions
@item
interfaces for runtime arithmetic support
@item
object file formats
@end itemize

In addition, the application binary interface implemented by a C++ compiler
affects code generation and runtime support for:
@itemize @bullet
@item
name mangling
@item
exception handling
@item
invoking constructors and destructors
@item
layout, alignment, and padding of classes
@item
layout and alignment of virtual tables
@end itemize

Some GCC compilation options cause the compiler to generate code that
does not conform to the platform's default ABI@.  Other options cause
different program behavior for implementation-defined features that are
not covered by an ABI@.  These options are provided for consistency with
other compilers that do not follow the platform's default ABI or the
usual behavior of implementation-defined features for the platform.
Be very careful about using such options.

Most platforms have a well-defined ABI that covers C code, but ABIs
that cover C++ functionality are not yet common.

Starting with GCC 3.2, GCC binary conventions for C++ are based on a
written, vendor-neutral C++ ABI that was designed to be specific to
64-bit Itanium but also includes generic specifications that apply to
any platform.
This C++ ABI is also implemented by other compiler vendors on some
platforms, notably GNU/Linux and BSD systems.
We have tried hard to provide a stable ABI that will be compatible with
future GCC releases, but it is possible that we will encounter problems
that make this difficult.  Such problems could include different
interpretations of the C++ ABI by different vendors, bugs in the ABI, or
bugs in the implementation of the ABI in different compilers.
GCC's @option{-Wabi} switch warns when G++ generates code that is
probably not compatible with the C++ ABI@.

The C++ library used with a C++ compiler includes the Standard C++
Library, with functionality defined in the C++ Standard, plus language
runtime support.  The runtime support is included in a C++ ABI, but there
is no formal ABI for the Standard C++ Library.  Two implementations
of that library are interoperable if one follows the de-facto ABI of the
other and if they are both built with the same compiler, or with compilers
that conform to the same ABI for C++ compiler and runtime support.

When G++ and another C++ compiler conform to the same C++ ABI, but the
implementations of the Standard C++ Library that they normally use do not
follow the same ABI for the Standard C++ Library, object files built with
those compilers can be used in the same program only if they use the same
C++ library.  This requires specifying the location of the C++ library
header files when invoking the compiler whose usual library is not being
used.  The location of GCC's C++ header files depends on how the GCC
build was configured, but can be seen by using the G++ @option{-v} option.
With default configuration options for G++ 3.3 the compile line for a
different C++ compiler needs to include

@smallexample
    -I@var{gcc_install_directory}/include/c++/3.3
@end smallexample

Similarly, compiling code with G++ that must use a C++ library other
than the GNU C++ library requires specifying the location of the header
files for that other library.

The most straightforward way to link a program to use a particular
C++ library is to use a C++ driver that specifies that C++ library by
default.  The @command{g++} driver, for example, tells the linker where
to find GCC's C++ library (@file{libstdc++}) plus the other libraries
and startup files it needs, in the proper order.

If a program must use a different C++ library and it's not possible
to do the final link using a C++ driver that uses that library by default,
it is necessary to tell @command{g++} the location and name of that
library.  It might also be necessary to specify different startup files
and other runtime support libraries, and to suppress the use of GCC's
support libraries with one or more of the options @option{-nostdlib},
@option{-nostartfiles}, and @option{-nodefaultlibs}.