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@node Exported Symbols of Shared Libraries
@section Controlling the Exported Symbols of Shared Libraries
@c Documentation of gnulib module 'lib-symbol-visibility'.
@c Copyright (C) 2005-2006, 2009-2018 Free Software Foundation, Inc.
@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, no Front-Cover Texts, and no Back-Cover
@c Texts. A copy of the license is included in the ``GNU Free
@c Documentation License'' file as part of this distribution.
The @code{lib-symbol-visibility} module allows precise control of the
symbols exported by a shared library. This is useful because
@itemize @bullet
@item
It prevents abuse of undocumented APIs of your library. Symbols that
are not exported from the library cannot be used. This eliminates the
problem that when the maintainer of the library changes internals of the
library, maintainers of other projects cry ``breakage''. Instead, these
maintainers are forced to negotiate the desired API from the maintainer
of the library.
@item
It reduces the risk of symbol collision between your library and other
libraries. For example, the symbol @samp{readline} is defined in several
libraries, most of which don't have the same semantics and the same calling
convention as the GNU readline library.
@item
It reduces the startup time of programs linked to the library. This is
because the dynamic loader has less symbols to process.
@item
It allows the compiler to generate better code. Within a shared library,
a call to a function that is a global symbol costs a ``call'' instruction
to a code location in the so-called PLT (procedure linkage table) which
contains a ``jump'' instruction to the actual function's code. (This is
needed so that the function can be overridden, for example by a function
with the same name in the executable or in a shared library interposed
with @code{LD_PRELOAD}.) Whereas a call to a function for which the compiler
can assume that it is in the same shared library is just a direct ``call''
instructions. Similarly for variables: A reference to a global variable
fetches a pointer in the so-called GOT (global offset table); this is a
pointer to the variable's memory. So the code to access it is two memory
load instructions. Whereas for a variable which is known to reside in the
same shared library, it is just a direct memory access: one memory load
instruction.
@end itemize
There are traditionally three ways to specify the exported symbols of a
shared library.
@itemize @bullet
@item
The programmer specifies the list of symbols to be exported when the
shared library is created. Usually a command-line option is passed
to the linker, with the name of a file containing the symbols.
The upside of this approach is flexibility: it allows the same code to
be used in different libraries with different export lists. The downsides
are: 1. it's a lot of maintenance overhead when the symbol list is platform
dependent, 2. it doesn't work well with C++, due to name mangling.
@item
The programmer specifies a ``hidden'' attribute for every variable and
function that shall not be exported.
The drawbacks of this approach are: Symbols are still exported from
the library by default. It's a lot of maintenance work to mark every non-
exported variable and function. But usually the exported API is quite small,
compared to the internal API of the library. And it's the wrong paradigm:
It doesn't force thinking when introducing new exported API.
@item
The programmer specifies a ``hidden'' attribute for all files that make up
the shared library, and an ``exported'' attribute for those symbols in these
files that shall be exported.
This is perfect: It burdens the maintainer only for exported API, not
for library-internal API. And it keeps the annotations in the source code.
@end itemize
GNU libtool's @option{-export-symbols} option implements the first approach.
This gnulib module implements the third approach. For this it relies on
GNU GCC 4.0 or newer, namely on its @samp{-fvisibility=hidden} command-line
option and the ``visibility'' attribute. (The ``visibility'' attribute
was already supported in GCC 3.4, but without the command line option,
introduced in GCC 4.0, the third approach could not be used.)
More explanations on this subject can be found in
@url{https://gcc.gnu.org/wiki/Visibility}, which contains more details
on the GCC features and additional advice for C++ libraries, and in
Ulrich Drepper's paper @url{https://www.akkadia.org/drepper/dsohowto.pdf},
which also explains other tricks for reducing the startup time impact
of shared libraries.
The gnulib autoconf macro @code{gl_VISIBILITY} tests for GCC 4.0 or newer.
It defines a Makefile variable @code{@@CFLAG_VISIBILITY@@} containing
@samp{-fvisibility=hidden} or nothing. It also defines as a C macro and
as a substituted variable: @@HAVE_VISIBILITY@@. Its value is 1 when symbol
visibility control is supported, and 0 otherwise.
To use this module in a library, say libfoo, you will do these steps:
@enumerate
@item
Add @code{@@CFLAG_VISIBILITY@@} or (in a Makefile.am)
@code{$(CFLAG_VISIBILITY)} to the CFLAGS for the compilation of the sources
that make up the library.
@item
Add a C macro definition, say @samp{-DBUILDING_LIBFOO}, to the CPPFLAGS
for the compilation of the sources that make up the library.
@item
Define a macro specific to your library like this.
@smallexample
#if BUILDING_LIBFOO && HAVE_VISIBILITY
#define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
#else
#define LIBFOO_DLL_EXPORTED
#endif
@end smallexample
This macro should be enabled in all public header files of your library.
@item
Annotate all variable, function and class declarations in all public header
files of your library with @samp{LIBFOO_DLL_EXPORTED}. This annotation
can occur at different locations: between the @samp{extern} and the
type or return type, or just before the entity being declared, or after
the entire declarator. My preference is to put it right after @samp{extern},
so that the declarations in the header files remain halfway readable.
@end enumerate
Note that the precise control of the exported symbols will not work with
other compilers than GCC >= 4.0, and will not work on systems where the
assembler or linker lack the support of ``hidden'' visibility. Therefore,
it's good if, in order to reduce the risk of collisions with symbols in
other libraries, you continue to use a prefix specific to your library
for all non-static variables and functions and for all C++ classes in
your library.
Note about other compilers: MSVC support can be added easily, by extending
the definition of the macro mentioned above, to something like this:
@smallexample
#if BUILDING_LIBFOO && HAVE_VISIBILITY
#define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
#elif BUILDING_LIBFOO && defined _MSC_VER
#define LIBFOO_DLL_EXPORTED __declspec(dllexport)
#elif defined _MSC_VER
#define LIBFOO_DLL_EXPORTED __declspec(dllimport)
#else
#define LIBFOO_DLL_EXPORTED
#endif
@end smallexample
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