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103a6eb94f
Symbols of pretty much all member functions of types that are meant to
be "private" to translation units that contribute to libabigail.so
were exported because we didn't do much to prevent that.
This patch starts controlling the set of symbols that are exported.
By default, symbols of any entity declared in a translation unit that
contributes to libabigail.so are hidden by default. Only symbols of
entities declared in public headers (headers in include/*.h) are
exported.
There are many ways to achieve that. This patch chooses to avoid
cluttering declarations of entities in the public header by adding
__attribute__((visibility="default")) to every declared type of
function in there.
Rather, the patch uses "#pragma GCC visibility push(default)" before
entities declared on those headers. By doing so, all those entities
have their symbol marked as "visible" by the compiler. Once the
header are #included, the #pragma GCC visibility pop" is used, so that
anything else has its symbol be hidden from that point on.
Note that for ease of maintenance the patch uses the macros
ABG_BEGIN_EXPORT_DECLARATIONS and ABG_END_EXPORT_DECLARATIONS rather
than using the pragma directive directly.
I believe this is a more elegant way of handling visibility, compared
to cluttering every single declaration in public headers with a
"__attribute__((visibility=("default")))" or with a macro which
expands to it.
This reduces the the set of symbols exported by libabigail.so from
20000+ to less than 5000.
* VISIBILITY: New documentation about this visiblity business.
* CONTRIBUTING: Update the "contributing guide" to refer to symbol
visibility issues.
* configure.ac: Define a variable VISIBILITY_FLAGS that is set to
the -fvisibility=hidden flag to pass to GCC, when its available.
* src/Makefile.am: Add VISIBILITY to source distribution. Also
add COMPILING and COMMIT-LOG-GUIDELINES that were missing.
* src/Makefile.am: Use the new $(VISIBILITY_FLAGS) when buiding
the library.
* tests/Makefile.am: Use the new $(VISIBILITY_FLAGS) when buiding
tests.
* tools/Makefile.am: Use the new $(VISIBILITY_FLAGS) when buiding
tools.
* src/abg-comp-filter.cc: Enclose inclusion of public headers in
ABG_BEGIN_EXPORT_DECLARATIONS and ABG_END_EXPORT_DECLARATIONS to
export the symbols of entities declared in there.
* src/abg-comparison.cc: Likewise.
* src/abg-config.cc: Likewise.
* src/abg-corpus.cc: Likewise.
* src/abg-diff-utils.cc: Likewise.
* src/abg-dwarf-reader.cc: Likewise.
* src/abg-hash.cc: Likewise.
* src/abg-ini.cc: Likewise.
* src/abg-ir.cc: Likewise.
* src/abg-libxml-utils.cc: Likewise.
* src/abg-libzip-utils.cc: Likewise.
* src/abg-reader.cc: Likewise.
* src/abg-suppression.cc: Likewise.
* src/abg-tools-utils.cc: Likewise.
* src/abg-traverse.cc: Likewise.
* src/abg-viz-common.cc: Likewise.
* src/abg-viz-dot.cc: Likewise.
* src/abg-viz-svg.cc: Likewise.
* src/abg-workers.cc: Likewise.
* src/abg-writer.cc: Likewise.
Signed-off-by: Dodji Seketeli <dodji@redhat.com>
69 lines
2.5 KiB
Plaintext
69 lines
2.5 KiB
Plaintext
PLEASE READ ALL OF THIS FILE, ESPECIALLY IF YOU ARE DEFINING A NEW
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PUBLIC HEADER IN LIBABIGAIL.
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How symbols that are exported are controlled in libabigail
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==========================================================
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We try to limit the number of ELF symbols that are exported by the
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libabigail.so shared library. We call this symbols visibility
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control.
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As GNU/Linux is our development platform, we control symbol visibility
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by using the visibility support of the G++ compiler.
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How to do so is properly explained at https://gcc.gnu.org/wiki/Visibility.
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All symbols are hidden by default
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=================================
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When building translation units that make up the libabigail.so shared
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library, G++ is invoked with the -fvisibility=hidden directive. Which
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instructs it to make symbols of functions and global variables
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*locally* defined in the shared library, *NOT* exported (or global).
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Exporting symbols of entities declared in public headers
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========================================================
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In a translation unit that is part of the libabigail.so shared
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library, before including a header file that is a public libabigail
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header (e.g, abg-ir.h), one need to declare:
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#include "abg-internal.h"
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ABG_BEGIN_EXPORT_DECLARATIONS
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then all the public header files inclusion (using #include directives)
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follow. At the end of these public header files inclusion, one need
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to declare:
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ABG_END_EXPORT_DECLARATIONS
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The ABG_BEGIN_EXPORT_DECLARATIONS is a macro defined in abg-internal.h
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which expands to:
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#pragma GCC visibility push(default)
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This instructs G++ to export the symbol of all global functions and
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variables definitions that are declared from that point on.
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The ABG_END_EXPORT_DECLARATIONS is a macro defined in abg-internal.h
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which expands to:
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#pragma GCC visibility pop
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It instructs G++ to stop exporting symbols of global functions and
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variable definition from that point on.
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In practice, the pair ABG_BEGIN_EXPORT_DECLARATIONS,
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ABG_END_EXPORT_DECLARATIONS allows us to only export symbols of
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global functions and variables declared in the block denoted by these
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two macros. Symbols of anything else that is declared outside of that block
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are going to be hidden, thanks to the -fvisibility=hidden option
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passed to G++.
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So whenever you are defining a new header file with declarations that
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ought to be part of the API of libabigail, the *definition* file which
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defines the declarations of the header file must use
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the ABG_BEGIN_EXPORT_DECLARATIONS and ABG_END_EXPORT_DECLARATIONS
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macro to include the public header.
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