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69 lines
2.5 KiB
69 lines
2.5 KiB
4 months ago
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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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