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/*
* Copyright 2010-2020, Tarantool AUTHORS, please see AUTHORS file.
*
* Redistribution and use in source and binary forms, with or
* without modification, are permitted provided that the following
* conditions are met:
*
* 1. Redistributions of source code must retain the above
* copyright notice, this list of conditions and the
* following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above
* copyright notice, this list of conditions and the following
* disclaimer in the documentation and/or other materials
* provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY <COPYRIGHT HOLDER> ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
* <COPYRIGHT HOLDER> OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
* BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
* THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/**
* The file is a hack to force the linker keep the needed symbols
* in the result tarantool executable file.
*
* Problem is that if a symbol is defined inside a static library,
* but never used in the final executable, the linker may throw it
* away. But many symbols are needed for Lua FFI and for the
* public C API used by dynamic modules.
*
* This file creates a 'false usage' of needed symbols. It stores
* pointers at them into a big array, and returns it like if the
* caller will use them somehow. Call, write by their address, or
* anything else.
*
* In reality the symbols are never touched after export from
* here, but the compiler and the linker can't tell that.
*
* Below are some alternatives, which may replace the current
* solution in future. Both are better in being able to declare a
* symbol as exported right where it is defined. Or sometimes in
* another place if necessary. For example, when the needed
* symbol is defined somewhere in a third_party library.
*
* ** Solution 1 - user-defined ELF sections. **
*
* That way is similar to what is used by the Linux kernel. To
* implement it there is a macros, lets call it EXPORT_SYMBOL. The
* macros takes one parameter - symbol name. In its implementation
* the macros defines a global struct keeping pointer at that
* symbol, and stored in a special section. For example, .tntexp
* section. Later when all is complied into the final executable,
* there is a linker script, which takes all the symbols defined
* in that section, and creates a reference at them, which is then
* somehow used in the code.
*
* A pseudocode example of how can it look in theory:
*
* struct tnt_exported_symbol {
* void *sym;
* };
*
* #define EXPORT_SYMBOL(symbol) \
* __attribute__((section(".tntexp")))
* struct tnt_exported_symbol tnt_exported_##sym = { \
* .sym = (void *) symbol \
* };
*
* For more info see EXPORT_SYMBOL() macros in
* include/linux/export.h file in the kernel sources.
*
* ** Solution 2 - precompile script which would find all exported
* functions and generate this file automatically. **
*
* Not much to explain. Introduce a macros EXPORT_SYMBOL, and walk
* all the source code, looking for it. When see a symbol marked
* so, remember it. Then generate the exports.c like it is defined
* below. But automatically.
*/
/**
* Symbol is just an address. No need to know its definition or
* even type to get that address. Even an integer global variable
* can be referenced as extern void(*)(void).
*/
#define EXPORT(symbol) extern void symbol(void);
#include "exports.h"
#undef EXPORT
void **
export_syms(void)
{
/*
* Compiler should think the exported symbols are
* reachable. When they are returned as an array, the
* compiler can't assume anything, and can't remove them.
*/
#define EXPORT(symbol) ((void *)symbol),
static void *symbols[] = {
#include "exports.h"
};
#undef EXPORT
return symbols;
}
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