fix: fix symbol export on Windows + use target properties

[aminya]

This changes the export definition to only happen when the tinyxml2 itself is built as a shared library.

It fixes an issue where the tinyxml2 symbols are exported even when it is compiled statically and privately linked into another shared library. This allows the compiler to inline these symbols. Otherwise, the symbols of tinyxml2 are exported in the final shared library.

[leethomason]

@aminya any thoughts / responses to @alexreinking comments?

[aminya]

I already discussed many things in https://github.com/leethomason/tinyxml2/pull/922#discussion_r1008769079

[alexreinking]

I already discussed many things in (comment)

@aminya - but you have not replied to my review comments, which were written more recently. In particular, the DEFINE_SYMBOL change is absolutely irrelevant.

[aminya]

TINYXML2_EXPORT is the gateway to forcing the symbol in the final shared library. What do you mean it is irrelevant?

[alexreinking]

@aminya - I tried to reproduce the issue you were facing. Please tell me if any of the steps I followed differ from yours.

Step 0: create a working directory

$ cd ~
$ mkdir test
$ cd test

Now we have somewhere to work.

Step 1: download, patch, and build tinyxml2

$ git clone https://github.com/leethomason/tinyxml2.git
$ cd tinyxml2
$ sed -i 's/\(__GNUC__ >= 4\)/defined(TINYXML2_EXPORT) \&\& \1/g' tinyxml2.h

That last line applies the same change you made to tinyxml2.h. The CMakeLists.txt file is untouched.

Now we'll build tinyxml2 as a static library, with PIC enabled since it will be part of a shared library later.

$ cmake -G Ninja -S . -B build -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=NO -DCMAKE_POSITION_INDEPENDENT_CODE=YES
$ cmake --build build/
$ cmake --install build --prefix ../_local

We're installing it to ~/test/_local, which will be convenient later.

Step 2: create a simple project:

Create two files in ~/test:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.23)
project(example)

find_package(tinyxml2 REQUIRED)

set(CMAKE_CXX_VISIBILITY_PRESET hidden)
set(CMAKE_VISIBILITY_INLINES_HIDDEN YES)

add_library(example SHARED example.cpp)
target_link_libraries(example PRIVATE tinyxml2::tinyxml2)

example.cpp:

#include <tinyxml2.h>
using namespace tinyxml2;

__attribute__((visibility("default")))
int example()
{
	static const char* xml = "<element/>";
	XMLDocument doc;
	doc.Parse( xml );

	return doc.ErrorID();
}

The build forces the example library to be shared, and it links to tinyxml2. We also hide symbols by default, but expose example as our one API.

Step 3: build the project

$ cmake -G Ninja -S . -B build -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$PWD/_local
$ cmake --build build

Here we set CMAKE_PREFIX_PATH to point to the patched tinyxml2 we just built.

Now we can see that the only exported symbol is example.

$ nm -D build/libexample.so | grep ' T ' | c++filt
00000000000035b0 T example()

We only care about the exported (capital), text ("T") symbols.

---

Step 3b: trying again

If I repeat all the same steps as before but without the patch to tinyxml2.h, then I see this:

alex@alex-ubuntu:~/test$ nm -D build/libexample.so | grep ' T ' | c++filt
0000000000009280 T example()
000000000000ffd0 T tinyxml2::XMLComment::ParseDeep(char*, tinyxml2::StrPair*, int*)
000000000000be70 T tinyxml2::XMLComment::XMLComment(tinyxml2::XMLDocument*)
... dozens more ...

This shows that no changes to the CMakeLists.txt are necessary, but your change to the header file is good and correct.

[aminya]

Thanks for the example. I moved the setting of the visibility properties of the tinyxml2 target to avoid any confusion.

[alexreinking]

Please revert the changes to CMakeLists.txt as they do not change the functionality. Keep only the change to tinyxml2.h. Then I will approve the PR. ATTN @leethomason

[leethomason]

Closing. Can re-open if the thread picks back up.

[aminya]

@leethomason This was ready to be merged. Are you closing this because of a simple documentation comment?

[leethomason]

@alexreinking had requested removing the changes to cmakelist.txt. There are still changes - were those intended? Is there any reason not to revert them?

[aminya]

@alexreinking had requested removing the changes to cmakelist.txt. There are still changes - were those intended? Is there any reason not to revert them?

Yes, they were intended. The CMake hidden variable was being set globally, but this makes it specific to the tinyxml target.

[alexreinking]

Yes, they were intended. The CMake hidden variable was being set globally, but this makes it specific to the tinyxml target.

@aminya -- I'm not sure what you mean by "globally". The variables would be local to the project, so if one were to FetchContent or add_subdirectory tinyxml2, they would not see these variables. So they are not global in that sense (as they would be were they set as cache variables).

Furthermore, this build has only one library (tinyxml2) which is affected by the two visibility variables you set. The only other target is xmltest, which is an executable; the visibility variables do not meaningfully affect executables unless ENABLE_EXPORTS is set, which is not the case here.

Thus the changes to the CMakeLists.txt are totally unnecessary and should not be included.

[dg0yt]

With https://github.com/leethomason/tinyxml2/pull/922/commits/240e5c10a17475c4e805a445ce3f53f5af35f2b4, the PR lost the if(BUILD_SHARED_LIBS) guard which was the key contribution IIUC.

[aminya]

Thus the changes to the CMakeLists.txt are totally unnecessary and should not be included.

I suggest you read the Effective Modern Cmake as it is the recommended approach by the CMake experts.

https://gist.github.com/mbinna/c61dbb39bca0e4fb7d1f73b0d66a4fd1#think-in-terms-of-targets-and-properties

[aminya]

With https://github.com/leethomason/tinyxml2/pull/922/commits/240e5c10a17475c4e805a445ce3f53f5af35f2b4, the PR lost the if(BUILD_SHARED_LIBS) guard which was the key contribution IIUC.

I checked the documentation of CMake. Based on the doc, that check is done automatically for a target.

[dg0yt]

I checked the documentation again now.

• IMO the variables are perfect modern CMake: They determine the default value for the target properties.
  So moving the value from the variable to the main target makes no difference.
• Modern CMake (cmake_minimum_required version >= 3.3) applies the property to all targets (CMP0063). So if you want to have different visibility settings for static libraries, you must use an explicit guard.

[alexreinking]

I suggest you read the Effective Modern Cmake as it is the recommended approach by the CMake experts.

@aminya -- Please do not resort to insults. I posted a demonstration above showing that your header edit alone (which is good and worth merging!) is sufficient to address the symbol export issue. Please re-examine this and either:

1. Show steps to reproduce the issue without the CMake edits (to demonstrate they are indeed necessary), or
2. Remove the CMake edits

---

@dg0yt - thank you for reading the documentation. The variables are indeed fine. They have the added benefit of applying to new targets within this project should the need ever arise; this reduces the risk of accidental symbol mismanagement.

So if you want to have different visibility settings for static libraries, you must use an explicit guard.

IIUC - we actually don't want this. We want the non-API symbols to be hidden in static libraries. The header fix makes exactly the API symbols visible (by checking TINYXML2_EXPORT) when building statically.