Add snippets.symbol_visibility_header() method

Defining public API in a cross platform library is painful, especially
on Windows. Since every library have to define pretty much the same
macros, better do it in Meson.

2 files changed, 121 insertions, 0 deletions

diff --git a/docs/markdown/Snippets-module.md b/docs/markdown/Snippets-module.md
new file mode 100644
index 000000000..15d5e9f74
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+++ b/docs/markdown/Snippets-module.md
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+---
+short-description: Code snippets module
+...
+
+# Snippets module
+
+*(new in 1.10.0)*
+
+This module provides helpers to generate commonly useful code snippets.
+
+## Functions
+
+### symbol_visibility_header()
+
+```meson
+snippets.symbol_visibility_header(header_name,
+  namespace: str
+  api: str
+  compilation: str
+  static_compilation: str
+  static_only: bool
+)
+```
+
+Generate a header file that defines macros to be used to mark all public APIs
+of a library. Depending on the platform, this will typically use
+`__declspec(dllexport)`, `__declspec(dllimport)` or
+`__attribute__((visibility("default")))`. It is compatible with C, C++,
+ObjC and ObjC++ languages. The content of the header is static regardless
+of the compiler used.
+
+The first positional argument is the name of the header file to be generated.
+It also takes the following keyword arguments:
+
+- `namespace`: Prefix for generated macros, defaults to the current project name.
+  It will be converted to upper case with all non-alphanumeric characters replaced
+  by an underscore `_`. It is only used for `api`, `compilation` and
+  `static_compilation` default values.
+- `api`: Name of the macro used to mark public APIs. Defaults to `<NAMESPACE>_API`.
+- `compilation`: Name of a macro defined only when compiling the library.
+   Defaults to `<NAMESPACE>_COMPILATION`.
+- `static_compilation`: Name of a macro defined only when compiling or using
+  static library. Defaults to `<NAMESPACE>_STATIC_COMPILATION`.
+- `static_only`: If set to true, `<NAMESPACE>_STATIC_COMPILATION` is defined
+  inside the generated header. In that case the header can only be used for
+  building a static library. By default it is `true` when `default_library=static`,
+  and `false` otherwise. [See below for more information](#static_library)
+
+Projects that define multiple shared libraries should typically have
+one header per library, with a different namespace.
+
+The generated header file should be installed using `install_headers()`.
+
+`meson.build`:
+```meson
+project('mylib', 'c')
+subdir('mylib')
+```
+
+`mylib/meson.build`:
+```meson
+snippets = import('snippets')
+apiheader = snippets.symbol_visibility_header('apiconfig.h')
+install_headers(apiheader, 'lib.h', subdir: 'mylib')
+lib = library('mylib', 'lib.c',
+  gnu_symbol_visibility: 'hidden',
+  c_args: ['-DMYLIB_COMPILATION'],
+)
+```
+
+`mylib/lib.h`
+```c
+#include <mylib/apiconfig.h>
+MYLIB_API int do_stuff();
+```
+
+`mylib/lib.c`
+```c
+#include "lib.h"
+int do_stuff() {
+  return 0;
+}
+```
+
+#### Static library
+
+When building both static and shared libraries on Windows (`default_library=both`),
+`-D<NAMESPACE>_STATIC_COMPILATION` must be defined only for the static library,
+using `c_static_args`. This causes Meson to compile the library twice.
+
+```meson
+if host_system == 'windows'
+  static_arg = ['-DMYLIB_STATIC_COMPILATION']
+else
+  static_arg = []
+endif
+lib = library('mylib', 'lib.c',
+  gnu_symbol_visibility: 'hidden',
+  c_args: ['-DMYLIB_COMPILATION'],
+  c_static_args: static_arg
+)
+```
+
+`-D<NAMESPACE>_STATIC_COMPILATION` C-flag must be defined when compiling
+applications that use the library API. It typically should be defined in
+`declare_dependency(..., compile_args: [])` and
+`pkgconfig.generate(..., extra_cflags: [])`.
+
+Note that when building both shared and static libraries on Windows,
+applications cannot currently rely on `pkg-config` to define this macro.
+See https://github.com/mesonbuild/meson/pull/14829.
diff --git a/docs/markdown/snippets/symbol_visibility_header.md b/docs/markdown/snippets/symbol_visibility_header.md
new file mode 100644
index 000000000..75aae786d
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+++ b/docs/markdown/snippets/symbol_visibility_header.md
@@ -0,0 +1,10 @@
+## New method to handle GNU and Windows symbol visibility for C/C++/ObjC/ObjC++
+
+Defining public API of a cross platform C/C++/ObjC/ObjC++ library is often
+painful and requires copying macro snippets into every projects, typically using
+`__declspec(dllexport)`, `__declspec(dllimport)` or
+`__attribute__((visibility("default")))`.
+
+Meson can now generate a header file that defines exactly what's needed for
+all supported platforms:
+[`snippets.symbol_visibility_header()`](Snippets-module.md#symbol_visibility_header).