GNOME Bugzilla – Bug 572536
Rework header file generation
Last modified: 2009-03-30 22:36:23 UTC
From mailing list discussion http://mail.gnome.org/archives/vala-list/2009-January/msg00058.html I want to finally solve issues we're having related to generation of C header files. The current situation is that valac generates a .h file in addition to a .c file for each .vala source file. The .h files describe both, the public and the internal, C API. There are various issues with the current approach: * Header file interdependencies Header files often depend on other header files and sometimes these dependencies form cycles. These cycles are currently detected by the compiler and resolved as far as possible - for example, by moving typedefs to other header files and reordering includes. While this works in many cases, there are situations that cannot be solved with the current approach, depending on what type you declare in what .vala file. There are also situations that could be solved with the current approach but are not implemented yet; it would complicate the code even further. * Internal API The whole point of an internal API is that it is internal, and we should therefore aim to separate public from internal header files. * Build performance As the internal API is in the header file as well, small changes can lead to a rebuild of the whole library or application. As the C compilation takes more time than the Vala compilation for larger projects, developers could profit from a vastly increased rebuild performance if we can solve this issue. * No single header file for users The trend in GLib-based libraries is to only provide a single header file per library for public use. The advantage is that it is easier to provide a stable include mechanism and provides more flexibility to the internal header organization of libraries. This is currently not directly supported by the Vala compiler, leading to extra work for people that want to provide that. It is important that we find a permanent solution as changes to this can lead to broken build systems, and I intend to finish upstream automake integration as soon as we have a stable solution. Possible solution steps and their issues: * Centralize type declarations Interdependencies only occur among type declarations (typedefs and struct declarations). If we move type declarations to a central place and include that before any function and global variable declarations, we solve the dependency issues. However, this change might decrease build performance, especially if we also do this for internal types. * Generate two sets of header files For each source file foo.vala, we could generate foo-priv.h in addition to foo.c and foo.h and move the internal C API there. One issue with this approach is that it clutters the source directory with even more files. The compiler will always have to generate all three files, even if some are empty, to not break build system integration. * Generate one additional header file for the internal API We could avoid generating many -priv.h by just generating one big foo-priv.h file that contains the full internal. However, in some projects you have a large internal API, for example, in applications without plugin support. This means that rebuild performance would get a lot worse. * Minimize use of header files A more radical approach would be to not use header files where not necessary. The Vala compiler could insert required declarations at the top of each generated .c file and only use include directives for external libraries. This would lead to optimal rebuild performance at the cost of a bit larger .c files. We obviously still need to support generating header files to use libraries written in Vala. I would propose to add an option -H foo.h to valac to generate a single header file with the full public C API. I'm leaning towards the last solution as this solves all of the issues mentioned above and should also provide the simplest way to integrate into build systems such as automake. I can understand that some people might not like that solution as it deviates from the traditional way how you would do it in C. However, Vala is about practical solutions, not traditions.
commit 2d4a4a264677b42710914907f63e1624e25e560f Author: Jürg Billeter <j@bitron.ch> Date: Tue Mar 31 00:31:07 2009 +0200 Rework header file generation Generate single C header file for public API, do not use header files for internal API. Fixes bug 471244, bug 571037, bug 572536, and bug 575629.