Compile multiple source files in an Autoconf test

Question

I'm working on a test for GCC init_priority and MSC init_seg. I want to craft a test to compile and link two source files in a specific order.

I'm having trouble finding information on compiling and linking two source files using AC_COMPILE_IFELSE or AC_LINK_IFELSE. The Autoconf docs don't appear to discuss the use case. Searching for terms like "Autoconf test with two source files" are returning irrelevant results. I think I might be missing good search terms.

Is it possible to use two source files during an Autoconf test? If so, then how do I do it?

Answer 1

AC_COMPILE_IFELSE and AC_LINK_IFELSE do not serve the purpose you describe, and they are not intended to do. Each takes one source, performs its test, and cleans up after itself. I didn't even find a macro for what you want at the Autoconf archive, which provides a number of sometimes-useful macros that don't ship with Autoconf itself. What you're proposing to do is far out of the ordinary, so if you want to proceed then I think you'll need to roll your own. Possibly it would help to consult the implementations of AC_COMPILE_IFELSE and AC_LINK_IFELSE.

But do sit back and have a think first, because I don't think such a test would be meaningful anyway. C++ relative initialization order across translation units is completely unspecified without use of an extension such as init_priority or init_seg. The language provides no justification for an assumption that it has anything to do with the order in which objects appear on a linker command line, nor even for an assumption that initializations arising from different TUs won't be intermixed. Thus, even if a test of cross-TU initialization happens to pass, that does not convey any actionable information, because there's no basis for certainty that whatever characteristic you are trying to test is responsible for the success.

I considered suggesting just a test for in-TU initialization reordering via init_priority, which you could perform with AC_RUN_IFELSE. That's not useful to you either, however, because you apparently need to support MSVC++, which does not provide a mechanism to modulate initialization order at that granularity.

Ultimately, then, I think the best you can do is to test whether init_priority or init_seg is accepted at all by the compiler (which you can do with AC_COMPILE_IFELSE), and suppose that if so, it in fact does the job it is supposed to do.