cross compile with my own libc

[anand-msft-fun]

How do I cross compile (for mips/arm) with my own version of libc using its .a and headers?
I'd like to disable features like exceptions, replace new/delete, no gnu extension etc

[phillipjohnston]

Some notes:

• You can already disable exceptions (among other things) as a project option: https://github.com/embeddedartistry/libcpp/blob/master/meson_options.txt
• The easiest ways to supply a libc are: using the libc-subproject option provided in the list above, or by supplying a cross-file when configuring the meson project (like this one) . In both cases you'll need a meson project wrapper for your libc so that it can be included as a subproject, with a dependency target to reference. If you just had the precompiled .a and headers, you could create a simple meson wrapper that would define a dependency target which specifies the link arguments and include directory path to point to those outputs.
• You can supply your own cross files to use for cross-compiling other projects. Look into the meson-buildsystem/cross folder for examples. I tend to use a layered approach: I have a base toolchain file (arm.txt, avr.txt, etc.), a processor-specific file with flags/settings/etc., and layer on any others I might need for specialization. But you could do it all in one file as well for convenience.
• I don't think this library requires GNU extensions; certainly my libc doesn't turn them on by default.

[anand-msft-fun]

@phillipjohnston Thanks for the quick response. I've been going through the meson.build file and the options to understand how to do it. Your tips will be of great help! Much appreciated!

I saw _GNU_SOURCE defined in the build file and was wondering if it was enabling some GNU stuff. With your libc's config as example where you've turned it off, I'll try to understand more of it. I'll reach out if I have more questions.

[phillipjohnston]

Thanks, that detail slipped my mind. I do recall now vaguely thinking it was easier to just go that route than rework the library to break all the deps, but it's been a few years so the specific details are fuzzy n ow.

[anand-msft-fun]

Can you explain more about what you mean it's easier to go that route? Keep _GNU_SOURCE defined?

From your libc's config I believe, with a libc built without_GNU_SOURCE , I should be able to build libcpp as it is in master branch.

However I see at
https://stackoverflow.com/questions/11670581/why-is-gnu-source-defined-by-default-and-how-to-turn-it-off
that this flag needs to be enabled for cpp. So am at a loss here. (Is this what you meant by avoiding rework? It's more effort to get rid of it?)

So libc can be built without _GNU_SOURCE but libcpp can't?

Anyway, I went ahead and built libcpp by itself without libc subproject and it built fine. After your tip, I made this following change in the build options and now it won't build.

diff --git a/meson_options.txt b/meson_options.txt
index 70a5475..5e1bbf6 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -19,7 +19,7 @@ option('libcxx-enable-stdinout', type: 'boolean', value: true, yield: true)
 option('libcxx-default-newdelete', type: 'boolean', value: true, yield: true)
 option('libcxx-silent-terminate', type: 'boolean', value: true, yield: true)
 option('libcxx-monotonic-clock', type: 'boolean', value: true, yield: true)
-option('use-libc-subproject', type: 'boolean', value: false, yield: true,
+option('use-libc-subproject', type: 'boolean', value: true, yield: true,
     description: 'When true, use the subproject defined in the libc-subproject option. An alternate approach is to override c_stdlib in your cross files.')
 option('libc-subproject', type: 'array',
     value: ['libc', 'libc_dep', 'libc_hosted_native_dep'],

Snippet of the error:

-o libc++abi.a.p/libcxxabi_src_fallback_malloc.cpp.o -c ../libcxxabi/src/fallback_malloc.cpp
In file included from /usr/include/pthread.h:21,
                 from include/c++/__threading_support:27,
                 from ../libcxxabi/src/fallback_malloc.cpp:14:
/usr/include/features.h:461:12: fatal error: sys/cdefs.h: No such file or directory
  461 | #  include <sys/cdefs.h>
      |            ^~~~~~~~~~~~~

btw, the below config is sorta what am aiming for. I might disable more. but this's what am starting with.

diff --git a/meson_options.txt b/meson_options.txt
index 70a5475..93d9045 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -1,25 +1,25 @@
 option('force-32-bit', type: 'boolean', value: false, yield: true,
     description: 'Force 32-bit compilation for libcpp')
-option('disable-rtti', type : 'boolean', value: false, yield: true)
-option('disable-exceptions', type : 'boolean', value: false, yield: true)
+option('disable-rtti', type : 'boolean', value: true, yield: true)
+option('disable-exceptions', type : 'boolean', value: true, yield: true)
 option('enable-pedantic', type: 'boolean', value: false, yield: true,
     description: 'Enable pedantic warnings')
 option('enable-pedantic-error', type: 'boolean', value: false, yield: true,
     description: 'Enable pedantic warnings and make them errors.')
-option('enable-threading', type: 'boolean', value: true, yield: true)
+option('enable-threading', type: 'boolean', value: false, yield: true)
 option('libcxx-use-compiler-rt', type: 'boolean', value: false, yield: true,
     description: 'Use compiler-rt to supply builtin functionality')
 option('libcxx-use-llvm-libunwind', type: 'boolean', value: true, yield: true)
-option('libcxx-thread-library', type: 'combo', choices: ['', 'none', 'pthread', 'ea-framework'], value: 'pthread', yield: true)
+option('libcxx-thread-library', type: 'combo', choices: ['', 'none', 'pthread', 'ea-framework'], value: 'none', yield: true)
 option('libcxx-has-external-thread-api', type: 'boolean', value: false, yield: true)
 option('libcxx-build-external-thread-api', type: 'boolean', value: false, yield: true)
 option('libcxx-enable-chrono', type: 'boolean', value: true, yield: true)
 option('libcxx-enable-filesystem', type: 'boolean', value: false, yield: true)
 option('libcxx-enable-stdinout', type: 'boolean', value: true, yield: true)
-option('libcxx-default-newdelete', type: 'boolean', value: true, yield: true)
+option('libcxx-default-newdelete', type: 'boolean', value: false, yield: true)
 option('libcxx-silent-terminate', type: 'boolean', value: true, yield: true)
 option('libcxx-monotonic-clock', type: 'boolean', value: true, yield: true)
-option('use-libc-subproject', type: 'boolean', value: false, yield: true,
+option('use-libc-subproject', type: 'boolean', value: true, yield: true,
     description: 'When true, use the subproject defined in the libc-subproject option. An alternate approach is to override c_stdlib in your cross files.')
 option('libc-subproject', type: 'array',
     value: ['libc', 'libc_dep', 'libc_hosted_native_dep'],