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jabamaus
Nov 2, 2024

Hi,

My MRuby::Build looks like this:

MRuby::Build.new do |conf| conf.toolchain conf.enable_cxx_exception pdb = "/Fd\"#{ENV['MRUBY_PDB']}\"" conf.cc {|c| c.flags << pdb} conf.cxx {|c| c.flags << pdb} GEMS.each { |g| conf.gem :core => g } DEFINES.each { |d| conf.defines << d } end

How could I modify this to register some additional source files for presym scanning? I don't want them to take part in the build in any way except to add some new symbols. I was expecting something like:

config.presym do |p| p.src_files << "my_src_file" end

Also is it possible to manually inject specific symbols directly into the build from here, without needing to scan source files?

I've messed about for a while but I'm not getting anywhere...

Thank you!
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Replies: 5 comments · 2 replies

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dearblue
Nov 4, 2024

I too have explored methods for adding supplemental presyms in the period before mruby 3.1.0.
But I remember that it was not possible to do it in the usual way.
If it really is needed, a mechanism probably needs to be added for that.

If you are already writing a gem and using C in it, there is a workaround for now.
Take advantage of the fact that MRB_PRESYM_SCANNING is defined when the mruby build system collects presym from the gem's C source code.

void
mrb_mruby_MYGEM_gem_init(mrb_state *mrb)
{
#ifdef MRB_PRESYM_SCANNING
  MRB_SYM(extra_symbol1);
  MRB_SYM(extra_symbol2);
  mrb_intern_lit(mrb, "$global_variable");
  mrb_intern_lit(mrb, "\x01\x02\x03");
#endif

  ...
}
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dearblue
Nov 4, 2024

psst!

These are not official practices. They may not be available in the future.

  • (approach 1) Specify a file in the build configuration file that contains an additional presym.

    Interrupt the task with the file task method and intervene with the Binding#local_variable_get method to the local variable ppps.
    The ppps variable is passed to presym.scan.

    mruby/tasks/presym.rake

    Line 36 in 4845575

    presyms = presym.scan(ppps) 

    MRuby::Build.new do
  ...

  mypresyms = "path/to/extra-presyms.txt"
  presym_path = File.join(self.build_dir, "presym")
  file presym_path => mypresyms do |t|
    e = t.actions[1].binding
    e.local_variable_get(:ppps) << mypresyms
  end
end

    As for the contents of the extra-presyms.txt file, symbols must be enclosed in <@! " and " !@>.

    <@! "extra_symbol1" !@>
<@! "extra_symbol2" !@>
<@! "$global_variable" !@>
<@! "\x01\x02\x03" !@>
  • (approach 2) Interrupt the `presym.scan` method.

    In the block of MRuby::Build.new, presym is still undefined.
    Interrupt MRuby::Build#define_rules to interrupt the scan method of MRuby::Build#presym.

    MRuby::Build.new do
  ...

  def self.define_rules
    super

    class << self.presym
      def scan(*args)
        presyms = super(*args)
        presyms << "extra_symbol1"
        presyms << "extra_symbol2"
        presyms << "$global_variable"
        presyms << "\x01\x02\x03"
        presyms.uniq!
        presyms.sort_by! { |e| [e.bytesize, e] }
        presyms
      end
    end
  end
end
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1 reply
@jabamaus
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jabamaus Nov 4, 2024
Author

Thanks dear blue, you are most helpful and I appreciate it 😃 I am writing an app that embeds mruby not a gem. Strange that this is not officially supported. I would probably go with option 2 in the meantime. Thank you!
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jabamaus
Nov 10, 2024
Author

Is there a reason global variables are not supported in presym? When I add eg $GLOBAL to presym tokens it appears in table.h but not in id.h. I note that globals do not appear in:

SYMBOL_TO_MACRO = {
#      Symbol      =>      Macro
# [prefix, suffix] => [prefix, suffix]
  ["@@"  , ""    ] => ["CV"  , ""    ],
  ["@"   , ""    ] => ["IV"  , ""    ],
  [""    , "!"   ] => [""    , "_B"  ],
  [""    , "?"   ] => [""    , "_Q"  ],
  [""    , "="   ] => [""    , "_E"  ],
  [""    , ""    ] => [""    , ""    ],
}.freeze
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matz
Nov 11, 2024
Maintainer

The reason is that the number of symbols used in the application and the symbols corresponding to global variables is small, so we thought the impact would be small. Another reason is that presym scan and allocation are done during the libmruby build, so it was difficult to apply to symbols in the app.
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@jabamaus
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jabamaus Nov 11, 2024
Author

I'm happily adding my app symbols into presym as per dearblue's suggestion:

MRuby::Build.new do
...

def self.define_rules
super

class << self.presym
  def scan(*args)
    presyms = super(*args)
    presyms << "extra_symbol1"
    presyms << "extra_symbol2"
    presyms << "$global_variable"
    presyms << "\x01\x02\x03"
    presyms.uniq!
    presyms.sort_by! { |e| [e.bytesize, e] }
    presyms
  end
end

end
end

The only problem is that it doesn't work with global variables. It doesn't look hard to, maybe I'll have a go.

I think the above code should be available via something like
conf.presym do |ps|
ps.symbols << "extra_symbol1"
end

As it seems like it is a generally useful thing.
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pusewicz
Aug 20, 2025

Yeah, having the ability to add extra presyms symbols when working with an already built libmruby.a would be ideal.
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