Modular•2mo ago

Is there something I’m not understanding about the Mojo memory model?

I hope this is an easy question and someone will know the obvious thing I'm doing wrong. I have this minimal example code that demonstrates an issue I’m having. main.mojo:

from command_line_mod import process_command_line_args

fn init_system():

  process_command_line_args()

fn main():
  init_system()

from command_line_mod import process_command_line_args

fn init_system():

  process_command_line_args()

fn main():
  init_system()

command_line_mod.mojo:

var bundle_file_name: String = "test"

fn process_command_line_args():

  print("bundle_file_name = ", bundle_file_name)

var bundle_file_name: String = "test"

fn process_command_line_args():

  print("bundle_file_name = ", bundle_file_name)

If I build and run this with mojo build main.mojo; ./main I get a segmentation fault. Why?

10 Replies

ModularBot•2mo ago

Congrats @nairby, you just advanced to level 1!

sb•2mo ago

Global variables are currently a bit of a sharp edge and aren't really supported 🙃 It's not an intentional omission, it's just a feature that needs some work to make happen that should work eventually roughly "as-is" once the feature lands

nairbyOP•2mo ago

Ah that explains why some of the global var use was working and some wasn't. Thanks

nairbyOP•2mo ago

Is there a rough timeline of when that might expected to be supported? Is this poor program design and therefore low priority? Considering migrating a Fortran application to mojo (in the future) so my ideas on storing data in global vars in modules may be outdated

sb•2mo ago

globals are definitely still useful, they are just difficult to deal with and init at times so it hasn't been at the top of the priority list yet I think you can try moving stuff into a "globals" object you pass around through the call tree

sora•2mo ago

And if your global data is imutable, you could use alias

sb•2mo ago

depending on how you treat it you can avoid most issues with the borrow checker

nairbyOP•2mo ago

Hmm ok interesting, I'll have to look into that more. Thanks again

sora•2mo ago

saying monad without saying monad?

sb•2mo ago

:squints: oh, yeah, huh lol

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Is there something I’m not understanding about the Mojo memory model?