CODEWITHSUNDEEP
gomemory-managementormgarbage-collectionpersistence
Sally Goldin
Sally Goldin

Reputation: 61

Golang: are global variables protected from garbage collection?

I'm fairly new to Golang. I'm working on an application that builds an in-memory object-oriented data model (basically an ORM) to support the application functionality. I realize this isn't really idiomatic Go but it makes sense in this situation.

All my core objects are allocated on the heap then stored in global (though not necessarily exported) map structures that allow the code to look them up based on database IDs. Objects that reference instances of other objects have pointer fields in their structure definitions.

I was under the impression that any data that can be reached from a global variable is protected from being garbage collected. However, I am seeing intermittent cases of pointer references apparently becoming nil over time. If I restart the application, and rebuild the object model, then try the same operation, the problem disappears.

Is GC freeing my memory out from under me? Or should I look elsewhere to understand this problem? And if the answer to my first question is yes... how can I stop this from happening?

Upvotes: 0

Views: 1803

Answers (1)

icza
icza

Reputation: 417757

The garbage collector does not free memory as long as it is reachable. Global or package level variables are accessible during the whole lifetime of your app, so they can't be freed by the GC.

If you see the opposite, that is definitely a bug / mistake on your part (unless the Go runtime itself has a bug). For example you may have a data race initializing / accessing your global variables, or you (or some library you use) may use package unsafe or the uintptr type incorrectly. For example, quoting from unsafe.Pointer:

A uintptr is an integer, not a reference. Converting a Pointer to a uintptr creates an integer value with no pointer semantics. Even if a uintptr holds the address of some object, the garbage collector will not update that uintptr's value if the object moves, nor will that uintptr keep the object from being reclaimed.

Upvotes: 2

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