Best Practices for Ownership in GLib

The GLib ecosystem provides a sophisticated framework for managing object ownership in C, transforming what would be error-prone manual memory management into a more declarative and readable system. This approach addresses one of C's most persistent challenges: the ambiguity of ownership when functions return or receive pointers. Without clear conventions, developers must rely on documentation to determine whether they should free a returned object or if it remains owned by the callee—a situation that frequently leads to memory leaks or use-after-free errors.

GLib's solution centers on the g_auto family of macros, which associate cleanup functions with types through G_DEFINE_AUTOPTR_CLEANUP_FUNC. When a variable is declared with g_autoptr(MyThing) thing = my_thing_new();, the compiler automatically ensures my_thing_release(thing) is called when the variable goes out of scope. This eliminates the need for complex control flow patterns like single-exit functions or goto cleanup blocks, while simultaneously serving as a visual annotation that the variable owns the object. The runtime overhead is negligible, and the readability benefit is substantial: ownership becomes explicit in the code itself rather than hidden in comments.

Ownership transfer presents a more subtle challenge. When a function takes ownership of a parameter, the caller must relinquish control to avoid double-free errors. With manual management, this is indistinguishable from a function that merely references the object. GLib's g_steal_pointer macro makes this transfer explicit: my_thing_finish_thing(g_steal_pointer(&thing)) clearly indicates that ownership has moved from the variable to the function. This explicitness prevents the common mistake of calling cleanup on an object that has already been freed elsewhere.

Scoping strategies further refine ownership management. Rather than declaring all variables at function scope—a pattern inherited from older C conventions—developers should limit variable lifetimes to natural scopes. This reduces the window during which objects are held and makes ownership relationships clearer. For example, introducing a block scope for a temporary object that's only needed to produce another object prevents that temporary from lingering unnecessarily.

The introduction of libdex fibers adds complexity to ownership management. Fibers enable asynchronous code that appears synchronous, using dex_await points that suspend execution. However, these suspension points create a critical vulnerability: any object not owned across an await point may be destroyed while the fiber is suspended, leaving a dangling pointer when execution resumes. The solution requires either taking ownership of objects needed across await points or re-acquiring them after suspension.

For long-running worker fibers, taking ownership can create a circular dependency where the fiber holds a reference to an object that should be disposed, preventing proper cleanup. The correct pattern uses weak references to re-acquire objects after each await, allowing the object to be destroyed when no longer needed while the fiber continues to run. This approach balances the need for valid references with proper resource management.

The broader lesson is that ownership annotations serve dual purposes: they enable automatic cleanup while simultaneously documenting programmer intent. This makes code more maintainable and less error-prone, particularly in complex asynchronous systems where object lifetimes span multiple execution contexts. By treating g_auto and g_steal as ownership annotations rather than mere cleanup mechanisms, developers can build more robust GLib applications that are easier to understand and modify.

For further exploration of these patterns, consult the GLib documentation on automatic cleanup functions and the libdex documentation on fibers and async patterns.