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5.4.2 Garbage Collection

As explained above, the SCM type can represent all Scheme values. Some values fit entirely into a SCM value (such as small integers), but other values require additional storage in the heap (such as strings and vectors). This additional storage is managed automatically by Guile. You don’t need to explicitly deallocate it when a SCM value is no longer used.

Two things must be guaranteed so that Guile is able to manage the storage automatically: it must know about all blocks of memory that have ever been allocated for Scheme values, and it must know about all Scheme values that are still being used. Given this knowledge, Guile can periodically free all blocks that have been allocated but are not used by any active Scheme values. This activity is called garbage collection.

It is easy for Guile to remember all blocks of memory that it has allocated for use by Scheme values, but you need to help it with finding all Scheme values that are in use by C code.

You do this when writing a SMOB mark function, for example (see section Garbage Collecting Smobs). By calling this function, the garbage collector learns about all references that your SMOB has to other SCM values.

Other references to SCM objects, such as global variables of type SCM or other random data structures in the heap that contain fields of type SCM, can be made visible to the garbage collector by calling the functions scm_gc_protect or scm_permanent_object. You normally use these functions for long lived objects such as a hash table that is stored in a global variable. For temporary references in local variables or function arguments, using these functions would be too expensive.

These references are handled differently: Local variables (and function arguments) of type SCM are automatically visible to the garbage collector. This works because the collector scans the stack for potential references to SCM objects and considers all referenced objects to be alive. The scanning considers each and every word of the stack, regardless of what it is actually used for, and then decides whether it could possibly be a reference to a SCM object. Thus, the scanning is guaranteed to find all actual references, but it might also find words that only accidentally look like references. These ‘false positives’ might keep SCM objects alive that would otherwise be considered dead. While this might waste memory, keeping an object around longer than it strictly needs to is harmless. This is why this technique is called “conservative garbage collection”. In practice, the wasted memory seems to be no problem.

The stack of every thread is scanned in this way and the registers of the CPU and all other memory locations where local variables or function parameters might show up are included in this scan as well.

The consequence of the conservative scanning is that you can just declare local variables and function parameters of type SCM and be sure that the garbage collector will not free the corresponding objects.

However, a local variable or function parameter is only protected as long as it is really on the stack (or in some register). As an optimization, the C compiler might reuse its location for some other value and the SCM object would no longer be protected. Normally, this leads to exactly the right behavior: the compiler will only overwrite a reference when it is no longer needed and thus the object becomes unprotected precisely when the reference disappears, just as wanted.

There are situations, however, where a SCM object needs to be around longer than its reference from a local variable or function parameter. This happens, for example, when you retrieve some pointer from a smob and work with that pointer directly. The reference to the SCM smob object might be dead after the pointer has been retrieved, but the pointer itself (and the memory pointed to) is still in use and thus the smob object must be protected. The compiler does not know about this connection and might overwrite the SCM reference too early.

To get around this problem, you can use scm_remember_upto_here_1 and its cousins. It will keep the compiler from overwriting the reference. For a typical example of its use, see Remembering During Operations.

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