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This means that the memory used to store a Scheme string, say, is |
This means that the memory used to store a Scheme string, say, is |
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automatically reclaimed when no one is using this string any longer. |
automatically reclaimed when no one is using this string any longer. |
8 |
This can work because Guile knows enough about its objects at run-time |
This can work because Guile knows enough about its objects at run-time |
9 |
to be able to trace all references between them. Thus, it can find |
to be able to trace all references between them. Thus, it can find all |
10 |
all 'live' objects (objects that are still in use) by starting from a |
'live' objects (objects that are still in use) by starting from a known |
11 |
known set of 'root' objects and following the links that these objects |
set of 'root' objects and following the links that these objects have to |
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have to other objects, and so on. The objects that are not reached by |
other objects, and so on. The objects that are not reached by this |
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this recursive process can be considered 'dead' and their memory can |
recursive process can be considered 'dead' and their memory can be |
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be reused for new objects. |
reused for new objects. |
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When you are programming in Scheme, you don't need to worry about the |
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garbage collector. When programming in C, there are a few rules that |
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you must follow so that the garbage collector can do its job. |
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@menu |
@menu |
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* Garbage Collection:: |
* Garbage Collection:: |
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@node Garbage Collection |
@node Garbage Collection |
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@section Garbage Collection |
@section Garbage Collection |
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|
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The general process of collecting dead objects outlined above relies on |
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the fact that the garbage collector is able to find all references to |
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SCM objects that might be used by the program in the future. When you |
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are programming in Scheme, you don't need to worry about this: The |
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collector is automatically aware of all objects in use by Scheme code. |
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When programming in C, you must help the garbage collector a bit so that |
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it can find all objects that are accessible from C. You do this when |
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writing a SMOB mark function, for example. By calling this function, |
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the garbage collector learns about all references that your SMOB has to |
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other SCM objects. |
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Other references to SCM objects, such as global variables of type SCM or |
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other random data structures in the heap that contain fields of type |
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SCM, can be made visible to the garbage collector by calling the |
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functions @code{scm_gc_protect} or @code{scm_permanent_object}. You |
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normally use these funtions for long lived objects such as a hash table |
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that is stored in a global variable. For temporary references in local |
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variables or function arguments, using these functions would be too |
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expensive. |
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These references are handled differently: Local variables (and function |
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arguments) of type SCM are automatically visible to the garbage |
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collector. This works because the collector scans the stack for |
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potential references to SCM objects and considers all referenced objects |
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to be alive. The scanning considers each and every word of the stack, |
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regardless of what it is actually used for, and then decides whether it |
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could possible be a reference to a SCM object. Thus, the scanning is |
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guaranteed to find all actual references, but it might also find words |
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that only accidentally look like references. These `false positives' |
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might keep SCM objects alive that would otherwise be considered dead. |
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While this might waste memory, keeping an object around longer than it |
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strictly needs to is harmless. This is why this technique is called |
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``conservative garbage collection''. In practice, the wasted memory |
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seems to be no problem. |
62 |
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The stack of every thread is scanned in this way and the registers of |
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the CPU and all other memory locations where local variables or function |
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parameters might show up are included in this scan as well. |
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The consequence of the conservative scanning is that you can just |
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declare local variables and function parameters of type SCM and be sure |
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that the garbage collector will not free the corresponding objects. |
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However, a local variable or function parameter is only protected as |
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long as it is really on the stack (or in some register). As an |
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optimization, the C compiler might reuse its location for some other |
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value and the SCM object would no longer be protected. Normally, this |
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leads to exactly the right behabvior: the compiler will only overwrite a |
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reference when it is no longer needed and thus the object becomes |
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unprotected precisely when the reference disappears, just as wanted. |
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|
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There are situations, however, where a SCM object needs to be around |
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longer than its reference from a local variable or function parameter. |
81 |
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This happens, for example, when you retrieve the array of characters |
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from a Scheme string and work on that array directly. The reference to |
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the SCM string object might be dead after the character array has been |
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retrieved, but the array itself is still in use and thus the string |
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object must be protected. The compiler does not know about this |
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connection and might overwrite the SCM reference too early. |
87 |
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To get around this problem, you can use @code{scm_remember_upto_here_1} |
89 |
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and its cousins. It will keep the compiler from overwriting the |
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reference. For an example of its use, see @ref{Remembering During |
91 |
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Operations}. |
92 |
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|
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@deffn {Scheme Procedure} gc |
@deffn {Scheme Procedure} gc |
94 |
@deffnx {C Function} scm_gc () |
@deffnx {C Function} scm_gc () |
95 |
Scans all of SCM objects and reclaims for further use those that are |
Scans all of SCM objects and reclaims for further use those that are |
97 |
explicitly. It is called automatically when appropriate. |
explicitly. It is called automatically when appropriate. |
98 |
@end deffn |
@end deffn |
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|
100 |
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@deftypefn {C Function} SCM scm_gc_protect_object (SCM @var{obj}) |
101 |
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Protects @var{obj} from being freed by the garbage collector, when it |
102 |
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otherwise might be. When you are done with the object, call |
103 |
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@code{scm_gc_unprotect_object} on the object. Calls to |
104 |
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@code{scm_gc_protect}/@code{scm_gc_unprotect_object} can be nested, and |
105 |
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the object remains protected until it has been unprotected as many times |
106 |
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as it was protected. It is an error to unprotect an object more times |
107 |
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than it has been protected. Returns the SCM object it was passed. |
108 |
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@end deftypefn |
109 |
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110 |
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@deftypefn {C Function} SCM scm_gc_unprotect_object (SCM @var{obj}) |
111 |
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|
112 |
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Unprotects an object from the garbage collector which was protected by |
113 |
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@code{scm_gc_unprotect_object}. Returns the SCM object it was passed. |
114 |
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@end deftypefn |
115 |
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116 |
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@deftypefn {C Function} SCM scm_permanent_object (SCM @var{obj}) |
117 |
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|
118 |
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Similar to @code{scm_gc_protect_object} in that it causes the |
119 |
|
collector to always mark the object, except that it should not be |
120 |
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nested (only call @code{scm_permanent_object} on an object once), and |
121 |
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it has no corresponding unpermanent function. Once an object is |
122 |
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declared permanent, it will never be freed. Returns the SCM object it |
123 |
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was passed. |
124 |
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@end deftypefn |
125 |
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|
126 |
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@c NOTE: The varargs scm_remember_upto_here is deliberately not |
127 |
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@c documented, because we don't think it can be implemented as a nice |
128 |
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@c inline compiler directive or asm block. New _3, _4 or whatever |
129 |
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@c forms could certainly be added though, if needed. |
130 |
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|
131 |
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@deftypefn {C Macro} void scm_remember_upto_here_1 (SCM obj) |
132 |
|
@deftypefnx {C Macro} void scm_remember_upto_here_2 (SCM obj1, SCM obj2) |
133 |
|
Create a reference to the given object or objects, so they're certain |
134 |
|
to be present on the stack or in a register and hence will not be |
135 |
|
freed by the garbage collector before this point. |
136 |
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|
137 |
|
Note that these functions can only be applied to ordinary C local |
138 |
|
variables (ie.@: ``automatics''). Objects held in global or static |
139 |
|
variables or some malloced block or the like cannot be protected with |
140 |
|
this mechanism. |
141 |
|
@end deftypefn |
142 |
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|
143 |
@deffn {Scheme Procedure} gc-stats |
@deffn {Scheme Procedure} gc-stats |
144 |
@deffnx {C Function} scm_gc_stats () |
@deffnx {C Function} scm_gc_stats () |
145 |
Return an association list of statistics about Guile's current |
Return an association list of statistics about Guile's current |