When an extension allocates resources that must be explicitly freed (in the same way that a port must be explicitly closed), a Scheme object associated with the resource should be placed into the management of the current custodian with scheme_add_managed.

Before allocating the resource, call scheme_custodian_check_available to ensure that the relevant custodian is not already shut down. If it is, scheme_custodian_check_available will raise an exception. If the custodian is shut down when scheme_add_managed is called, the close function provided to scheme_add_managed will be called immediately, and no exception will be reported.

16.1  Library Functions

¤ Scheme_Custodian *scheme_make_custodian(Scheme_Custodian *m)

Creates a new custodian as a subordinate of m. If m is NULL, then the main custodian is used as the new custodian's supervisor. Do not use NULL for m unless you intend to create an especially privileged custodian.

¤ Scheme_Custodian_Reference *scheme_add_managed(Scheme_Custodian *m, Scheme_Object *o,
    Scheme_Close_Custodian_Client *f, void *data,
    int strong)

Places the value o into the management of the custodian m. If m is NULL, the current custodian is used.

The f function is called by the custodian if it is ever asked to ``shutdown'' its values; o and data are passed on to f, which has the type

typedef void (*Scheme_Close_Custodian_Client)(Scheme_Object *o, void *data);

If strong is non-zero, then the newly managed value will be remembered until either the custodian shuts it down or scheme_remove_managed is called. If strong is zero, the value is allowed to be garbaged collected (and automatically removed from the custodian).

The return value from scheme_add_managed can be used to refer to the value's custodian later in a call to scheme_remove_managed. A value can be registered with at most one custodian.

If m (or the current custodian if m is NULL)is shut down, then f is called immediately, and the result is NULL.

¤ void scheme_custodian_check_available(Scheme_Custodian *m, const char *name,
const char *resname)

Checks whether m is already shut down, and raises an error if so. If m is NULL, the current custodian is used. The name argument is used for error reporting. The resname argument will likely be used for checking pre-set limits in the future; pre-set limits will have symbolic names, and the resname string will be compared to the symbols.

¤ void scheme_remove_managed(Scheme_Custodian_Reference *mref, Scheme_Object *o)

Removes o from the management of its custodian. The mref argument must be a value returned by scheme_add_managed or NULL.

¤ void scheme_close_managed(Scheme_Custodian *m)

Instructs the custodian m to shutdown all of its managed values.

¤ void scheme_add_atexit_closer(Scheme_Exit_Closer_Func f)

Installs a function to be called on each custodian-registered item and its closer when MzScheme is about to exit. The registered function has the type

   typedef void (*Scheme_Exit_Closer_Func)(Scheme_Object *o,
                                           Scheme_Close_Custodian_Client *f, 
                                           void *d);

where d is the second argument for f.