Document how CRYPTO_EX_DATA stuff works for

RSA structures. Other structures behave in
a similar way.

doc/crypto/RSA_get_ex_new_index.pod

@@ -2,26 +2,114 @@
 
 =head1 NAME
 
-RSA_get_ex_new_index, RSA_set_ex_data, RSA_get_ex_data - ...
+RSA_get_ex_new_index, RSA_set_ex_data, RSA_get_ex_data - add application specific data to RSA structures.
 
 =head1 SYNOPSIS
 
  #include <openssl/rsa.h>
 
- int RSA_get_ex_new_index(long argl, char *argp, int (*new_func)(),
-    int (*dup_func)(), void (*free_func)());
+ int RSA_get_ex_new_index(long argl, void *argp,
+		CRYPTO_EX_new *new_func,
+		CRYPTO_EX_dup *dup_func,
+		CRYPTO_EX_free *free_func);
+
+ int RSA_set_ex_data(RSA *r,int idx,void *arg);
+
+ void *RSA_get_ex_data(RSA *r, int idx);
+
+ int new_func(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
+					int idx, long argl, void *argp);
+
+ void free_func(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
+					int idx, long argl, void *argp);
+ int dup_func(CRYPTO_EX_DATA *to, CRYPTO_EX_DATA *from, void *from_d, 
+					int idx, long argl, void *argp);
 
- int RSA_set_ex_data(RSA *r,int idx,char *arg);
 
- char *RSA_get_ex_data(RSA *r, int idx);
 
 =head1 DESCRIPTION
 
-...
+Several OpenSSL structures can have application specific data attached to them.
+This has several potential uses, it can be used to cache data associated with
+a structure (for example the hash of some part of the structure) or some
+additional data (for example a handle to the data in an external library).
+
+Since the application data can be anything at all it is passed and retrieved
+as a B<void *> type.
+
+The B<RSA_get_ex_new_index()> function is initially called to "register" some
+new application specific data. It takes three optional function pointers which
+are called when the parent structure (in this case an RSA structure) is
+initially created, when it is copied and when it is freed up. If any or all of
+these function pointer arguments are not used they should be set to NULL. The
+precise manner in which these function pointer are called is described in more
+detail below. B<RSA_get_ex_new_index()> also takes additional long and pointer
+parameters which will be passed to the supplied functions but which otherwise
+have no special meaning. It returns an B<index> which should be stored
+(typically in a static variable) and passed used in the B<idx> parameter in
+the remaining functions. Each successful call to B<RSA_get_ex_new_index()>
+will return an index greater than any previously returned, this is important
+because the optional functions are called in order of increasing index value.
+
+B<RSA_set_ex_data()> is used to set application specific data, the data is
+supplied in the B<arg> parameter and its precise meaning is up to the
+application.
+
+B<RSA_get_ex_data()> is used to retrieve application specific data. The data
+is returned to the application, this will be the same value as supplied to
+a previous B<RSA_set_ex_data()> call.
+
+B<new_func()> is called when a structure is initially allocated (for example
+with B<RSA_new()>. The parent structure members will not have any meaningful
+values at this point. This function will typically be used to allocate any
+application specific structure.
+
+B<free_func()> is called when a structure is being freed up. The dynamic parent
+structure members should not be accessed because they will be freed up when
+this function is called.
+
+B<new_func()> and B<free_func()> take the same parameters. B<parent> is a
+pointer to the parent RSA structure. B<ptr> is a the application specific data
+(this wont be of much use in B<new_func()>. B<ad> is a pointer to the
+B<CRYPTO_EX_DATA> structure from the parent RSA structure: the functions
+B<CRYPTO_get_ex_data()> and B<CRYPTO_get_ex_data()> can be called to manipulate
+it. The B<idx> parameter is the index: this will be the same value returned by
+B<RSA_get_ex_new_index()> when the functions were initially registered. Finally
+the B<argl> and B<argp> parameters are the values originally passed to the same
+corresponding parameters when B<RSA_get_ex_new_index()> was called.
+
+B<dup_func()> is called when a structure is being copied. Pointers to the
+destination and source B<CRYPTO_EX_DATA> structures are passed in the B<to> and
+B<from> parameters respectively. The B<from_d> parameter is passed a pointer to
+the source application data when the function is called, when the function returns
+the value is copied to the destination: the application can thus modify the data
+pointed to by B<from_d> and have different values in the source and destination.
+The B<idx>, B<argl> and B<argp> parameters are the same as those in B<new_func()>
+and B<free_func()>.
 
 =head1 RETURN VALUES
 
-...
+B<RSA_get_ex_new_index()> returns a new index or -1 on failure (note 0 is a valid
+index value).
+
+B<RSA_set_ex_data()> returns 1 on success or 0 on failure.
+
+B<RSA_get_ex_data()> returns the application data or 0 on failure. 0 may also
+be valid application data but currently it can only fail if given an invalid B<idx>
+parameter.
+
+B<new_func()> and B<dup_func()> should return 0 for failure and 1 for success.
+
+On failure an error code can be obtained from B<ERR_get_error(3)>.
+
+=head1 BUGS
+
+B<dup_func()> is currently never called.
+
+The return value of B<new_func()> is ignored.
+
+The B<new_func()> function isn't very useful because no meaningful values are
+present in the parent RSA structure when it is called.
 
 =head1 SEE ALSO