1=pod
2
3=head1 NAME
4
5CRYPTO_EX_new, CRYPTO_EX_free, CRYPTO_EX_dup,
6CRYPTO_free_ex_index, CRYPTO_get_ex_new_index, CRYPTO_set_ex_data,
7CRYPTO_get_ex_data, CRYPTO_free_ex_data, CRYPTO_new_ex_data
8- functions supporting application-specific data
9
10=head1 SYNOPSIS
11
12 #include <openssl/crypto.h>
13
14 int CRYPTO_get_ex_new_index(int class_index,
15                             long argl, void *argp,
16                             CRYPTO_EX_new *new_func,
17                             CRYPTO_EX_dup *dup_func,
18                             CRYPTO_EX_free *free_func);
19
20 typedef void CRYPTO_EX_new(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
21                            int idx, long argl, void *argp);
22 typedef void CRYPTO_EX_free(void *parent, void *ptr, CRYPTO_EX_DATA *ad,
23                             int idx, long argl, void *argp);
24 typedef int CRYPTO_EX_dup(CRYPTO_EX_DATA *to, const CRYPTO_EX_DATA *from,
25                           void *from_d, int idx, long argl, void *argp);
26
27 int CRYPTO_new_ex_data(int class_index, void *obj, CRYPTO_EX_DATA *ad)
28
29 int CRYPTO_set_ex_data(CRYPTO_EX_DATA *r, int idx, void *arg);
30
31 void *CRYPTO_get_ex_data(CRYPTO_EX_DATA *r, int idx);
32
33 void CRYPTO_free_ex_data(int class_index, void *obj, CRYPTO_EX_DATA *r);
34
35 int CRYPTO_free_ex_index(int class_index, int idx);
36
37=head1 DESCRIPTION
38
39Several OpenSSL structures can have application-specific data attached to them,
40known as "exdata."
41The specific structures are:
42
43    APP
44    BIO
45    DH
46    DRBG
47    DSA
48    EC_KEY
49    ENGINE
50    RSA
51    SSL
52    SSL_CTX
53    SSL_SESSION
54    UI
55    UI_METHOD
56    X509
57    X509_STORE
58    X509_STORE_CTX
59
60Each is identified by an B<CRYPTO_EX_INDEX_xxx> define in the B<crypto.h>
61header file.  In addition, B<CRYPTO_EX_INDEX_APP> is reserved for
62applications to use this facility for their own structures.
63
64The API described here is used by OpenSSL to manipulate exdata for specific
65structures.  Since the application data can be anything at all it is passed
66and retrieved as a B<void *> type.
67
68The B<CRYPTO_EX_DATA> type is opaque.  To initialize the exdata part of
69a structure, call CRYPTO_new_ex_data(). This is only necessary for
70B<CRYPTO_EX_INDEX_APP> objects.
71
72Exdata types are identified by an B<index>, an integer guaranteed to be
73unique within structures for the lifetime of the program.  Applications
74using exdata typically call B<CRYPTO_get_ex_new_index> at startup, and
75store the result in a global variable, or write a wrapper function to
76provide lazy evaluation.  The B<class_index> should be one of the
77B<CRYPTO_EX_INDEX_xxx> values. The B<argl> and B<argp> parameters are saved
78to be passed to the callbacks but are otherwise not used.  In order to
79transparently manipulate exdata, three callbacks must be provided. The
80semantics of those callbacks are described below.
81
82When copying or releasing objects with exdata, the callback functions
83are called in increasing order of their B<index> value.
84
85If a dynamic library can be unloaded, it should call CRYPTO_free_ex_index()
86when this is done.
87This will replace the callbacks with no-ops
88so that applications don't crash.  Any existing exdata will be leaked.
89
90To set or get the exdata on an object, the appropriate type-specific
91routine must be used.  This is because the containing structure is opaque
92and the B<CRYPTO_EX_DATA> field is not accessible.  In both API's, the
93B<idx> parameter should be an already-created index value.
94
95When setting exdata, the pointer specified with a particular index is saved,
96and returned on a subsequent "get" call.  If the application is going to
97release the data, it must make sure to set a B<NULL> value at the index,
98to avoid likely double-free crashes.
99
100The function B<CRYPTO_free_ex_data> is used to free all exdata attached
101to a structure. The appropriate type-specific routine must be used.
102The B<class_index> identifies the structure type, the B<obj> is
103a pointer to the actual structure, and B<r> is a pointer to the
104structure's exdata field.
105
106=head2 Callback Functions
107
108This section describes how the callback functions are used. Applications
109that are defining their own exdata using B<CYPRTO_EX_INDEX_APP> must
110call them as described here.
111
112When a structure is initially allocated (such as RSA_new()) then the
113new_func() is called for every defined index. There is no requirement
114that the entire parent, or containing, structure has been set up.
115The new_func() is typically used only to allocate memory to store the
116exdata, and perhaps an "initialized" flag within that memory.
117The exdata value should be set by calling CRYPTO_set_ex_data().
118
119When a structure is free'd (such as SSL_CTX_free()) then the
120free_func() is called for every defined index.  Again, the state of the
121parent structure is not guaranteed.  The free_func() may be called with a
122NULL pointer.
123
124Both new_func() and free_func() take the same parameters.
125The B<parent> is the pointer to the structure that contains the exdata.
126The B<ptr> is the current exdata item; for new_func() this will typically
127be NULL.  The B<r> parameter is a pointer to the exdata field of the object.
128The B<idx> is the index and is the value returned when the callbacks were
129initially registered via CRYPTO_get_ex_new_index() and can be used if
130the same callback handles different types of exdata.
131
132dup_func() is called when a structure is being copied.  This is only done
133for B<SSL>, B<SSL_SESSION>, B<EC_KEY> objects and B<BIO> chains via
134BIO_dup_chain().  The B<to> and B<from> parameters
135are pointers to the destination and source B<CRYPTO_EX_DATA> structures,
136respectively.  The B<from_d> parameter needs to be cast to a B<void **pptr>
137as the API has currently the wrong signature; that will be changed in a
138future version.  The B<*pptr> is a pointer to the source exdata.
139When the dup_func() returns, the value in B<*pptr> is copied to the
140destination ex_data.  If the pointer contained in B<*pptr> is not modified
141by the dup_func(), then both B<to> and B<from> will point to the same data.
142The B<idx>, B<argl> and B<argp> parameters are as described for the other
143two callbacks.  If the dup_func() returns B<0> the whole CRYPTO_dup_ex_data()
144will fail.
145
146=head1 RETURN VALUES
147
148CRYPTO_get_ex_new_index() returns a new index or -1 on failure.
149
150CRYPTO_free_ex_index() and
151CRYPTO_set_ex_data() return 1 on success or 0 on failure.
152
153CRYPTO_get_ex_data() returns the application data or NULL on failure;
154note that NULL may be a valid value.
155
156dup_func() should return 0 for failure and 1 for success.
157
158=head1 COPYRIGHT
159
160Copyright 2015-2019 The OpenSSL Project Authors. All Rights Reserved.
161
162Licensed under the OpenSSL license (the "License").  You may not use
163this file except in compliance with the License.  You can obtain a copy
164in the file LICENSE in the source distribution or at
165L<https://www.openssl.org/source/license.html>.
166
167=cut
168