=pod

=head1 NAME

CRYPTO_EX_new, CRYPTO_EX_free, CRYPTO_EX_dup,

CRYPTO_free_ex_index, CRYPTO_get_ex_new_index, CRYPTO_set_ex_data,

CRYPTO_get_ex_data, CRYPTO_free_ex_data, CRYPTO_new_ex_data

- functions supporting application-specific data

=head1 SYNOPSIS

 #include <openssl/crypto.h>

 int CRYPTO_get_ex_new_index(int class_index,

                             long argl, void *argp,

                             CRYPTO_EX_new *new_func,

                             CRYPTO_EX_dup *dup_func,

                             CRYPTO_EX_free *free_func);

 typedef void CRYPTO_EX_new(void *parent, void *ptr, CRYPTO_EX_DATA *ad,

                            int idx, long argl, void *argp);

 typedef void CRYPTO_EX_free(void *parent, void *ptr, CRYPTO_EX_DATA *ad,

                             int idx, long argl, void *argp);

 typedef int CRYPTO_EX_dup(CRYPTO_EX_DATA *to, const CRYPTO_EX_DATA *from,

                           void *from_d, int idx, long argl, void *argp);

 int CRYPTO_new_ex_data(int class_index, void *obj, CRYPTO_EX_DATA *ad)

 int CRYPTO_set_ex_data(CRYPTO_EX_DATA *r, int idx, void *arg);

 void *CRYPTO_get_ex_data(CRYPTO_EX_DATA *r, int idx);

 void CRYPTO_free_ex_data(int class_index, void *obj, CRYPTO_EX_DATA *r);

 int CRYPTO_free_ex_index(int class_index, int idx);

=head1 DESCRIPTION

Several OpenSSL structures can have application-specific data attached to them,

known as "exdata."

The specific structures are:

    APP

    BIO

    DH

    DRBG

    DSA

    EC_KEY

    ENGINE

    RSA

    SSL

    SSL_CTX

    SSL_SESSION

    UI

    UI_METHOD

    X509

    X509_STORE

    X509_STORE_CTX

Each is identified by an B<CRYPTO_EX_INDEX_xxx> define in the B<crypto.h>

header file.  In addition, B<CRYPTO_EX_INDEX_APP> is reserved for

applications to use this facility for their own structures.

The API described here is used by OpenSSL to manipulate exdata for specific

structures.  Since the application data can be anything at all it is passed

and retrieved as a B<void *> type.

The B<CRYPTO_EX_DATA> type is opaque.  To initialize the exdata part of

a structure, call CRYPTO_new_ex_data(). This is only necessary for

B<CRYPTO_EX_INDEX_APP> objects.

Exdata types are identified by an B<index>, an integer guaranteed to be

unique within structures for the lifetime of the program.  Applications

using exdata typically call B<CRYPTO_get_ex_new_index> at startup, and

store the result in a global variable, or write a wrapper function to

provide lazy evaluation.  The B<class_index> should be one of the

B<CRYPTO_EX_INDEX_xxx> values. The B<argl> and B<argp> parameters are saved

to be passed to the callbacks but are otherwise not used.  In order to

transparently manipulate exdata, three callbacks must be provided. The

semantics of those callbacks are described below.

When copying or releasing objects with exdata, the callback functions

are called in increasing order of their B<index> value.

If a dynamic library can be unloaded, it should call CRYPTO_free_ex_index()

when this is done.

This will replace the callbacks with no-ops

so that applications don't crash.  Any existing exdata will be leaked.

To set or get the exdata on an object, the appropriate type-specific

routine must be used.  This is because the containing structure is opaque

and the B<CRYPTO_EX_DATA> field is not accessible.  In both API's, the

B<idx> parameter should be an already-created index value.

When setting exdata, the pointer specified with a particular index is saved,

and returned on a subsequent "get" call.  If the application is going to

release the data, it must make sure to set a B<NULL> value at the index,

to avoid likely double-free crashes.

The function B<CRYPTO_free_ex_data> is used to free all exdata attached

to a structure. The appropriate type-specific routine must be used.

The B<class_index> identifies the structure type, the B<obj> is

a pointer to the actual structure, and B<r> is a pointer to the

structure's exdata field.

=head2 Callback Functions

This section describes how the callback functions are used. Applications

that are defining their own exdata using B<CYPRTO_EX_INDEX_APP> must

call them as described here.

When a structure is initially allocated (such as RSA_new()) then the

new_func() is called for every defined index. There is no requirement

that the entire parent, or containing, structure has been set up.

The new_func() is typically used only to allocate memory to store the

exdata, and perhaps an "initialized" flag within that memory.

The exdata value should be set by calling CRYPTO_set_ex_data().

When a structure is free'd (such as SSL_CTX_free()) then the

free_func() is called for every defined index.  Again, the state of the

parent structure is not guaranteed.  The free_func() may be called with a

NULL pointer.

Both new_func() and free_func() take the same parameters.

The B<parent> is the pointer to the structure that contains the exdata.

The B<ptr> is the current exdata item; for new_func() this will typically

be NULL.  The B<r> parameter is a pointer to the exdata field of the object.

The B<idx> is the index and is the value returned when the callbacks were

initially registered via CRYPTO_get_ex_new_index() and can be used if

the same callback handles different types of exdata.

dup_func() is called when a structure is being copied.  This is only done

for B<SSL>, B<SSL_SESSION>, B<EC_KEY> objects and B<BIO> chains via

BIO_dup_chain().  The B<to> and B<from> parameters

are pointers to the destination and source B<CRYPTO_EX_DATA> structures,

respectively.  The B<from_d> parameter needs to be cast to a B<void **pptr>

as the API has currently the wrong signature; that will be changed in a

future version.  The B<*pptr> is a pointer to the source exdata.

When the dup_func() returns, the value in B<*pptr> is copied to the

destination ex_data.  If the pointer contained in B<*pptr> is not modified

by the dup_func(), then both B<to> and B<from> will point to the same data.

The B<idx>, B<argl> and B<argp> parameters are as described for the other

two callbacks.  If the dup_func() returns B<0> the whole CRYPTO_dup_ex_data()

will fail.

=head1 RETURN VALUES

CRYPTO_get_ex_new_index() returns a new index or -1 on failure.

CRYPTO_free_ex_index() and

CRYPTO_set_ex_data() return 1 on success or 0 on failure.

CRYPTO_get_ex_data() returns the application data or NULL on failure;

note that NULL may be a valid value.

dup_func() should return 0 for failure and 1 for success.

=head1 COPYRIGHT

Copyright 2015-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

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