NAME

       CRYPTO_EX_new, CRYPTO_EX_free, CRYPTO_EX_dup, CRYPTO_free_ex_index,
       CRYPTO_get_ex_new_index, CRYPTO_alloc_ex_data, CRYPTO_set_ex_data, CRYPTO_get_ex_data,
       CRYPTO_free_ex_data, CRYPTO_new_ex_data - functions supporting application-specific data

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_alloc_ex_data(int class_index, void *obj, CRYPTO_EX_DATA *ad,
                                 int idx);

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

        void *CRYPTO_get_ex_data(const 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);

DESCRIPTION

       Several OpenSSL structures can have application-specific data attached to them, known as
       "exdata."  The specific structures are:

           BIO
           DH
           DSA
           EC_KEY
           ENGINE
           EVP_PKEY
           RSA
           SSL
           SSL_CTX
           SSL_SESSION
           UI
           UI_METHOD
           X509
           X509_STORE
           X509_STORE_CTX

       In addition, the APP name is reserved for use by application code.

       Each is identified by an CRYPTO_EX_INDEX_xxx define in the header file <openssl/crypto.h>.
       In addition, 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 void *
       type.

       The CRYPTO_EX_DATA type is opaque.  To initialize the exdata part of a structure, call
       CRYPTO_new_ex_data(). This is only necessary for CRYPTO_EX_INDEX_APP objects.

       Exdata types are identified by an index, an integer guaranteed to be unique within
       structures for the lifetime of the program.  Applications using exdata typically call
       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 class_index should be one of the
       CRYPTO_EX_INDEX_xxx values. The argl and 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 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 CRYPTO_EX_DATA field is not
       accessible.  In both API's, the 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 NULL value at the index, to avoid likely double-free crashes.

       The function CRYPTO_free_ex_data is used to free all exdata attached to a structure. The
       appropriate type-specific routine must be used.  The class_index identifies the structure
       type, the obj is a pointer to the actual structure, and r is a pointer to the structure's
       exdata field.

   Callback Functions
       This section describes how the callback functions are used. Applications that are defining
       their own exdata using 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
       may be allocated later on with CRYPTO_alloc_ex_data(), or may 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 parent is the pointer to
       the structure that contains the exdata.  The ptr is the current exdata item; for
       new_func() this will typically be NULL.  The r parameter is a pointer to the exdata field
       of the object.  The 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 SSL,
       SSL_SESSION, EC_KEY objects and BIO chains via BIO_dup_chain().  The to and from
       parameters are pointers to the destination and source CRYPTO_EX_DATA structures,
       respectively.  The *from_d parameter is a pointer to the source exdata.  When the
       dup_func() returns, the value in *from_d is copied to the destination ex_data.  If the
       pointer contained in *pptr is not modified by the dup_func(), then both to and from will
       point to the same data.  The idx, argl and argp parameters are as described for the other
       two callbacks.  If the dup_func() returns 0 the whole CRYPTO_dup_ex_data() will fail.

RETURN VALUES

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

       CRYPTO_free_ex_index(), CRYPTO_alloc_ex_data() 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.

HISTORY

       CRYPTO_alloc_ex_data() was added in OpenSSL 3.0.

       The signature of the dup_func() callback was changed in OpenSSL 3.0 to use the type void
       ** for from_d.  Previously this parameter was of type void *.

       Support for ENGINE "exdata" was deprecated in OpenSSL 3.0.

COPYRIGHT

       Copyright 2015-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.