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DESCRIPTION

    BIO
    DH
    DSA
    EC_KEY
    ECDH
    ECDSA
    ENGINE
    RSA
    SSL
    SSL_CTX
    SSL_SESSION
    UI
    X509
    X509_STORE
    X509_STORE_CTX

Each is identified by a CRYPTO_EX_INDEX_* constant defined in the <openssl/crypto.h> header file.

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().

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_* 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.

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 APIs, 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.

The callback functions are used as follows.

When a structure is initially allocated (such as by RSA_new(3)), then 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 by SSL_CTX_free(3)), 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 and SSL_SESSION objects. 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 dup_func returns, the value in from_d is copied to the destination ex_data. If the pointer contained in from_d 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.

CRYPTO_set_ex_data() is used to set application specific data. The data is supplied in the arg parameter and its precise meaning is up to the application.

CRYPTO_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 CRYPTO_set_ex_data() call.