ASN1_EXTERN_FUNCS, ASN1_ex_d2i, ASN1_ex_d2i_ex, ASN1_ex_i2d, ASN1_ex_new_func,
  ASN1_ex_new_ex_func, ASN1_ex_free_func, ASN1_ex_print_func,
  IMPLEMENT_EXTERN_ASN1 - ASN.1 external function support
 #include <openssl/asn1t.h>
 typedef int ASN1_ex_d2i(ASN1_VALUE **pval, const unsigned char **in, long len,
                         const ASN1_ITEM *it, int tag, int aclass, char opt,
                         ASN1_TLC *ctx);
 typedef int ASN1_ex_d2i_ex(ASN1_VALUE **pval, const unsigned char **in, long len,
                            const ASN1_ITEM *it, int tag, int aclass, char opt,
                            ASN1_TLC *ctx, OSSL_LIB_CTX *libctx,
                            const char *propq);
 typedef int ASN1_ex_i2d(const ASN1_VALUE **pval, unsigned char **out,
                         const ASN1_ITEM *it, int tag, int aclass);
 typedef int ASN1_ex_new_func(ASN1_VALUE **pval, const ASN1_ITEM *it);
 typedef int ASN1_ex_new_ex_func(ASN1_VALUE **pval, const ASN1_ITEM *it,
                                 OSSL_LIB_CTX *libctx, const char *propq);
 typedef void ASN1_ex_free_func(ASN1_VALUE **pval, const ASN1_ITEM *it);
 typedef int ASN1_ex_print_func(BIO *out, const ASN1_VALUE **pval,
                                int indent, const char *fname,
                                const ASN1_PCTX *pctx);
 struct ASN1_EXTERN_FUNCS_st {
    void *app_data;
    ASN1_ex_new_func *asn1_ex_new;
    ASN1_ex_free_func *asn1_ex_free;
    ASN1_ex_free_func *asn1_ex_clear;
    ASN1_ex_d2i *asn1_ex_d2i;
    ASN1_ex_i2d *asn1_ex_i2d;
    ASN1_ex_print_func *asn1_ex_print;
    ASN1_ex_new_ex_func *asn1_ex_new_ex;
    ASN1_ex_d2i_ex *asn1_ex_d2i_ex;
 };
 typedef struct ASN1_EXTERN_FUNCS_st ASN1_EXTERN_FUNCS;
 #define IMPLEMENT_EXTERN_ASN1(sname, tag, fptrs)
ASN.1 data structures templates are typically defined in OpenSSL using a series
  of macros such as ASN1_SEQUENCE(), ASN1_SEQUENCE_END() and so
  on. Instead templates can also be defined based entirely on external
  functions. These external functions are called to perform operations such as
  creating a new ASN1_VALUE or converting an ASN1_VALUE to or from
  DER encoding.
The macro IMPLEMENT_EXTERN_ASN1() can be used to create
    such an externally defined structure. The name of the structure should be
    supplied in the sname parameter. The tag for the structure (e.g.
    typically V_ASN1_SEQUENCE) should be supplied in the tag
    parameter. Finally a pointer to an ASN1_EXTERN_FUNCS structure should
    be supplied in the fptrs parameter.
The ASN1_EXTERN_FUNCS structure has the following
  entries.
  - app_data
- A pointer to arbitrary application specific data.
- asn1_ex_new
- A "new" function responsible for constructing a new
      ASN1_VALUE object. The newly constructed value should be stored in
      *pval. The it parameter is a pointer to the ASN1_ITEM
      template object created via the IMPLEMENT_EXTERN_ASN1() macro.
    Returns a positive value on success or 0 on error. 
- asn1_ex_free
- A "free" function responsible for freeing the ASN1_VALUE
      passed in *pval that was previously allocated via a "new"
      function. The it parameter is a pointer to the ASN1_ITEM
      template object created via the IMPLEMENT_EXTERN_ASN1() macro.
- asn1_ex_clear
- A "clear" function responsible for clearing any data in the
      ASN1_VALUE passed in *pval and making it suitable for reuse.
      The it parameter is a pointer to the ASN1_ITEM template
      object created via the IMPLEMENT_EXTERN_ASN1() macro.
- asn1_ex_d2i
- A "d2i" function responsible for converting DER data with the
      tag tag and class class into an ASN1_VALUE. If
      *pval is non-NULL then the ASN_VALUE it points to should be
      reused. Otherwise a new ASN1_VALUE should be allocated and stored
      in *pval. *in points to the DER data to be decoded and
      len is the length of that data. After decoding *in should be
      updated to point at the next byte after the decoded data. If the
      ASN1_VALUE is considered optional in this context then opt
      will be nonzero. Otherwise it will be zero. The it parameter is a
      pointer to the ASN1_ITEM template object created via the
      IMPLEMENT_EXTERN_ASN1() macro. A pointer to the current
      ASN1_TLC context (which may be required for other ASN1 function
      calls) is passed in the ctx parameter.
    The asn1_ex_d2i entry may be NULL if
        asn1_ex_d2i_ex has been specified instead. Returns <= 0 on error or a positive value on success. 
- asn1_ex_i2d
- An "i2d" function responsible for converting an
      ASN1_VALUE into DER encoding. On entry *pval will contain
      the ASN1_VALUE to be encoded. If default tagging is to be used then
      tag will be -1 on entry. Otherwise if implicit tagging should be
      used then tag and aclass will be the tag and associated
      class.
    If out is not NULL then this function should write the
        DER encoded data to the buffer in *out, and then increment
        *out to point to immediately after the data just written. If out is NULL then no data should be written but the
        length calculated and returned as if it were. The asn1_ex_i2d entry may be NULL if
        asn1_ex_i2d_ex has been specified instead. The return value should be negative if a fatal error occurred,
        or 0 if a non-fatal error occurred. Otherwise it should return the
        length of the encoded data. 
- asn1_ex_print
- A "print" function. out is the BIO to print the output
      to. *pval is the ASN1_VALUE to be printed. indent is
      the number of spaces of indenting to be printed before any data is
      printed. fname is currently unused and is always "".
      pctx is a pointer to the ASN1_PCTX for the print operation.
    Returns 0 on error or a positive value on success. If the
        return value is 2 then an additional newline will be printed after the
        data printed by this function. 
- asn1_ex_new_ex
- This is the same as asn1_ex_new except that it is additionally
      passed the OSSL_LIB_CTX to be used in libctx and any property query
      string to be used for algorithm fetching in the propq parameter.
      See "ALGORITHM FETCHING" in crypto(7) for further
      details. If asn1_ex_new_ex is non NULL, then it will always be
      called in preference to asn1_ex_new.
- asn1_ex_d2i_ex
- This is the same as asn1_ex_d2i except that it is additionally
      passed the OSSL_LIB_CTX to be used in libctx and any property query
      string to be used for algorithm fetching in the propq parameter.
      See "ALGORITHM FETCHING" in crypto(7) for further
      details. If asn1_ex_d2i_ex is non NULL, then it will always be
      called in preference to asn1_ex_d2i.
Return values for the various callbacks are as described above.
The asn1_ex_new_ex and asn1_ex_d2i_ex callbacks were added in
  OpenSSL 3.0.
Copyright 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>.