doc/man3/d2i_X509.pod

   1 =pod
   2
   3 =begin comment
   4
   5 Any keypair function here that gets deprecated should be moved to
   6 d2i_RSAPrivateKey.pod.
   7
   8 =end comment
   9
  10 =head1 NAME
  11
  12 d2i_ACCESS_DESCRIPTION,
  13 d2i_ADMISSIONS,
  14 d2i_ADMISSION_SYNTAX,
  15 d2i_ASIdOrRange,
  16 d2i_ASIdentifierChoice,
  17 d2i_ASIdentifiers,
  18 d2i_ASN1_BIT_STRING,
  19 d2i_ASN1_BMPSTRING,
  20 d2i_ASN1_ENUMERATED,
  21 d2i_ASN1_GENERALIZEDTIME,
  22 d2i_ASN1_GENERALSTRING,
  23 d2i_ASN1_IA5STRING,
  24 d2i_ASN1_INTEGER,
  25 d2i_ASN1_NULL,
  26 d2i_ASN1_OBJECT,
  27 d2i_ASN1_OCTET_STRING,
  28 d2i_ASN1_PRINTABLE,
  29 d2i_ASN1_PRINTABLESTRING,
  30 d2i_ASN1_SEQUENCE_ANY,
  31 d2i_ASN1_SET_ANY,
  32 d2i_ASN1_T61STRING,
  33 d2i_ASN1_TIME,
  34 d2i_ASN1_TYPE,
  35 d2i_ASN1_UINTEGER,
  36 d2i_ASN1_UNIVERSALSTRING,
  37 d2i_ASN1_UTCTIME,
  38 d2i_ASN1_UTF8STRING,
  39 d2i_ASN1_VISIBLESTRING,
  40 d2i_ASRange,
  41 d2i_AUTHORITY_INFO_ACCESS,
  42 d2i_AUTHORITY_KEYID,
  43 d2i_BASIC_CONSTRAINTS,
  44 d2i_CERTIFICATEPOLICIES,
  45 d2i_CMS_ContentInfo,
  46 d2i_CMS_ReceiptRequest,
  47 d2i_CMS_bio,
  48 d2i_CRL_DIST_POINTS,
  49 d2i_DHxparams,
  50 d2i_DIRECTORYSTRING,
  51 d2i_DISPLAYTEXT,
  52 d2i_DIST_POINT,
  53 d2i_DIST_POINT_NAME,
  54 d2i_DSA_SIG,
  55 d2i_ECDSA_SIG,
  56 d2i_EDIPARTYNAME,
  57 d2i_ESS_CERT_ID,
  58 d2i_ESS_CERT_ID_V2,
  59 d2i_ESS_ISSUER_SERIAL,
  60 d2i_ESS_SIGNING_CERT,
  61 d2i_ESS_SIGNING_CERT_V2,
  62 d2i_EXTENDED_KEY_USAGE,
  63 d2i_GENERAL_NAME,
  64 d2i_GENERAL_NAMES,
  65 d2i_IPAddressChoice,
  66 d2i_IPAddressFamily,
  67 d2i_IPAddressOrRange,
  68 d2i_IPAddressRange,
  69 d2i_ISSUER_SIGN_TOOL,
  70 d2i_ISSUING_DIST_POINT,
  71 d2i_NAMING_AUTHORITY,
  72 d2i_NETSCAPE_CERT_SEQUENCE,
  73 d2i_NETSCAPE_SPKAC,
  74 d2i_NETSCAPE_SPKI,
  75 d2i_NOTICEREF,
  76 d2i_OCSP_BASICRESP,
  77 d2i_OCSP_CERTID,
  78 d2i_OCSP_CERTSTATUS,
  79 d2i_OCSP_CRLID,
  80 d2i_OCSP_ONEREQ,
  81 d2i_OCSP_REQINFO,
  82 d2i_OCSP_REQUEST,
  83 d2i_OCSP_RESPBYTES,
  84 d2i_OCSP_RESPDATA,
  85 d2i_OCSP_RESPID,
  86 d2i_OCSP_RESPONSE,
  87 d2i_OCSP_REVOKEDINFO,
  88 d2i_OCSP_SERVICELOC,
  89 d2i_OCSP_SIGNATURE,
  90 d2i_OCSP_SINGLERESP,
  91 d2i_OSSL_CMP_MSG,
  92 d2i_OSSL_CMP_PKIHEADER,
  93 d2i_OSSL_CMP_PKISI,
  94 d2i_OSSL_CRMF_CERTID,
  95 d2i_OSSL_CRMF_CERTTEMPLATE,
  96 d2i_OSSL_CRMF_ENCRYPTEDVALUE,
  97 d2i_OSSL_CRMF_MSG,
  98 d2i_OSSL_CRMF_MSGS,
  99 d2i_OSSL_CRMF_PBMPARAMETER,
 100 d2i_OSSL_CRMF_PKIPUBLICATIONINFO,
 101 d2i_OSSL_CRMF_SINGLEPUBINFO,
 102 d2i_OTHERNAME,
 103 d2i_PBE2PARAM,
 104 d2i_PBEPARAM,
 105 d2i_PBKDF2PARAM,
 106 d2i_PKCS12,
 107 d2i_PKCS12_BAGS,
 108 d2i_PKCS12_MAC_DATA,
 109 d2i_PKCS12_SAFEBAG,
 110 d2i_PKCS12_bio,
 111 d2i_PKCS12_fp,
 112 d2i_PKCS7,
 113 d2i_PKCS7_DIGEST,
 114 d2i_PKCS7_ENCRYPT,
 115 d2i_PKCS7_ENC_CONTENT,
 116 d2i_PKCS7_ENVELOPE,
 117 d2i_PKCS7_ISSUER_AND_SERIAL,
 118 d2i_PKCS7_RECIP_INFO,
 119 d2i_PKCS7_SIGNED,
 120 d2i_PKCS7_SIGNER_INFO,
 121 d2i_PKCS7_SIGN_ENVELOPE,
 122 d2i_PKCS7_bio,
 123 d2i_PKCS7_fp,
 124 d2i_PKCS8_PRIV_KEY_INFO,
 125 d2i_PKCS8_PRIV_KEY_INFO_bio,
 126 d2i_PKCS8_PRIV_KEY_INFO_fp,
 127 d2i_PKCS8_bio,
 128 d2i_PKCS8_fp,
 129 d2i_PKEY_USAGE_PERIOD,
 130 d2i_POLICYINFO,
 131 d2i_POLICYQUALINFO,
 132 d2i_PROFESSION_INFO,
 133 d2i_PROXY_CERT_INFO_EXTENSION,
 134 d2i_PROXY_POLICY,
 135 d2i_RSA_OAEP_PARAMS,
 136 d2i_RSA_PSS_PARAMS,
 137 d2i_SCRYPT_PARAMS,
 138 d2i_SCT_LIST,
 139 d2i_SXNET,
 140 d2i_SXNETID,
 141 d2i_TS_ACCURACY,
 142 d2i_TS_MSG_IMPRINT,
 143 d2i_TS_MSG_IMPRINT_bio,
 144 d2i_TS_MSG_IMPRINT_fp,
 145 d2i_TS_REQ,
 146 d2i_TS_REQ_bio,
 147 d2i_TS_REQ_fp,
 148 d2i_TS_RESP,
 149 d2i_TS_RESP_bio,
 150 d2i_TS_RESP_fp,
 151 d2i_TS_STATUS_INFO,
 152 d2i_TS_TST_INFO,
 153 d2i_TS_TST_INFO_bio,
 154 d2i_TS_TST_INFO_fp,
 155 d2i_USERNOTICE,
 156 d2i_X509,
 157 d2i_X509_ALGOR,
 158 d2i_X509_ALGORS,
 159 d2i_X509_ATTRIBUTE,
 160 d2i_X509_CERT_AUX,
 161 d2i_X509_CINF,
 162 d2i_X509_CRL,
 163 d2i_X509_CRL_INFO,
 164 d2i_X509_CRL_bio,
 165 d2i_X509_CRL_fp,
 166 d2i_X509_EXTENSION,
 167 d2i_X509_EXTENSIONS,
 168 d2i_X509_NAME,
 169 d2i_X509_NAME_ENTRY,
 170 d2i_X509_PUBKEY,
 171 d2i_X509_PUBKEY_bio,
 172 d2i_X509_PUBKEY_fp,
 173 d2i_X509_REQ,
 174 d2i_X509_REQ_INFO,
 175 d2i_X509_REQ_bio,
 176 d2i_X509_REQ_fp,
 177 d2i_X509_REVOKED,
 178 d2i_X509_SIG,
 179 d2i_X509_VAL,
 180 i2d_ACCESS_DESCRIPTION,
 181 i2d_ADMISSIONS,
 182 i2d_ADMISSION_SYNTAX,
 183 i2d_ASIdOrRange,
 184 i2d_ASIdentifierChoice,
 185 i2d_ASIdentifiers,
 186 i2d_ASN1_BIT_STRING,
 187 i2d_ASN1_BMPSTRING,
 188 i2d_ASN1_ENUMERATED,
 189 i2d_ASN1_GENERALIZEDTIME,
 190 i2d_ASN1_GENERALSTRING,
 191 i2d_ASN1_IA5STRING,
 192 i2d_ASN1_INTEGER,
 193 i2d_ASN1_NULL,
 194 i2d_ASN1_OBJECT,
 195 i2d_ASN1_OCTET_STRING,
 196 i2d_ASN1_PRINTABLE,
 197 i2d_ASN1_PRINTABLESTRING,
 198 i2d_ASN1_SEQUENCE_ANY,
 199 i2d_ASN1_SET_ANY,
 200 i2d_ASN1_T61STRING,
 201 i2d_ASN1_TIME,
 202 i2d_ASN1_TYPE,
 203 i2d_ASN1_UNIVERSALSTRING,
 204 i2d_ASN1_UTCTIME,
 205 i2d_ASN1_UTF8STRING,
 206 i2d_ASN1_VISIBLESTRING,
 207 i2d_ASN1_bio_stream,
 208 i2d_ASRange,
 209 i2d_AUTHORITY_INFO_ACCESS,
 210 i2d_AUTHORITY_KEYID,
 211 i2d_BASIC_CONSTRAINTS,
 212 i2d_CERTIFICATEPOLICIES,
 213 i2d_CMS_ContentInfo,
 214 i2d_CMS_ReceiptRequest,
 215 i2d_CMS_bio,
 216 i2d_CRL_DIST_POINTS,
 217 i2d_DHxparams,
 218 i2d_DIRECTORYSTRING,
 219 i2d_DISPLAYTEXT,
 220 i2d_DIST_POINT,
 221 i2d_DIST_POINT_NAME,
 222 i2d_DSAPrivateKey,
 223 i2d_DSAPrivateKey_bio,
 224 i2d_DSAPrivateKey_fp,
 225 i2d_DSAPublicKey,
 226 i2d_DSA_PUBKEY,
 227 i2d_DSA_PUBKEY_bio,
 228 i2d_DSA_PUBKEY_fp,
 229 i2d_DSA_SIG,
 230 i2d_DSAparams,
 231 i2d_ECDSA_SIG,
 232 i2d_EDIPARTYNAME,
 233 i2d_ESS_CERT_ID,
 234 i2d_ESS_CERT_ID_V2,
 235 i2d_ESS_ISSUER_SERIAL,
 236 i2d_ESS_SIGNING_CERT,
 237 i2d_ESS_SIGNING_CERT_V2,
 238 i2d_EXTENDED_KEY_USAGE,
 239 i2d_GENERAL_NAME,
 240 i2d_GENERAL_NAMES,
 241 i2d_IPAddressChoice,
 242 i2d_IPAddressFamily,
 243 i2d_IPAddressOrRange,
 244 i2d_IPAddressRange,
 245 i2d_ISSUER_SIGN_TOOL,
 246 i2d_ISSUING_DIST_POINT,
 247 i2d_NAMING_AUTHORITY,
 248 i2d_NETSCAPE_CERT_SEQUENCE,
 249 i2d_NETSCAPE_SPKAC,
 250 i2d_NETSCAPE_SPKI,
 251 i2d_NOTICEREF,
 252 i2d_OCSP_BASICRESP,
 253 i2d_OCSP_CERTID,
 254 i2d_OCSP_CERTSTATUS,
 255 i2d_OCSP_CRLID,
 256 i2d_OCSP_ONEREQ,
 257 i2d_OCSP_REQINFO,
 258 i2d_OCSP_REQUEST,
 259 i2d_OCSP_RESPBYTES,
 260 i2d_OCSP_RESPDATA,
 261 i2d_OCSP_RESPID,
 262 i2d_OCSP_RESPONSE,
 263 i2d_OCSP_REVOKEDINFO,
 264 i2d_OCSP_SERVICELOC,
 265 i2d_OCSP_SIGNATURE,
 266 i2d_OCSP_SINGLERESP,
 267 i2d_OSSL_CMP_MSG,
 268 i2d_OSSL_CMP_PKIHEADER,
 269 i2d_OSSL_CMP_PKISI,
 270 i2d_OSSL_CRMF_CERTID,
 271 i2d_OSSL_CRMF_CERTTEMPLATE,
 272 i2d_OSSL_CRMF_ENCRYPTEDVALUE,
 273 i2d_OSSL_CRMF_MSG,
 274 i2d_OSSL_CRMF_MSGS,
 275 i2d_OSSL_CRMF_PBMPARAMETER,
 276 i2d_OSSL_CRMF_PKIPUBLICATIONINFO,
 277 i2d_OSSL_CRMF_SINGLEPUBINFO,
 278 i2d_OTHERNAME,
 279 i2d_PBE2PARAM,
 280 i2d_PBEPARAM,
 281 i2d_PBKDF2PARAM,
 282 i2d_PKCS12,
 283 i2d_PKCS12_BAGS,
 284 i2d_PKCS12_MAC_DATA,
 285 i2d_PKCS12_SAFEBAG,
 286 i2d_PKCS12_bio,
 287 i2d_PKCS12_fp,
 288 i2d_PKCS7,
 289 i2d_PKCS7_DIGEST,
 290 i2d_PKCS7_ENCRYPT,
 291 i2d_PKCS7_ENC_CONTENT,
 292 i2d_PKCS7_ENVELOPE,
 293 i2d_PKCS7_ISSUER_AND_SERIAL,
 294 i2d_PKCS7_NDEF,
 295 i2d_PKCS7_RECIP_INFO,
 296 i2d_PKCS7_SIGNED,
 297 i2d_PKCS7_SIGNER_INFO,
 298 i2d_PKCS7_SIGN_ENVELOPE,
 299 i2d_PKCS7_bio,
 300 i2d_PKCS7_fp,
 301 i2d_PKCS8PrivateKeyInfo_bio,
 302 i2d_PKCS8PrivateKeyInfo_fp,
 303 i2d_PKCS8_PRIV_KEY_INFO,
 304 i2d_PKCS8_PRIV_KEY_INFO_bio,
 305 i2d_PKCS8_PRIV_KEY_INFO_fp,
 306 i2d_PKCS8_bio,
 307 i2d_PKCS8_fp,
 308 i2d_PKEY_USAGE_PERIOD,
 309 i2d_POLICYINFO,
 310 i2d_POLICYQUALINFO,
 311 i2d_PROFESSION_INFO,
 312 i2d_PROXY_CERT_INFO_EXTENSION,
 313 i2d_PROXY_POLICY,
 314 i2d_RSA_OAEP_PARAMS,
 315 i2d_RSA_PSS_PARAMS,
 316 i2d_SCRYPT_PARAMS,
 317 i2d_SCT_LIST,
 318 i2d_SXNET,
 319 i2d_SXNETID,
 320 i2d_TS_ACCURACY,
 321 i2d_TS_MSG_IMPRINT,
 322 i2d_TS_MSG_IMPRINT_bio,
 323 i2d_TS_MSG_IMPRINT_fp,
 324 i2d_TS_REQ,
 325 i2d_TS_REQ_bio,
 326 i2d_TS_REQ_fp,
 327 i2d_TS_RESP,
 328 i2d_TS_RESP_bio,
 329 i2d_TS_RESP_fp,
 330 i2d_TS_STATUS_INFO,
 331 i2d_TS_TST_INFO,
 332 i2d_TS_TST_INFO_bio,
 333 i2d_TS_TST_INFO_fp,
 334 i2d_USERNOTICE,
 335 i2d_X509,
 336 i2d_X509_ALGOR,
 337 i2d_X509_ALGORS,
 338 i2d_X509_ATTRIBUTE,
 339 i2d_X509_CERT_AUX,
 340 i2d_X509_CINF,
 341 i2d_X509_CRL,
 342 i2d_X509_CRL_INFO,
 343 i2d_X509_CRL_bio,
 344 i2d_X509_CRL_fp,
 345 i2d_X509_EXTENSION,
 346 i2d_X509_EXTENSIONS,
 347 i2d_X509_NAME,
 348 i2d_X509_NAME_ENTRY,
 349 i2d_X509_PUBKEY,
 350 i2d_X509_PUBKEY_bio,
 351 i2d_X509_PUBKEY_fp,
 352 i2d_X509_REQ,
 353 i2d_X509_REQ_INFO,
 354 i2d_X509_REQ_bio,
 355 i2d_X509_REQ_fp,
 356 i2d_X509_REVOKED,
 357 i2d_X509_SIG,
 358 i2d_X509_VAL,
 359 - convert objects from/to ASN.1/DER representation
 360
 361 =head1 SYNOPSIS
 362
 363 =for openssl generic
 364
 365  TYPE *d2i_TYPE(TYPE **a, const unsigned char **ppin, long length);
 366  TYPE *d2i_TYPE_bio(BIO *bp, TYPE **a);
 367  TYPE *d2i_TYPE_fp(FILE *fp, TYPE **a);
 368
 369  int i2d_TYPE(const TYPE *a, unsigned char **ppout);
 370  int i2d_TYPE(TYPE *a, unsigned char **ppout);
 371  int i2d_TYPE_fp(FILE *fp, const TYPE *a);
 372  int i2d_TYPE_fp(FILE *fp, TYPE *a);
 373  int i2d_TYPE_bio(BIO *bp, const TYPE *a);
 374  int i2d_TYPE_bio(BIO *bp, TYPE *a);
 375
 376 =head1 DESCRIPTION
 377
 378 In the description here, B<I<TYPE>> is used a placeholder
 379 for any of the OpenSSL datatypes, such as B<X509_CRL>.
 380 The function parameters I<ppin> and I<ppout> are generally
 381 either both named I<pp> in the headers, or I<in> and I<out>.
 382
 383 These functions convert OpenSSL objects to and from their ASN.1/DER
 384 encoding.  Unlike the C structures which can have pointers to sub-objects
 385 within, the DER is a serialized encoding, suitable for sending over the
 386 network, writing to a file, and so on.
 387
 388 B<d2i_I<TYPE>>() attempts to decode I<len> bytes at I<*ppin>. If successful a
 389 pointer to the B<I<TYPE>> structure is returned and I<*ppin> is incremented to
 390 the byte following the parsed data.  If I<a> is not NULL then a pointer
 391 to the returned structure is also written to I<*a>.  If an error occurred
 392 then NULL is returned.
 393
 394 On a successful return, if I<*a> is not NULL then it is assumed that I<*a>
 395 contains a valid B<I<TYPE>> structure and an attempt is made to reuse it. This
 396 "reuse" capability is present for historical compatibility but its use is
 397 B<strongly discouraged> (see BUGS below, and the discussion in the RETURN
 398 VALUES section).
 399
 400 B<d2i_I<TYPE>_bio>() is similar to B<d2i_I<TYPE>>() except it attempts
 401 to parse data from BIO I<bp>.
 402
 403 B<d2i_I<TYPE>_fp>() is similar to B<d2i_I<TYPE>>() except it attempts
 404 to parse data from FILE pointer I<fp>.
 405
 406 B<i2d_I<TYPE>>() encodes the structure pointed to by I<a> into DER format.
 407 If I<ppout> is not NULL, it writes the DER encoded data to the buffer
 408 at I<*ppout>, and increments it to point after the data just written.
 409 If the return value is negative an error occurred, otherwise it
 410 returns the length of the encoded data.
 411
 412 If I<*ppout> is NULL memory will be allocated for a buffer and the encoded
 413 data written to it. In this case I<*ppout> is not incremented and it points
 414 to the start of the data just written.
 415
 416 B<i2d_I<TYPE>_bio>() is similar to B<i2d_I<TYPE>>() except it writes
 417 the encoding of the structure I<a> to BIO I<bp> and it
 418 returns 1 for success and 0 for failure.
 419
 420 B<i2d_I<TYPE>_fp>() is similar to B<i2d_I<TYPE>>() except it writes
 421 the encoding of the structure I<a> to FILE pointer I<fp> and it
 422 returns 1 for success and 0 for failure.
 423
 424 These routines do not encrypt private keys and therefore offer no
 425 security; use L<PEM_write_PrivateKey(3)> or similar for writing to files.
 426
 427 =head1 NOTES
 428
 429 The letters B<i> and B<d> in B<i2d_I<TYPE>>() stand for
 430 "internal" (that is, an internal C structure) and "DER" respectively.
 431 So B<i2d_I<TYPE>>() converts from internal to DER.
 432
 433 The functions can also understand B<BER> forms.
 434
 435 The actual TYPE structure passed to B<i2d_I<TYPE>>() must be a valid
 436 populated B<I<TYPE>> structure -- it B<cannot> simply be fed with an
 437 empty structure such as that returned by TYPE_new().
 438
 439 The encoded data is in binary form and may contain embedded zeros.
 440 Therefore, any FILE pointers or BIOs should be opened in binary mode.
 441 Functions such as strlen() will B<not> return the correct length
 442 of the encoded structure.
 443
 444 The ways that I<*ppin> and I<*ppout> are incremented after the operation
 445 can trap the unwary. See the B<WARNINGS> section for some common
 446 errors.
 447 The reason for this-auto increment behaviour is to reflect a typical
 448 usage of ASN1 functions: after one structure is encoded or decoded
 449 another will be processed after it.
 450
 451 The following points about the data types might be useful:
 452
 453 =over 4
 454
 455 =item B<ASN1_OBJECT>
 456
 457 Represents an ASN1 OBJECT IDENTIFIER.
 458
 459 =item B<DHparams>
 460
 461 Represents a PKCS#3 DH parameters structure.
 462
 463 =item B<DHxparams>
 464
 465 Represents an ANSI X9.42 DH parameters structure.
 466
 467 =item B<ECDSA_SIG>
 468
 469 Represents an ECDSA signature.
 470
 471 =item B<X509_ALGOR>
 472
 473 Represents an B<AlgorithmIdentifier> structure as used in IETF RFC 6960 and
 474 elsewhere.
 475
 476 =item B<X509_Name>
 477
 478 Represents a B<Name> type as used for subject and issuer names in
 479 IETF RFC 6960 and elsewhere.
 480
 481 =item B<X509_REQ>
 482
 483 Represents a PKCS#10 certificate request.
 484
 485 =item B<X509_SIG>
 486
 487 Represents the B<DigestInfo> structure defined in PKCS#1 and PKCS#7.
 488
 489 =back
 490
 491 =head1 RETURN VALUES
 492
 493 B<d2i_I<TYPE>>(), B<d2i_I<TYPE>_bio>() and B<d2i_I<TYPE>_fp>() return a valid
 494 B<I<TYPE>> structure or NULL if an error occurs.  If the "reuse" capability has
 495 been used with a valid structure being passed in via I<a>, then the object is
 496 freed in the event of error and I<*a> is set to NULL.
 497
 498 B<i2d_I<TYPE>>() returns the number of bytes successfully encoded or a negative
 499 value if an error occurs.
 500
 501 B<i2d_I<TYPE>_bio>() and B<i2d_I<TYPE>_fp>() return 1 for success and 0 if an
 502 error occurs.
 503
 504 =head1 EXAMPLES
 505
 506 Allocate and encode the DER encoding of an X509 structure:
 507
 508  int len;
 509  unsigned char *buf;
 510
 511  buf = NULL;
 512  len = i2d_X509(x, &buf);
 513  if (len < 0)
 514      /* error */
 515
 516 Attempt to decode a buffer:
 517
 518  X509 *x;
 519  unsigned char *buf;
 520  const unsigned char *p;
 521  int len;
 522
 523  /* Set up buf and len to point to the input buffer. */
 524  p = buf;
 525  x = d2i_X509(NULL, &p, len);
 526  if (x == NULL)
 527      /* error */
 528
 529 Alternative technique:
 530
 531  X509 *x;
 532  unsigned char *buf;
 533  const unsigned char *p;
 534  int len;
 535
 536  /* Set up buf and len to point to the input buffer. */
 537  p = buf;
 538  x = NULL;
 539
 540  if (d2i_X509(&x, &p, len) == NULL)
 541      /* error */
 542
 543 =head1 WARNINGS
 544
 545 Using a temporary variable is mandatory. A common
 546 mistake is to attempt to use a buffer directly as follows:
 547
 548  int len;
 549  unsigned char *buf;
 550
 551  len = i2d_X509(x, NULL);
 552  buf = OPENSSL_malloc(len);
 553  ...
 554  i2d_X509(x, &buf);
 555  ...
 556  OPENSSL_free(buf);
 557
 558 This code will result in I<buf> apparently containing garbage because
 559 it was incremented after the call to point after the data just written.
 560 Also I<buf> will no longer contain the pointer allocated by OPENSSL_malloc()
 561 and the subsequent call to OPENSSL_free() is likely to crash.
 562
 563 Another trap to avoid is misuse of the I<a> argument to B<d2i_I<TYPE>>():
 564
 565  X509 *x;
 566
 567  if (d2i_X509(&x, &p, len) == NULL)
 568      /* error */
 569
 570 This will probably crash somewhere in d2i_X509(). The reason for this
 571 is that the variable I<x> is uninitialized and an attempt will be made to
 572 interpret its (invalid) value as an B<X509> structure, typically causing
 573 a segmentation violation. If I<x> is set to NULL first then this will not
 574 happen.
 575
 576 =head1 BUGS
 577
 578 In some versions of OpenSSL the "reuse" behaviour of B<d2i_I<TYPE>>() when
 579 I<*a> is valid is broken and some parts of the reused structure may
 580 persist if they are not present in the new one. Additionally, in versions of
 581 OpenSSL prior to 1.1.0, when the "reuse" behaviour is used and an error occurs
 582 the behaviour is inconsistent. Some functions behaved as described here, while
 583 some did not free I<*a> on error and did not set I<*a> to NULL.
 584
 585 As a result of the above issues the "reuse" behaviour is strongly discouraged.
 586
 587 B<i2d_I<TYPE>>() will not return an error in many versions of OpenSSL,
 588 if mandatory fields are not initialized due to a programming error
 589 then the encoded structure may contain invalid data or omit the
 590 fields entirely and will not be parsed by B<d2i_I<TYPE>>(). This may be
 591 fixed in future so code should not assume that B<i2d_I<TYPE>>() will
 592 always succeed.
 593
 594 Any function which encodes a structure (B<i2d_I<TYPE>>(),
 595 B<i2d_I<TYPE>>() or B<i2d_I<TYPE>>()) may return a stale encoding if the
 596 structure has been modified after deserialization or previous
 597 serialization. This is because some objects cache the encoding for
 598 efficiency reasons.
 599
 600 =head1 COPYRIGHT
 601
 602 Copyright 1998-2020 The OpenSSL Project Authors. All Rights Reserved.
 603
 604 Licensed under the Apache License 2.0 (the "License").  You may not use
 605 this file except in compliance with the License.  You can obtain a copy
 606 in the file LICENSE in the source distribution or at
 607 L<https://www.openssl.org/source/license.html>.
 608
 609 =cut