ber_decode

, ber_alloc_t

, ber_free

, ber_bvdup

, ber_init

, ber_flatten

, ber_get_next

, ber_skip_tag

, ber_peek_tag

, ber_scanf

, ber_get_int

, ber_get_stringa

, ber_get_stringal

, ber_get_stringb

, ber_get_null

, ber_get_boolean

, ber_get_bitstring

, ber_first_element

, ber_next_element

, ber_bvfree

, ber_bvecfree

- Basic Encoding Rules library decoding functions

Synopsis

cc [ flag... ] file... -lldap [ library... ]
#include <lber.h>

BerElement *ber_alloc_t(int options);

struct berval *ber_bvdup(const struct berval *bv);

void ber_free(BerElement *ber, int freebuf);

BerElement *ber_init(const struct berval *bv);

int ber_flatten(BerElement *ber, struct berval **bvPtr);

ber_tag_t ber_get_next(Sockbuf *sb, ber_len_t *len, BerElement *ber);

ber_tag_t ber_skip_tag(BerElement *ber, ber_len_t *len);

ber_tag_t ber_peek_tag(BerElement *ber, ber_len_t *len);

ber_tag_t ber_get_int(BerElement *ber, ber_int_t *num);

ber_tag_t ber_get_stringb(BerElement *ber, char *buf,
     ber_len_t *len);

ber_tag_t ber_get_stringa(BerElement *ber, char **buf);

ber_tag_t ber_get_stringal(BerElement *ber, struct berval **bv);

ber_tag_t ber_get_null(BerElement *ber);

ber_tag_t ber_get_boolean(BerElement *ber, int *boolval);

ber_tag_t ber_get_bitstringa(BerElement *ber, char **buf,
     ber_len_t *len);

ber_tag_t ber_first_element(BerElement *ber, ber_len_t *len,
     char **last);

ber_tag_t ber_next_element(BerElement *ber, ber_len_t *len,
     char *last);

ber_tag_t ber_scanf(BerElement *ber, const char *fmt [, arg...]);

void ber_bvfree(struct berval *bv);

void ber_bvecfree(struct berval **bvec);

Description

These functions provide a subfunction interface to a simplified implementation of the Basic Encoding Rules of ASN.1. The version of BER these functions support is the one defined for the LDAP protocol. The encoding rules are the same as BER, except that only definite form lengths are used, and bitstrings and octet strings are always encoded in primitive form. In addition, these lightweight BER functions restrict tags and class to fit in a single octet (this means the actual tag must be less than 31). When a “tag”is specified in the descriptions below, it refers to the tag, class, and primitive or constructed bit in the first octet of the encoding. This man page describes the decoding functions in the lber library. See ber_encode(3LDAP) for details on the corresponding encoding functions.

Normally, the only functions that need be called by an application are ber_get_next() to get the next BER element and ber_scanf() to do the actual decoding. In some cases, ber_peek_tag() may also need to be called in normal usage. The other functions are provided for those applications that need more control than ber_scanf() provides. In general, these functions return the tag of the element decoded, or -1 if an error occurred.

The ber_get_next() function is used to read the next BER element from the given Sockbuf, sb. A Sockbuf consists of the descriptor (usually socket, but a file descriptor works just as well) from which to read, and a BerElement structure used to maintain a buffer. On the first call, the sb_ber struct should be zeroed. It strips off and returns the leading tag byte, strips off and returns the length of the entire element in len, and sets up ber for subsequent calls to ber_scanf(), and all to decode the element.

The ber_peek_tag() function returns the tag of the next element to be parsed in the BerElement argument. The length of this element is stored in the *lenPtr argument. LBER_DEFAULT is returned if there is no further data to be read. The decoding position within the ber argument is unchanged by this call; that is, the fact that ber_peek_tag() has been called does not affect future use of ber.

The ber_skip_tag() function is similar to ber_peek_tag(), except that the state pointer in the BerElement argument is advanced past the first tag and length, and is pointed to the value part of the next element. This function should only be used with constructed types and situations when a BER encoding is used as the value of an OCTET STRING. The length of the value is stored in *lenPtr.

The ber_scanf() function is used to decode a BER element in much the same way that scanf(3C) works. It reads from ber, a pointer to a BerElement such as returned by ber_get_next(), interprets the bytes according to the format string fmt, and stores the results in its additional arguments. The format string contains conversion specifications which are used to direct the interpretation of the BER element. The format string can contain the following characters.

a

Octet string. A char ** should be supplied. Memory is allocated, filled with the contents of the octet string, null-terminated, and returned in the parameter.

s

Octet string. A char * buffer should be supplied, followed by a pointer to an integer initialized to the size of the buffer. Upon return, the null-terminated octet string is put into the buffer, and the integer is set to the actual size of the octet string.

O

Octet string. A struct ber_val ** should be supplied, which upon return points to a memory allocated struct berval containing the octet string and its length. ber_bvfree() can be called to free the allocated memory.

b

Boolean. A pointer to an integer should be supplied.

i

Integer. A pointer to an integer should be supplied.

B

Bitstring. A char ** should be supplied which will point to the memory allocated bits, followed by an unsigned long *, which will point to the length (in bits) of the bitstring returned.

n

Null. No parameter is required. The element is simply skipped if it is recognized.

v

Sequence of octet strings. A char *** should be supplied, which upon return points to a memory allocated null-terminated array of char *'s containing the octet strings. NULL is returned if the sequence is empty.

V

Sequence of octet strings with lengths. A struct berval *** should be supplied, which upon return points to a memory allocated, null-terminated array of struct berval *'s containing the octet strings and their lengths. NULL is returned if the sequence is empty. ber_bvecfree() can be called to free the allocated memory.

x

Skip element. The next element is skipped.

{

Begin sequence. No parameter is required. The initial sequence tag and length are skipped.

}

End sequence. No parameter is required and no action is taken.

[

Begin set. No parameter is required. The initial set tag and length are skipped.

]

End set. No parameter is required and no action is taken.

The ber_get_int() function tries to interpret the next element as an integer, returning the result in num. The tag of whatever it finds is returned on success, –1 on failure.

The ber_get_stringb() function is used to read an octet string into a pre-allocated buffer. The len parameter should be initialized to the size of the buffer, and will contain the length of the octet string read upon return. The buffer should be big enough to take the octet string value plus a terminating NULL byte.

The ber_get_stringa() function is used to allocate memory space into which an octet string is read.

The ber_get_stringal() function is used to allocate memory space into which an octet string and its length are read. It takes a struct berval **, and returns the result in this parameter.

The ber_get_null() function is used to read a NULL element. It returns the tag of the element it skips over.

The ber_get_boolean() function is used to read a boolean value. It is called the same way that ber_get_int() is called.

The ber_get_bitstringa() function is used to read a bitstring value. It takes a char ** which will hold the allocated memory bits, followed by an unsigned long *, which will point to the length (in bits) of the bitstring returned.

The ber_first_element() function is used to return the tag and length of the first element in a set or sequence. It also returns in last a magic cookie parameter that should be passed to subsequent calls to ber_next_element(), which returns similar information.

The ber_alloc_t() function constructs and returns BerElement. A null pointer is returned on error. The options field contains a bitwise-OR of options which are to be used when generating the encoding of this BerElement. One option is defined and must always be supplied:

#define LBER_USE_DER 0x01

When this option is present, lengths will always be encoded in the minimum number of octets. Note that this option does not cause values of sets and sequences to be rearranged in tag and byte order, so these functions are not suitable for generating DER output as defined in X.509 and X.680

The ber_init function constructs a BerElement and returns a new BerElement containing a copy of the data in the bv argument. The ber_init function returns the null pointer on error.

The ber_free() function frees a BerElement which is returned from the API calls ber_alloc_t() or ber_init(). Each BerElement must be freed by the caller. The second argument freebuf should always be set to 1 to ensure that the internal buffer used by the BER functions is freed as well as the BerElement container itself.

The ber_bvdup() function returns a copy of a berval. The bv_val field in the returned berval points to a different area of memory as the bv_val field in the argument berval. The null pointer is returned on error (that is, is out of memory).

The ber_flatten() function allocates a struct berval whose contents are BER encoding taken from the ber argument. The bvPtr pointer points to the returned berval, which must be freed using ber_bvfree(). This function returns 0 on success and -1 on error.

Examples

Example 1 Assume the variable ber contains a lightweight BER encoding of the following ASN.1 object:

      AlmostASearchRequest := SEQUENCE {
          baseObject      DistinguishedName,
          scope           ENUMERATED {
              baseObject    (0),
              singleLevel   (1),
              wholeSubtree  (2)
          },
          derefAliases    ENUMERATED {
              neverDerefaliases   (0),
              derefInSearching    (1),
              derefFindingBaseObj (2),
              alwaysDerefAliases  (3N)
          },
          sizelimit       INTEGER (0 .. 65535),
          timelimit       INTEGER (0 .. 65535),
          attrsOnly       BOOLEAN,
          attributes      SEQUENCE OF AttributeType
      }

Example 2 The element can be decoded using ber_scanf() as follows.

      int    scope, ali, size, time, attrsonly;
      char   *dn, **attrs;
      if ( ber_scanf( ber, "{aiiiib{v}}", &dn, &scope, &ali,
          &size, &time, &attrsonly, &attrs ) == –1 )
              /* error */
      else
              /* success */

Errors

If an error occurs during decoding, generally these functions return -1.

Notes

The return values for all of these functions are declared in the <lber.h> header. Some functions may allocate memory which must be freed by the calling application.

Attributes

See attributes(5) for a description of the following attributes:

See Also

ber_encode(3LDAP), attributes(5)

Yeong, W., Howes, T., and Hardcastle-Kille, S., “Lightweight Directory Access Protocol”, OSI-DS-26, April 1992.

Information Processing - Open Systems Interconnection - Model and Notation - Service Definition - Specification of Basic Encoding Rules for Abstract Syntax Notation One, International Organization for Standardization, International Standard 8825.