LBER-DECODE(3)

NAME
ber_get_next, ber_skiptag, ber_peek_tag, ber_scanf, ber_get_int, ber_get_stringb, ber_get_stringa, ber_get_null, ber_get_boolean, ber_get_bitstring, ber_first_element, ber_next_element - LBER simplified Basic Encoding Rules library routines for decoding

SYNOPSIS
#include <lber.h>

typedef struct berelement {
    char *ber_buf;
    char *ber_ptr;
    char *ber_end;
    struct seqorset *ber_sos;
    int ber_tag;
    int ber_usertag;
} BerElement;

typedef struct sockbuf {
    int sb_sd;
    BerElement sb_ber;
} Sockbuf;

typedef struct berval {
    unsigned long bv_len;
    char *bv_val;
};

ber_get_next(sb, len, ber)
Sockbuf *sb;
unsigned long *len;
BerElement *ber;

ber_skip_tag(ber, len)
BerElement *ber;
unsigned long *len;

ber_peek_tag(ber, len)
BerElement *ber;
unsigned long *len;

ber_get_int(ber, num)
BerElement *ber;
long *num;

ber_get_stringb(ber, buf, len)
BerElement *ber;
char *buf;
unsigned long *len;

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

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

ber_get_null(ber)
BerElement *ber;

ber_get_boolean(ber, bool)
BerElement *ber;
int *bool;

ber_get_bitstringa(ber, buf, blen)
BerElement *ber;
char **buf;
unsigned long *blen;

ber_first_element(ber, len, cookie)
BerElement *ber;
unsigned long *len;
char **cookie;

ber_next_element(ber, len, cookie)
BerElement *ber;
unsigned long *len;
char *cookie;

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

ber_bvfree(bv)
struct berval *bv;

ber_bvecfree(bvec)
struct berval **bvec;

DESCRIPTION

These routines provide a subroutine interface to a simplified implementation of the Basic Encoding Rules of ASN.1. The version of BER these routines 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 routines 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 routines in the lber library. See lber-encode(3) for details on the corresponding encoding routines.

Normally, the only routines 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 routines are provided for those applications that need more control than ber_scanf() provides. In general, these routines return the tag of the element decoded, or -1 if an error occurred.

The ber_get_next() routine 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() et al to decode the element.

The ber_scanf() routine is used to decode a BER element in much the same way that scanf(3) 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 malloced struct berval containing the octet string and its length. ber_bvfree() can be called to free the malloced 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 malloced 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 malloced 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 malloced 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 malloced 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() routine 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() routine is used to read an octet string into a preallocated 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() routine is used to malloc space into which an octet string is read.

The ber_get_stringal() routine is used to malloc 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() routine is used to read a NULL element. It returns the tag of the element it skips over.

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

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

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

EXAMPLES
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  (3)
          },
          sizelimit       INTEGER (0 .. 65535),
          timelimit       INTEGER (0 .. 65535),
          attrsOnly       BOOLEAN,
          attributes      SEQUENCE OF AttributeType
      }

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 routines return -1.

NOTES

The return values for all of these functions are declared in the <lber.h> header file. Some routines may malloc memory.

SEE ALSO
lber-encode(3) ldap-parse(3) ldap-sync(3) ldap-async(3)

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.

AUTHOR
Tim Howes, University of Michigan