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LBER_DECODE(3)             Library Functions Manual             LBER_DECODE(3)



NAME
       ber_get_next, ber_skip_tag, ber_peek_tag, ber_scanf, ber_get_int,
       ber_get_enum, ber_get_stringb, ber_get_stringa, ber_get_stringal,
       ber_get_stringbv, ber_get_null, ber_get_boolean, ber_get_bitstring,
       ber_first_element, ber_next_element - OpenLDAP LBER simplified Basic
       Encoding Rules library routines for decoding

LIBRARY
       OpenLDAP LBER (liblber, -llber)

SYNOPSIS
       #include <&lt;lber.h>&gt;

       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_scanf(BerElement *ber, const char *fmt, ...);

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

       ber_tag_t ber_get_enum(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_stringbv(BerElement *ber, struct berval *bv, int
       alloc);

       ber_tag_t ber_get_null(BerElement *ber);

       ber_tag_t ber_get_boolean(BerElement *ber, ber_int_t *bool);

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

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

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

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.  This man page describes the decoding routines in
       the lber library.  See lber-encode(3) for details on the corresponding
       encoding routines.  Consult lber-types(3) for information about types,
       allocators, and deallocators.

       Normally, the only routines that need to 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
       LBER_ERROR if an error occurred.

       The ber_get_next() routine is used to read the next BER element from
       the given Sockbuf, sb.  It strips off and returns the leading tag,
       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. See lber-sockbuf(3) for details of the Sockbuf implementation
       of the sb parameter.

       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.  The caller
                 should free the returned string using ber_memfree().

              A  Octet string.  A variant of "a".  A char ** should be
                 supplied.  Memory is allocated, filled with the contents of
                 the octet string, null-terminated, and returned in the
                 parameter, unless a zero-length string would result; in that
                 case, the arg is set to NULL.  The caller should free the
                 returned string using ber_memfree().

              s  Octet string.  A char * buffer should be supplied, followed
                 by a pointer to a ber_len_t initialized to the size of the
                 buffer.  Upon return, the null-terminated octet string is put
                 into the buffer, and the ber_len_t 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 dynamically allocated struct berval
                 containing the octet string and its length.  The caller
                 should free the returned structure using ber_bvfree().

              o  Octet string.  A struct ber_val * should be supplied, which
                 upon return contains the dynamically allocated octet string
                 and its length.  The caller should free the returned octet
                 string using ber_memfree().

              m  Octet string.  A struct ber_val * should be supplied, which
                 upon return contains the octet string and its length.  The
                 string resides in memory assigned to the BerElement, and must
                 not be freed by the caller.

              b  Boolean.  A pointer to a ber_int_t should be supplied.

              e  Enumeration.  A pointer to a ber_int_t should be supplied.

              i  Integer.  A pointer to a ber_int_t should be supplied.

              B  Bitstring.  A char ** should be supplied which will point to
                 the dynamically allocated bits, followed by a ber_len_t *,
                 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 dynamically allocated null-
                 terminated array of char *'s containing the octet strings.
                 NULL is returned if the sequence is empty.  The caller should
                 free the returned array and octet strings using
                 ber_memvfree().

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

              W  Sequence of octet strings with lengths.  A BerVarray * should
                 be supplied, which upon return points to a dynamically
                 allocated array of struct berval's containing the octet
                 strings and their lengths. The array is terminated by a
                 struct berval with a NULL bv_val string pointer.  NULL is
                 returned if the sequence is empty.  The caller should free
                 the returned structures using ber_bvarray_free().

              M  Sequence of octet strings with lengths.  This is a
                 generalized form of the previous three formats.  A void **
                 (ptr) should be supplied, followed by a ber_len_t * (len) and
                 a ber_len_t (off).  Upon return (ptr) will point to a
                 dynamically allocated array whose elements are all of size
                 (*len).  A struct berval will be filled starting at offset
                 (off) in each element.  The strings in each struct berval
                 reside in memory assigned to the BerElement and must not be
                 freed by the caller.  The array is terminated by a struct
                 berval with a NULL bv_val string pointer.  NULL is returned
                 if the sequence is empty.  The number of elements in the
                 array is also stored in (*len) on return.  The caller should
                 free the returned array using ber_memfree().

              l  Length of the next element.  A pointer to a ber_len_t should
                 be supplied.

              t  Tag of the next element.  A pointer to a ber_tag_t should be
                 supplied.

              T  Skip element and return its tag.  A pointer to a ber_tag_t
                 should be supplied.

              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, LBER_ERROR (-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 dynamically allocate space
       into which an octet string is read.  The caller should free the
       returned string using ber_memfree().

       The ber_get_stringal() routine is used to dynamically allocate space
       into which an octet string and its length are read.  It takes a struct
       berval **, and returns the result in this parameter.  The caller should
       free the returned structure using ber_bvfree().

       The ber_get_stringbv() routine is used to read an octet string and its
       length into the provided struct berval *. If the alloc parameter is
       zero, the string will reside in memory assigned to the BerElement, and
       must not be freed by the caller. If the alloc parameter is non-zero,
       the string will be copied into dynamically allocated space which should
       be returned using ber_memfree().

       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_enum() routine is used to read a enumeration 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 dynamically allocated bits,
       followed by an ber_len_t *, which will point to the length (in bits) of
       the bitstring returned.  The caller should free the returned string
       using ber_memfree().

       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.

             ber_int_t    scope, deref, size, time, attrsonly;
             char   *dn, **attrs;
             ber_tag_t tag;

             tag = ber_scanf( ber, "{aeeiib{v}}",
                 &dn, &scope, &deref,
                 &size, &time, &attrsonly, &attrs );

             if( tag == LBER_ERROR ) {
                     /* error */
             } else {
                     /* success */
             }

             ber_memfree( dn );
             ber_memvfree( attrs );

ERRORS
       If an error occurs during decoding, generally these routines return
       LBER_ERROR ((ber_tag_t)-1).

NOTES
       The return values for all of these functions are declared in the
       <&lt;lber.h>&gt; header file.  Some routines may dynamically allocate memory
       which must be freed by the caller using supplied deallocation routines.

SEE ALSO
       lber-encode(3), lber-memory(3), lber-sockbuf(3), lber-types(3)

ACKNOWLEDGEMENTS
       OpenLDAP Software is developed and maintained by The OpenLDAP Project
       <http://www.openldap.org/>;.  OpenLDAP Software is derived from
       University of Michigan LDAP 3.3 Release.



OpenLDAP 2.4.23                   2010/06/30                    LBER_DECODE(3)