LBER_DECODE(3)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 - LBER simplified
Basic Encoding Rules library routines for decoding
LIBRARY
OpenLDAP LBER (liblber, -llber)
SYNOPSIS
#include <lber.h>
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 simpli-
fied 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 gen-
eral, 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 subse-
quent calls to ber_scanf() et al to decode the element.
See lber-sockbuf(3) for details of the Sockbuf implementa-
tion 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 for-
mat string fmt, and stores the results in its additional
arguments. The format string contains conversion specifi-
cations 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().
s Octet string. A char * buffer should be sup-
plied, followed by a pointer to a ber_len_t ini-
tialized 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 dynami-
cally 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 sup-
plied, which upon return contains the dynami-
cally 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 sup-
plied, 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 dynami-
cally 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 for-
mats. A void ** (ptr) should be supplied, fol-
lowed by a ber_len_t * (len) and a ber_len_t
(off). Upon return (ptr) will point to a dynam-
ically 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 ini-
tial 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 ele-
ment 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 allo-
cate 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 bit-
string 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_ele-
ment(), which returns similar information.
EXAMPLES
Assume the variable ber contains a lightweight BER encod-
ing 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 rou-
tines return LBER_ERROR ((ber_tag_t)-1).
NOTES
The return values for all of these functions are declared
in the <lber.h> header file. Some routines may dynami-
cally allocate memory which must be freed by the caller
using supplied deallocation routines.
SEE ALSOlber-encode(3), lber-memory(3), lber-sockbuf(3), lber-
types(3)ACKNOWLEDGEMENTS
OpenLDAP is developed and maintained by The OpenLDAP Pro-
ject (http://www.openldap.org/). OpenLDAP is derived from
University of Michigan LDAP 3.3 Release.
OpenLDAP LDVERSION RELEASEDATE LBER_DECODE(3)
