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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_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>
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 long *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_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. 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. Consult
lber-types(3) for information about types, allocators, and deallocators.
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. The caller should free the returned
ber_val 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 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 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 points containing the dynamically allocated octet string and
its length. The caller should free the returned octet string using
ber_memfree().
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 an 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().
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_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 (-1).
NOTES
The return values for all of these functions are declared in the <lber.h>
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-types(3) ldap-parse(3) ldap-sync(3)
ldap-async(3)
ACKNOWLEDGEMENTS
OpenLDAP is developed and maintained by The OpenLDAP Project
(http://www.openldap.org/). OpenLDAP is derived from University of
Michigan LDAP 3.3 Release.
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