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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_bit-string, ber_get_bitstring,
string, ber_first_element, ber_next_element - OpenLDAP 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 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 bit-strings bitstrings
strings 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-sock-buf(3) lber-sockbuf(3)
buf(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 ele-ment. element.
ment. 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 parame-ter, parameter,
ter, 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 dynam-ically dynamically
ically 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 dynam-ically dynamically
ically 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 con-taining containing
taining 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 ele-ments elements
ments 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 BerEle-ment BerElement
ment 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 ter-minating terminating
minating 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 <lber.h> header file. Some routines
may dynamically allocate memory which must be freed by the caller using supplied deallocation rou-tines. routines.
tines.
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 2.4.28 2011/11/24 LBER_DECODE(3)
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