ber_get_next(3)
LBER_DECODE(3) C LIBRARY FUNCTIONS 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, BerEle-
ment *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
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(3)
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 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 ele-
ment 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 parame-
ter. 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
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(3)
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 sup-
plied, 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 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 sup-
plied.
e Enumeration. A pointer to a ber_int_t should be
supplied.
i Integer. A pointer to a ber_int_t should be sup-
plied.
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 contain-
ing 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
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(3)
dynamically allocated array of struct berval's con-
taining 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 ini-
tial 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 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.
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(3)
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 allo-
cate 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:
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(3)
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 sup-
plied deallocation routines.
SEE ALSO
lber-encode(3), lber-memory(3), lber-sockbuf(3), lber-
types(3)
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LBER_DECODE(3) C LIBRARY FUNCTIONS LBER_DECODE(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.
OpenLDAP LDVERSION Last change: RELEASEDATE 7
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