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Manpages LBER_DECODESection: C Library Functions (3)Updated: 12 July 2000 Index Return to Main Contents NAMEber_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 decodingSYNOPSIS#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); DESCRIPTIONThese 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.
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. EXAMPLESAssume 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 ); ERRORSIf an error occurs during decoding, generally these routines return LBER_ERROR (-1).NOTESThe 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 ALSOlber-encode(3) lber-memory(3) lber-types(3) ldap-parse(3) ldap-sync(3) ldap-async(3)ACKNOWLEDGEMENTSis developed and maintained by The OpenLDAP Project (http://www.openldap.org/). is derived from University of Michigan LDAP 3.3 Release.
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