lber-encode(3ldap)

lber-encode -- LBER simplified encoding library routines

Synopsis

#include <lber.h> 
 
typedef struct berelement { 
    char             ber_buf; 
    char             ber_ptr; 
    char             ber_end; 
    struct seqorset  ber_sos; 
    int              ber_tag; 
    int              ber_usertag; 
} BerElement; 
 
typedef struct sockbuf { 
    int              sb_sd; 
    BerElement       sb_ber; 
} Sockbuf; 
 
BerElement ber_alloc(); 
 
ber_flush(Sockbuf sb, BerElement ber, int freeit); 
 
ber_printf(BerElement ber, char fmt [, arg... ] ); 
 
ber_put_int(BerElement ber, long num, char tag); 
 
ber_put_ostring(BerElement ber, char str, unsigned long len, char tag); 
 
ber_put_string(BerElement ber, char str, char tag); 
 
ber_put_null(BerElement ber, char tag); 
 
ber_put_boolean(BerElement ber, int bool, char tag); 
 
ber_put_bitstring(BerElement ber, char str, int blen, char tag); 
 
ber_start_seq(BerElement ber, char tag); 
 
ber_start_set(BerElement ber, char tag); 
 
ber_put_seq(BerElement ber); 
 
ber_put_set(BerElement ber);

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, it refers to the tag, class, and primitive or constructed bit in the first octet of the encoding. This manual page describes the encoding routines in the LBER library. See lber-decode(3ldap) for details on the corresponding decoding routines.

Normally, the only routines that need be called by an application are ber_alloc to allocate a BER element for encoding, ber_printf to do the actual encoding, and ber_flush to actually write the element. The other routines are provided for those applications that need more control than ber_printf provides. In general, these routines return the length of the element encoded, or -1 if an error occurred.

The ber_alloc routine is used to allocate a new BER element. The ber_flush routine is used to actually write the element to a socket (or file) descriptor, once it has been fully encoded (using ber_printf). The sb structure contains the descriptor and a BerElement used for input buffering. Only the sb_sd field is relevant to the ber_flush routine.

The ber_printf routine is used to encode a BER element in much the same way that the sprintf routine works (see the fprintf(3S) manual page). One important difference, though, is that some state information is kept with the ber parameter so that multiple calls can be made to ber_printf to append things to the end of the BER element. ber_printf writes to ber, a pointer to a BerElement such as returned by ber_alloc. It interprets and formats its arguments according to the format string fmt. The format string can contain the following characters:

b: Boolean. An integer parameter should be supplied. A boolean element is output.
i: Integer. An integer parameter should be supplied. An integer element is output.
B: Bitstring. A char pointer to the start of the bitstring is supplied, followed by the number of bits in the bitstring. A bitstring element is output.
n: Null. No parameter is required. A null element is output.
o: Octet string. A char is supplied, followed by the length of the string pointed to. An octet string element is output.
s: Octet string. A null-terminated string is supplied. An octet string element is output, not including the trailing NULL octet.
t: Tag. An int specifying the tag to give the next element is provided. This works across calls.
v: Several octet strings. A null-terminated array of char 's is supplied. Note that a construct like '{v}' is required to get an actual sequence of octet strings.
{: Begin sequence. No parameter is required.
}: End sequence. No parameter is required.
[: Begin set. No parameter is required.
]: End set. No parameter is required.

The ber_put_int routine writes the integer element num to the BER element ber.

The ber_put_boolean routine writes the boolean value given by bool to the BER element.

The ber_put_bitstring routine writes blen bits starting at str as a bitstring value to the given BER element. Note that blen is the length in bits of the bitstring.

The ber_put_ostring routine writes len bytes starting at str to the BER element as an octet string.

The ber_put_string routine writes the null-terminated string (minus the terminating '\0') to the BER element as an octet string.

The ber_put_null routine writes a NULL element to the BER element.

The ber_start_seq routine is used to start a sequence in the BER element. The ber_start_set routine works similarly. The end of the sequence or set is marked by the nearest matching call to ber_put_seq or ber_put_set, respectively.

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.

Return values

If an error occurs during encoding, generally these routines return -1.

The return values for all of these functions are declared in the <lber.h> header file.

Examples

Assuming the following variable declarations, and that the variables have been assigned appropriately, an LBER 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   3ldap 
       }, 
       sizelimit       INTEGER (0 .. 65535), 
       timelimit       INTEGER (0 .. 65535), 
       attrsOnly       BOOLEAN, 
       attributes      SEQUENCE OF AttributeType 
   }

can be achieved like so:

   int    scope, ali, size, time, attrsonly; 
   char   dn, attrs; 
    
   / ... fill in values ... / 
   if ( (ber = ber_alloc()) == NULLBER ) 
           / error / 
   
 
   if ( ber_printf( ber, "{siiiib{v}}", dn, scope, ali, 
       size, time, attrsonly, attrs ) == -1 ) 
           / error / 
   else 
           / success /

References

lber-decode(3ldap)

Yeong, W., Howes, T., and Hardcastle-Kille, S., Lightweight Directory Access Protocol, OSI-DS-26, April 1992.

Information Processing - Open Systems Interconnection - Model and Notation - Service Definition - Specification of Basic Encoding Rules for Abstract Syntax Notation One, International Organization for Standardization, International Standard 8825.