lber-decode.3

开放源码的ldap系统

.TH LBER_DECODE 3 "12 July 2000" "OpenLDAP LDVERSION"
.\" $OpenLDAP: pkg/ldap/doc/man/man3/lber-decode.3,v 1.4.8.2 2000/07/29 01:53:04 kurt Exp $
.\" Copyright 1998-2000 The OpenLDAP Foundation All Rights Reserved.
.\" Copying restrictions apply.  See COPYRIGHT/LICENSE.
.SH 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
.SH SYNOPSIS
.nf
.ft B
#include <lber.h>
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_next(
	Sockbuf *sb,
	ber_len_t \(**len,
	BerElement \(**ber);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_skip_tag(
	BerElement \(**ber,
	ber_len_t long \(**len);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_peek_tag(
	BerElement \(**ber,
	ber_len_t \(**len);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_scanf(
	BerElement \(**ber,
	const char \(**fmt, ...);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_int(
	BerElement \(**ber,
	ber_int_t \(**num);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_enum(
	BerElement \(**ber,
	ber_int_t \(**num);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_stringb(
	BerElement \(**ber,
	char \(**buf,
	ber_len_t \(**len);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_stringa(
	BerElement \(**ber,
	char \(***buf);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_stringal(
	BerElement \(**ber,
	struct berval \(***bv);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_null(
	BerElement \(**ber );
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_boolean(
	BerElement \(**ber,
	ber_int_t \(**bool);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_get_bitstringa(
	BerElement \(**ber,
	char \(***buf,
	ber_len_t \(**blen);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_first_element(
	BerElement \(**ber,
	ber_len_t \(**len,
	char \(***cookie);
.ft
.fi
.LP
.nf
.ft B
ber_tag_t ber_next_element(
	BerElement \(**ber,
	ber_len_t \(**len,
	const char \(**cookie);
.SH DESCRIPTION
.LP
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.
.LP
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.
.LP
The ber_get_next() routine is used to read the next BER element from
the given Sockbuf, \fIsb\fP.  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 \fIsb_ber\fP struct should
be zeroed.  It strips off and returns the
leading tag byte, strips off and returns the length of the
entire element in \fIlen\fP,
and sets up \fIber\fP for subsequent calls to ber_scanf() et al to decode
the element.
.LP
The ber_scanf() routine is used to decode a BER element in much the
same way that scanf(3) works.  It reads from \fIber\fP, a pointer to a
BerElement such as returned by ber_get_next(), interprets the
bytes according to the format string \fIfmt\fP, 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.
.RS
.LP
.TP 3
.SM 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().
.TP
.SM 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.
.TP
.SM 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().
.TP
.SM 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().
.TP
.SM b
Boolean.  A pointer to a ber_int_t should be supplied.
.TP
.SM e
Enumeration.  A pointer to a ber_int_t should be supplied.
.TP
.SM i
Integer.  A pointer to a ber_int_t should be supplied.
.TP
.SM 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.
.TP
.SM n
Null.  No parameter is required.  The element is simply skipped if
it is recognized.
.TP
.SM 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().
.TP
.SM 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().
.TP
.SM l
Length of the next element.  A pointer to a ber_len_t should be supplied.
.TP
.SM t
Tag of the next element.  A pointer to a ber_tag_t should be supplied.
.TP
.SM T
Skip element and return its tag.  A pointer to a ber_tag_t should be supplied.
.TP
.SM x
Skip element.  The next element is skipped.
.TP
.SM {
Begin sequence.  No parameter is required.  The initial sequence tag
and length are skipped.
.TP
.SM }
End sequence.  No parameter is required and no action is taken.
.TP
.SM [
Begin set.  No parameter is required.  The initial set tag
and length are skipped.
.TP
.SM ]
End set.  No parameter is required and no action is taken.
.RE
.LP
The ber_get_int() routine tries to interpret the next element as an integer,
returning the result in \fInum\fP.  The tag of whatever it finds is returned
on success, LBER_ERROR (\-1) on failure.
.LP
The ber_get_stringb() routine is used to read an octet string into a
preallocated buffer.  The \fIlen\fP 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.
.LP
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().
.LP
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().
.LP
The ber_get_null() routine is used to read a NULL element.  It returns
the tag of the element it skips over.
.LP
The ber_get_boolean() routine is used to read a boolean value.
It is called the same way that ber_get_int() is called.
.LP
The ber_get_enum() routine is used to read a enumeration value.
It is called the same way that ber_get_int() is called.
.LP
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().
.LP
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 \fIcookie\fP
a magic cookie parameter that should be passed to subsequent calls to
ber_next_element(), which returns similar information.
.SH EXAMPLES
Assume the variable \fIber\fP contains a lightweight BER encoding of
the following ASN.1 object:
.LP
.nf
      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
      }
.fi
.LP
The element can be decoded using ber_scanf() as follows.
.LP
.nf
      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 );
.fi
.SH ERRORS
If an error occurs during decoding, generally these routines return
LBER_ERROR (\-1).
.LP
.SH NOTES
.LP
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.
.SH SEE ALSO
.BR lber-encode (3)
.BR lber-memory (3)
.BR lber-types (3)
.BR ldap-parse (3)
.BR ldap-sync (3)
.BR ldap-async (3)
.SH ACKNOWLEDGEMENTS
.B	OpenLDAP
is developed and maintained by The OpenLDAP Project (http://www.openldap.org/).
.B	OpenLDAP
is derived from University of Michigan LDAP 3.3 Release.