Document revision date: 30 March 2001

The following is an example of encoding:

BerElement *ber_alloc_t(int options);

The ber_alloc_t() function constructs and returns BerElement. The null pointer is returned on error. The options field contains a bitwise-or of options which are to be used when generating the encoding of this BerElement. One option is defined and must always be supplied:

#define LBER_USE_DER 0x01

When this option is present, lengths will always be encoded in the minimum number of octets. Note that this option does not cause values of sets and sequences to be rearranged in tag and byte order, so these functions are not sufficient for generating DER output as defined in X.509 and X.680. If the caller takes responsibility for ordering values of sets and sequences correctly, DER output as defined in X.509 and X.680 can be produced.

Unrecognized option bits are ignored.

The BerElement returned by ber_alloc_t() is initially empty. Calls to ber_printf() will append bytes to the end of the BerElement.

int ber_printf(BerElement *ber, char *fmt, ... )

The ber_printf() function is used to encode a BER element in much the same way that sprintf() works. One important difference, though, is that state information is kept in the BER argument so that multiple calls can be made to ber_printf() to append to the end of the BER element. BER must be a pointer to a BerElement returned by ber_alloc_t() . The ber_printf() function interprets and formats its arguments according to the format string fmt . The ber_printf() function returns -1 if there is an error during encoding and a positive number if successful. As with sprintf() , each character in fmt refers to an argument to ber_printf() .

The format string can contain the following format characters:

t	Tag. The next argument is a ber_tag_t specifying the tag to override the next element to be written to the ber. This works across calls. The value must contain the tag class, constructed bit, and tag value. The tag value must fit in a single octet (tag value is less than 32). For example, a tag of "[3]" for a constructed type is 0xA3.
b	Boolean. The next argument is a ber_int_t , containing either 0 for FALSE or 0xff for TRUE. A boolean element is output. If this format character is not preceded by the 't' format modifier, the tag 0x01 is used for the element.
e	Enumerated. The next argument is a ber_int_t , containing the enumerated value in the host's byte order. An enumerated element is output. If this format character is not preceded by the 't' format modifier, the tag 0x0A is used for the element.
i	Integer. The next argument is a ber_int_t , containing the integer in the host's byte order. An integer element is output. If this format character is not preceded by the 't' format modifier, the tag 0x02 is used for the element.
B	Bitstring. The next two arguments are a char * pointer to the start of the bitstring, followed by a ber_len_t containing the number of bits in the bitstring. A bitstring element is output, in primitive form. If this format character is not preceded by the 't' format modifier, the tag 0x03 is used for the element.
n	Null. No argument is required. An ASN.1 NULL element is output. If this format character is not preceded by the 't' format modifier, the tag 0x05 is used for the element.
o	Octet string. The next two arguments are a char *, followed by a ber_len_t with the length of the string. The string may contain null bytes and need not by zero-terminated. An octet string element is output, in primitive form. If this format character is not preceded by the 't' format modifier, the tag 0x04 is used for the element.
s	Octet string. The next argument is a char * pointing to a zero-terminated string. An octet string element in primitive form is output, which does not include the trailing '\0' byte. If this format character is not preceded by the 't' format modifier, the tag 0x04 is used for the element.
v	Several octet strings. The next argument is a char **, an array of char * pointers to zero-terminated strings. The last element in the array must be a null pointer. The octet strings do not include the leading SEQUENCE OF octet strings. The 't' format modifier cannot be used with this format character.
V	Several octet strings. A NULL-terminated array of struct berval *'s is supplied. Note that a construct like '{V}' is required to get an actual SEQUENCE OF octet strings. The 't' format modifier cannot be used with this format character.
{	Begin sequence. No argument is required. If this format character is not preceded by the 't' format modifier, the tag 0x30 is used.
}	End sequence. No argument is required. The 't' format modifier cannot be used with this format character.
[	Begin set. No argument is required. If this format character is not preceded by the 't' format modifier, the tag 0x31 is used.
]	End set. No argument is required. The 't' format modifier cannot be used with this format character.

Each use of a '{' format character must be matched by a '}' character, either later in the format string, or in the format string of a subsequent call to ber_printf() for that BerElement. The same applies to the '[' and ']'.

Sequences and sets nest, and implementations of this API must maintain internal state to be able to properly calculate the lengths.

int ber_flatten (BerElement *ber, struct berval **bvPtr);

The ber_flatten() function allocates a struct berval whose contents are a BER encoding taken from the ber argument. The bvPtr pointer points to the returned berval, which must be freed using ber_bvfree() . This function returns 0 on success and -1 on error.

The ber_flatten() API call is not present in U-M LDAP 3.3.

The use of ber_flatten() on a BerElement in which all '{' and '}' format modifiers have not been properly matched is an error (that is, -1 will be returned by ber_flatten() if this situation is exists).

12.20.1.1 Encoding Example

The following is an example of encoding the following ASN.1 data type:

Example1Request ::= SEQUENCE { s OCTET STRING, -- must be printable val1 INTEGER, val2 [0] INTEGER DEFAULT 0 } int encode_example1(char *s,ber_int_t val1,ber_int_t val2, struct berval **bvPtr) { BerElement *ber; int rc; ber = ber_alloc_t(LBER_USE_DER); if (ber == NULL) return -1; if (ber_printf(ber,"{si",s,val1) == -1) { ber_free(ber,1); return -1; } if (val2 != 0) { if (ber_printf(ber,"ti",(ber_tag_t)0x80,val2) == -1) { ber_free(ber,1); return -1; } } if (ber_printf(ber,"}") == -1) { ber_free(ber,1); return -1; } rc = ber_flatten(ber,bvPtr); ber_free(ber,1); return rc; }

12.20.2 Decoding

The following two symbols are available to applications.

#define LBER_ERROR 0xffffffffL #define LBER_DEFAULT 0xffffffffL BerElement *ber_init (struct berval *bv);

The ber_init() function constructs a BerElement and returns a new BerElement containing a copy of the data in the bv argument. The ber_init() function returns the null pointer on error.

ber_tag_t ber_scanf (BerElement *ber, char *fmt, ... );

The ber_scanf() function is used to decode a BER element in much the same way that sscanf() works. One important difference, though, is that some state information is kept with the ber argument so that multiple calls can be made to ber_scanf() to sequentially read from the BER element. The ber argument must be a pointer to a BerElement returned by ber_init() . The ber_scanf() function interprets function the bytes according to the format string fmt, and stores the results in its additional arguments. The ber_scanf() function returns LBER_ERROR on error, and a different value on success.

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 ** argument should be supplied. Memory is allocated, filled with the contents of the octet string, null- terminated, and the pointer to the string is stored in the argument. The returned value must be freed using ldap_memfree() . The tag of the element must indicate the primitive form (constructed strings are not supported) but is otherwise ignored and discarded during the decoding. This format cannot be used with octet strings which could contain null bytes.
O	Octet string. A struct berval ** argument should be supplied, which upon return points to a allocated struct berval containing the octet string and its length. The ber_bvfree() function must be called to free the allocated memory. The tag of the element must indicate the primitive form (constructed strings are not supported) but is otherwise ignored during the decoding.
b	Boolean. A pointer to a ber_int_t should be supplied. The value stored will be 0 for FALSE or nonzero for TRUE. The tag of the element must indicate the primitive form but is otherwise ignored during the decoding.
e	Enumerated value stored will be in host byte order. The tag of the element must indicate the primitive form but is otherwise ignored during the decoding. The ber_scanf() function will return an error if the enumerated value cannot be stored in a ber_int_t .
i	Integer. A pointer to a ber_int_t should be supplied. The value stored will be in host byte order. The tag of the element must indicate the primitive form but is otherwise ignored during the decoding. The ber_scanf() function will return an error if the integer cannot be stored in a ber_int_t .
B	Bitstring. A char ** argument should be supplied which will point to the allocated bits, followed by a ber_len_t * argument, which will point to the length (in bits) of the bit-string returned. The ldap_memfree() function must be called to free the bit-string. The tag of the element must indicate the primitive form (constructed bitstrings are not supported) but is otherwise ignored during the decoding.
n	Null. No argument is required. The element is simply skipped if it is recognized as a zero-length element. The tag is ignored.
v	Several octet strings. A char *** argument should be supplied, which upon return points to a allocated null-terminated array of char *'s containing the octet strings. NULL is stored if the sequence is empty. The ldap_memfree() function must be called to free each element of the array and the array itself. The tag of the sequence and of the octet strings are ignored.
V	Several octet strings (which could contain null bytes). A struct berval *** should be supplied, which upon return points to a allocated null-terminated array of struct berval *'s containing the octet strings and their lengths. NULL is stored if the sequence is empty. The ber_bvecfree() function can be called to free the allocated memory. The tag of the sequence and of the octet strings are ignored.
x	Skip element. The next element is skipped. No argument is required.
{	Begin sequence. No argument is required. The initial sequence tag and length are skipped.
}	End sequence. No argument is required.
[	Begin set. No argument is required. The initial set tag and length are skipped.
]	End set. No argument is required.

ber_tag_t ber_peek_tag (BerElement *ber, ber_len_t *lenPtr);

The ber_peek_tag() function returns the tag of the next element to be parsed in the BerElement argument. The length of this element is stored in the *lenPtr argument. LBER_DEFAULT is returned if there is no further data to be read. The ber argument is not modified.

ber_tag_t ber_skip_tag (BerElement *ber, ber_len_t *lenPtr);

The ber_skip_tag() function is similar to ber_peek_tag() , except that the state pointer in the BerElement argument is advanced past the first tag and length, and is pointed to the value part of the next element. This function should only be used with constructed types and situations when a BER encoding is used as the value of an OCTET STRING. The length of the value is stored in *lenPtr.

ber_tag_t ber_first_element(BerElement *ber, ber_len_t *lenPtr, char **opaquePtr); ber_tag_t ber_next_element (BerElement *ber, ber_len_t *lenPtr, char *opaque);

The ber_first_element() and ber_next_element() functions are used to traverse a SET, SET OF, SEQUENCE or SEQUENCE OF data value. The ber_first_element() function calls ber_skip_tag() , stores internal information in *lenPtr and *opaquePtr, and calls ber_peek_tag() for the first element inside the constructed value. LBER_DEFAULT is returned if the constructed value is empty. The ber_next_element() function positions the state at the start of the next element in the constructed type. LBER_DEFAULT is returned if there are no further values.

The len and opaque values should not be used by applications other than as arguments to ber_next_element() , as shown in the following example:

12.20.2.1 Decoding Example

The following is an example of decoding an ASN.1 data type:

Example2Request ::= SEQUENCE { dn OCTET STRING, -- must be printable scope ENUMERATED { b (0), s (1), w (2) }, ali ENUMERATED { n (0), s (1), f (2), a (3) }, size INTEGER, time INTEGER, tonly BOOLEAN, attrs SEQUENCE OF OCTET STRING, -- must be printable [0] SEQUENCE OF SEQUENCE { type OCTET STRING -- must be printable, crit BOOLEAN DEFAULT FALSE, value OCTET STRING } OPTIONAL } #define TAG_CONTROL_LIST 0xA0U /* context specific cons 0 */ int decode_example2(struct berval *bv) { BerElement *ber; ber_len_t len; ber_tag_t res; ber_int_t scope, ali, size, time, tonly; char *dn = NULL, **attrs = NULL; int i,rc = 0; ber = ber_init(bv); if (ber == NULL) { fputs("ERROR ber_init failed\n", stderr); return -1; } res = ber_scanf(ber,"{aiiiib{v}",&dn,&scope,&ali, &size,&time,&tonly,&attrs); if (res == LBER_ERROR) { fputs("ERROR ber_scanf failed\n", stderr); ber_free(ber,1); return -1; } /* *** use dn */ ldap_memfree(dn); for (i = 0; attrs != NULL && attrs[i] != NULL; i++) { /* *** use attrs[i] */ ldap_memfree(attrs[i]); } ldap_memfree(attrs); if (ber_peek_tag(ber,&len) == TAG_CONTROL_LIST) { char *opaque; ber_tag_t tag; for (tag = ber_first_element(ber,&len,&opaque); tag != LBER_DEFAULT; tag = ber_next_element (ber,&len,opaque)) { ber_len_t tlen; ber_tag_t ttag; char *type; ber_int_t crit; struct berval *value; if (ber_scanf(ber,"{a",&type) == LBER_ERROR) { fputs("ERROR cannot parse type\n", stderr); break; } /* *** use type */ ldap_memfree(type); ttag = ber_peek_tag(ber,&tlen); if (ttag == 0x01U) { /* boolean */ if (ber_scanf(ber,"b", &crit) == LBER_ERROR){ fputs("ERROR cannot parse crit\n", stderr); rc = -1; break; } } else if (ttag == 0x04U) { /* octet string */ crit = 0; } else { fputs("ERROR extra field in controls\n", stderr ); break; } if (ber_scanf(ber,"O}",&value) == LBER_ERROR) { fputs("ERROR cannot parse value\n", stderr); rc = -1; break; } /* *** use value */ ber_bvfree(value); } } if ( rc == 0 ) { /* no errors so far */ if (ber_scanf(ber,"}") == LBER_ERROR) { rc = -1; } } ber_free(ber,1); return rc; }

12.21 Sample LDAP API Code

#include <ldap.h> main() { LDAP *ld; LDAPMessage *res, *e; int i, rc; char *a, *dn; BerElement *ptr; char **vals; /* open an LDAP session */ if ( (ld = ldap_init( "dotted.host.name", ldap_PORT )) == NULL ) exit( 1 ); /* authenticate as nobody */ if (( rc = ldap_simple_bind_s( ld, NULL, NULL )) != ldap_SUCCESS ) { fprintf( stderr, "ldap_simple_bind_s: %s\n", ldap_err2string( rc )); exit( 1 ); } /* search for entries with cn of "Babs Jensen", return all attrs */ if (( rc = ldap_search_s( ld, "o=University of Michigan, c=US", ldap_SCOPE_SUBTREE, "(cn=Babs Jensen)", NULL, 0, &res )) != ldap_SUCCESS ) { fprintf( stderr, "ldap_search_s: %s\n", ldap_err2string( rc )); exit( 1 ); } /* step through each entry returned */ for ( e = ldap_first_entry( ld, res ); e != NULL; e = ldap_next_entry( ld, e ) ) { /* print its name */ dn = ldap_get_dn( ld, e ); printf( "dn: %s\n", dn ); ldap_memfree( dn ); /* print each attribute */ for ( a = ldap_first_attribute( ld, e, &ptr ); a != NULL; a = ldap_next_attribute( ld, e, ptr ) ) { printf( "attribute: %s\n", a ); /* print each value */ vals = ldap_get_values( ld, e, a ); for ( i = 0; vals[i] != NULL; i++ ) { printf( "value: %s\n", vals[i] ); } ldap_value_free( vals ); ldap_memfree( a ); } if ( ptr != NULL ) { ber_free( ptr, 0 ); } } /* free the search results */ ldap_msgfree( res ); /* close and free connection resources */ ldap_unbind( ld ); }

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