diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/asn1/doc/src/asn1_ug.xml | 66 |
1 files changed, 4 insertions, 62 deletions
diff --git a/lib/asn1/doc/src/asn1_ug.xml b/lib/asn1/doc/src/asn1_ug.xml index a16aac0e03..dd08eb1817 100644 --- a/lib/asn1/doc/src/asn1_ug.xml +++ b/lib/asn1/doc/src/asn1_ug.xml @@ -949,68 +949,10 @@ ok <section> <marker id="SET"></marker> <title>SET</title> - <p>The SET type is an unusual construct and normally the SEQUENCE - type is more appropriate to use. Set is also inefficient compared with SEQUENCE, as the components can be in any order. Hence, it must be possible - to distinguish every component in 'SET', both when - encoding and decoding a value of a type defined to be a SET. - The tags of all components must be different from each other - in order to be easily recognizable.</p> - <p>A SET may be defined as:</p> - <pre> -Pdu2 ::= SET { - a INTEGER, - b BOOLEAN, - c ENUMERATED {on(0),off(1)} } </pre> - <p>A SET is represented as an Erlang record. - For each SEQUENCE and <c>SET</c> in - an ASN.1 module an Erlang record declaration is generated. For - <c>Pdu2</c> above a record is defined like this:</p> - <pre> --record('Pdu2',{a, b, c}). </pre> - <p>The record declarations for a module <c>M</c> are placed in a - separate <c>M.hrl</c> file.</p> - <p>Values can be assigned in Erlang as demonstrated below:</p> - <pre> -V = #'Pdu2'{a=44,b=false,c=off}. </pre> - <p>The decode functions will return a record as result when decoding - a SET. - </p> - <p>The difference between SET and SEQUENCE is that the order of - the components (in the BER encoded format) is undefined for SET - and defined as the lexical order from the ASN.1 definition for - SEQUENCE. The ASN.1 compiler for Erlang will always encode a - SET in the lexical order. The decode routines can handle SET - components encoded in any order but will always return the - result as a record. Since all components of the SET must be - distinguishable both in the encoding phase as well as the - decoding phase the following type is not allowed in a module - with EXPLICIT or IMPLICIT as tag-default :</p> - <p></p> - <pre> -Bad ::= SET {i INTEGER, - j INTEGER } </pre> - <p>The ASN.1 to Erlang compiler rejects the above type. We - shall not explain the concept of tag further here, we refer to - [<cite id="X.680"></cite>]. - </p> - <p>Encoding of a SET with components with DEFAULT values behaves - similar as a SEQUENCE. The DER encoding format restrictions on DEFAULT - values is the same for SET as for SEQUENCE, and is supported by - the compiler.</p> - <p>Moreover, in DER the elements of a SET will be sorted. If a - component is an un-tagged choice the sorting have to take place - in run-time. This fact emphasizes the following recommendation - if DER encoding format is used.</p> - <p>The concept of SET is an unusual - construct and one cannot think of one single application - where the set type is essential. (Imagine if someone - "invented'' the shuffled array in 'C') People tend to think - that 'SET' sounds nicer and more mathematical than 'SEQUENCE' - and hence use it when 'SEQUENCE' would have been more - appropriate. It is also most inefficient, since every correct - implementation of SET must always be prepared to accept the - components in any order. So, if possible use SEQUENCE instead - of SET.</p> + <p>In Erlang, the SET type is used exactly as SEQUENCE. Note + that if the BER or DER encoding rules are used, decoding a + SET is slower than decoding a SEQUENCE because the components + must be sorted.</p> </section> <section> |