Snacc1.2rj：高性能ASN.1到C/C++/IDL编译器

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"Snacc1.2rj是一个高性能的ASN.1到C/C++/IDL编译器，由Michael Sample开发并由Robert Joop增强。该文档是1995年9月22日关于Snacc 1.2rj.9的初步文档，其开发得到了加拿大电信研究所（CITR）和加拿大自然科学与工程研究委员会（NSERC）的资助。" asn.1是一种标准化的编码规范，用于在不同系统之间交换结构化数据，尤其在通信协议和数据库中广泛应用。ASN.1定义了一种抽象语法，可以将复杂的数据结构转换为二进制形式，便于在网络上传输。Snacc编译器的作用就是将asn.1定义的语法转换为C、C++或IDL（Interface Definition Language）代码，这些代码可以直接在编程语言中使用，从而简化了跨平台的数据交换。 Snacc是自由软件，遵循GNU General Public License（GPL）条款，允许用户自由地分发和修改源代码。这意味着任何获得Snacc的人都可以查看、修改其源码，并按照GPL的规定发布改进版本。运行时库则归UBC（University of British Columbia）和Michael Sample所有，同样允许自由分发和修改，但必须遵守相应的版权规定。文档作者Michael Sample和Robert Joop分别在1993年和1994至1995年间对Snacc进行了贡献，表明这是一个持续发展的项目，不断进行优化和更新以适应技术的发展。通过Snacc，开发者可以方便地处理asn.1定义的数据类型，如基本类型（整数、字符串等）、序列、选择和枚举等。编译后的C/C++/IDL代码能够帮助程序员快速实现asn.1数据的编码和解码，这对于实现网络通信协议（如TCP/IP上的各种协议）、数据存储和解析等任务至关重要。此外，由于asn.1的广泛接受性，使用Snacc可以帮助确保不同系统之间的数据互操作性。 Snacc是一个关键工具，它为asn.1编码提供了高效的实现，促进了跨平台的数据交换和通信协议的开发。作为自由软件，它鼓励社区参与和共享，促进了技术的进步和普及。

–mf number causes the names of the generated source ﬁles to have a maximum length

of number characters, including their sufﬁx. The number argument must be at

least 3. This option is useful for supporting operating systems that only support

short ﬁle names. A better solution is to shorten the module name of each ASN.1

module.

–l number this is fairly obscure but may be useful. Each error that the decoders can

report is given an id number. The number number is where the error ids start

decreasing from as they are assigned to errors. The default is -100 if this option

is not given. Avoid using a number in the range -100 to 0 since they may conﬂict

with the library routines' error ids. If you are re-compiling the useful types for

the library use -50. Another use of this option is to integrate newly generated

code with older code; if done correctly, the error ids will not conﬂict.

Since ASN.1 has different scoping rules than C and C++, some name munging is done

for types, named-numbers etc. to eliminate conﬂicts. Some capitalization schemes

were chosen to ﬁt common C programming style. For all names, dashes in the ASN.1

source are converted to underscores. See Sections 4.2 and 6.2 for more naming infor-

mation.

If the -mm switch has been given, the module name is used as a base name for the

generated source ﬁle names. It will be put into lowercase and dashes will be replaced

with underscores. Module names that result in ﬁle names longer than speciﬁed with

the -mf option will be truncated. If the -mf option was not given, ﬁle names will

be truncated if they are too long for the target ﬁle system. You may want to shorten

long module names to meaningful abbreviations. This will avoid ﬁle name conﬂicts

for module names that are truncated to the same substring. Any module name and ﬁle

name conﬂicts will be reported.

If your ASN.1 modules have syntactic or semantic errors, each error will be printed to

stderr along with the ﬁle name and line number of where it occurred. These errors are

usable by GNU emacs compiling tools. See the next chapter for more information on

the types of errors snacc can detect.

More errors can be detected and reported in a single compile if type and value deﬁni-

tions are separated by semi-colons. Separating type and value deﬁnitions with semi-

colons is not required, and if used, need not be used to separate all type and value

deﬁnitions. Semi-colons are necessary after some macros that introduce ambiguity. In

general, if you get a parse error you can't ﬁgure out, try separating the surrounding

type/value deﬁnitions with semicolons.

1.2.1 Known Bugs



Snacc has problems with the following case:

Foo ::= SEQUENCE

{

id IdType,

val ANY DEFINED BY id

}

Chapter 2

Introduction to Snacc

Release 1.1

Snacc compiles ASN.1 [1] (Abstract Syntax Notation One) modules into C, C++ or

type tables. The generated C or C++ code contains equivalent data structures and rou-

tines to convert values between the internal (C or C++) representation and the corre-

sponding BER [2] (Basic Encoding Rules) format. The name “snacc” is an acronym

for “Sample Neufeld ASN.1 to C/C++ Compiler”.

This compiler was written so I could do some encoding performance research for my

M.Sc. See [13], or [12] for the results of that research. A techreport will soo be avail-

able via ftp from UBC, in the same directory as snacc.

The ASN.1 data structure language can specify complex types such as lists and re-

cursively deﬁned types. BER data values are deﬁned independently of any computer

architecture, providing a universal data value representation that is useful for sharing

data in heterogeneous networks.

The process of converting an ASN.1 value from its C or C++ representation into an

equivalent BER data value is called encoding and the reverse process is called decod-

ing. This document was written assuming that the reader is familiar ASN.1 and BER.

Further information on ASN.1 and BER can be found in [14], [6], [1] and [2].

Compiling ASN.1 into C is not a new idea but many other tools such as UBC's CASN1

[7], ISODE's PEPY/POSY [11], and commercial tools either do not parse ASN.1 '90,

produce slow encoders and decoders or are outrageously expensive. The aim of this

tool is to provide an ASN.1 compiler that parses ASN.1 '90, produces efﬁcient encod-

ing and decoding routines and is freely available. Effort has been made to make the

generated encoders and decoders relatively easy to ﬁt into different software environ-

ments.

The table driven encoders are useful for certain applications such as protocol testing.

They are also useful if you need to dynamically load new ASN.1 deﬁnitions. It is also

fairly simple to write your own special ASN.1 tools based on tables (e.g. a protocol

tester that veriﬁes that values conform to a given ASN.1 type deﬁnition). The price

of the ﬂexibility is speed; they are slower ( 4 times) than the compiled C and C++

versions.

Some of snacc's features include:



parses CCITT ASN.1 '90 including subtype notation



can compile and link inter-dependent ASN.1 modules (IMPORTS/EXPORTS)



some X.400 and SNMP macros are parsed



macro deﬁnitions do not generate syntax errors but are are not processed. The

macro deﬁnitions are retained as a string internally (if you want to modify the

compiler to process them).



value notation is parsed. OBJECT IDENTIFIERs, INTEGERs and BOOLEANs

are translated to C/C++ values. Any other value in {}'s is kept as a string inter-

nally (if you want to modify the compiler to process them).



optionally supports “;” separated type or value deﬁnitions in the ASN.1 source.

This is useful for dealing with some macros and other language ambiguities that

introduce parsing problems.



ANY DEFINED BY types are supported via the SNMP OBJECT-TYPE macro

2.1 Installing snacc

First of all, if you haven't already done so, un-archive snacc to produce the directory

snacc and its contents. The following tools are required to compile snacc:



lex or GNU's flex (flex is recommended)



yacc or GNU's bison (bison is recommended)



a C compiler and make

Some versions of yacc will choke due to the large size of the asn1.yacc ﬁle, how-

ever, I have had no problems with bison. Our yacc grammar for ASN.1 has 61

shift/reduce errors and 2 reduce/reduce errors. Most of these errors were introduced

when certain macros were added to the compiler. Some of the shift/reduce errors will

require you to follow the offending macro in the ASN.1 module with a semi-colon.

The reduce/reduce errors were introduced by macros that have “Type or Value Lists”

because the NULL Type and NULL values use the same symbol, “NULL”. This is not

a problem since no real processing is done with the macros in question at the present.

Lex will work for the asn1.lex ﬁle but flex will typically produce a smaller ex-

ecutable. Most versions of lex have a small maximum token size that will cause

problems for long tokens in the ASN.1 source ﬁles, such as quoted strings. To avoid

this problem, increase the YYLMAX value in the generated lex.yy.c ﬁle to at least 2048.

Flex does not seem to have this problem.

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