4 The MIB Compiler

The chapter The MIB Compiler describes the MIB compiler and contains the following topics:

• Operation
• Import
• Consistency checking between MIBs
• .hrl file generation
• Emacs integration
• Deviations from the standard

4.1 Operation

The MIB must be written as a text file in SMIv1 or SMIv2 using an ASN.1 notation before it will be compiled. This text file must have the same name as the MIB, but with the suffix .mib. This is necessary for handling the IMPORT statement.

The association file, which contains the names of instrumentation functions for the MIB, should have the suffix .funcs. If the compiler does not find the association file, it gives a warning message and uses default instrumentation functions. (See Default Instrumentation for more details).

The MIB compiler is started with a call to snmpc:compile(<mibname>). For example:

snmpc:compile("RFC1213-MIB").
    

The output is a new file which is called <mibname>.bin.

The MIB compiler understands both SMIv1 and SMIv2 MIBs. It uses the MODULE-IDENTITY statement to determinate if the MIB is written in SMI version 1 or 2.

4.2 Importing MIBs

The compiler handles the IMPORT statement. It is important to import the compiled file and not the ASN.1 (source) file. A MIB must be recompiled to make changes visible to other MIBs importing it.

The compiled files of the imported MIBs must be present in the current directory, or a directory in the current path. The path is supplied with the {i, Path} option, for example:

snmpc:compile("MY-MIB", 
       [{i, ["friend_mibs/", "../standard_mibs/"]}]).
    

It is also possible to import MIBs from OTP applications in an "include_lib" like fashion with the il option. Example:

snmpc:compile("MY-MIB", 
       [{il, ["snmp/priv/mibs/", "myapp/priv/mibs/"]}]).
    

finds the latest version of the snmp and myapp applications in the OTP system and uses the expanded paths as include paths.

Note that an SMIv2 MIB can import an SMIv1 MIB and vice versa.

The following MIBs are built-ins of the Erlang SNMP compiler: SNMPv2-SMI, RFC-1215, RFC-1212, SNMPv2-TC, SNMPv2-CONF, and RFC1155-SMI. They cannot therefore be compiled separately.

4.3 MIB Consistency Checking

When an MIB is compiled, the compiler detects if several managed objects use the same OBJECT IDENTIFIER. If that is the case, it issues an error message. However, the compiler cannot detect Oid conflicts between different MIBs. These kinds of conflicts generate an error at load time. To avoid this, the following function can be used to do consistency checking between MIBs:

erl>snmpc:is_consistent(ListOfMibNames).
    

ListOfMibNames is a list of compiled MIBs, for example ["RFC1213-MIB", "MY-MIB"]. The function also performs consistency checking of trap definitions.

4.4 .hrl File Generation

It is possible to generate an .hrl file which contains definitions of Erlang constants from a compiled MIB file. This file can then be included in Erlang source code. The file will contain constants for:

• object Identifiers for tables, table entries and variables
• column numbers
• enumerated values
• default values for variables and table columns.

Use the following command to generate a .hrl file from an MIB:

erl>snmpc:mib_to_hrl(MibName).
    

4.5 Emacs Integration

With the Emacs editor, the next-error (C-X `) function can be used indicate where a compilation error occurred, provided the error message is described by a line number.

Use M-x compile to compile an MIB from inside Emacs, and enter:

erl -s snmpc compile <MibName> -noshell
   

An example of <MibName> is RFC1213-MIB.

4.6 Compiling from a Shell or a Makefile

The erlc commands can be used to compile SNMP MIBs. Example:

erlc MY-MIB.mib
   

All the standard erlc flags are supported, e.g.

erlc -I mymibs -o mymibs -W MY-MIB.mib
   

The flags specific to the MIB compiler can be specified by using the + syntax:

erlc +'{group_check,false}' MY-MIB.mib
   

4.7 Deviations from the Standard

In some aspects the Erlang MIB compiler does not follow or implement the SMI fully. Here are the differences:

• Tables must be written in the following order: tableObject, entryObject, column1, ..., columnN (in order).

• Integer values, for example in the SIZE expression must be entered in decimal syntax, not in hex or bit syntax.

• Symbolic names must be unique within a MIB and within a system.

• Hyphens are allowed in SMIv2 (a pragmatic approach). The reason for this is that according to SMIv2, hyphens are allowed for objects converted from SMIv1, but not for others. This is impossible to check for the compiler.

• If a word is a keyword in any of SMIv1 or SMIv2, it is a keyword in the compiler (deviates from SMIv1 only).

• Indexes in a table must be objects, not types (deviates from SMIv1 only).

• A subset of all semantic checks on types are implemented. For example, strictly the TimeTicks may not be sub-classed but the compiler allows this (standard MIBs must pass through the compiler) (deviates from SMIv2 only).

• The MIB.Object syntax is not implemented (since all objects must be unique anyway).

• Two different names cannot define the same OBJECT IDENTIFIER.

• The type checking in the SEQUENCE construct is non-strict (i.e. subtypes may be specified). The reason for this is that some standard MIBs use this.

• A definition has normally a status field. When the status field has the value deprecated, then the MIB-compiler will ignore this definition. With the MIB-compiler option {deprecated,true} the MIB-compiler does not ignore the deprecated definitions.
• An object has a DESCRIPTIONS field. The descriptions-field will not be included in the compiled mib by default. In order to get the description, the mib must be compiled with the option description.