Feature Marking and Selection

Requires Net-SNMP 5.7

Note: Features discussed on this page require Net-SNMP version 5.7 or higher.

Background

Many components of Net-SNMP are present to support certain features in the code. But if the code that requires a feature isn't compiled in, the supporting code still is and needlessly adds to the size of the running executable and libraries.

Design Criteria

• By default, everything must still be included. 3rd-party developers may be making use of code even if internal Net-SNMP code isn't.
• A configure flag (--with-minimialist) to enable minimal code
  • --enable-mini-agent should probably turn it on?
• Flags to request including and excluding of features
  • --with-features="foo bar"
  • --with-out-features="foo bar"

Feature Marking and Requiring Macros

To start with, include <net-snmp/features.h>

 #include <net-snmp/features.h>

If you're implementing code that is only needed in certain locations, declare the feature name using the netsnmp_feature_provide() macro:

 netsnmp_feature_provide(foo)

If the feature is dependent on the availability of another feature, use the netsnmp_feature_require() or netsnmp_feature_want() macros:

 netsnmp_feature_require(foo)
 netsnmp_feature_want(bar)

In this case, if foo is unavailable a hard-error will be triggered. However, if bar is unavailable the compilation will continue.

Coding Using Features

To mark code as removable if a feature is unneeded, surround it with #ifndef markings:

 #ifndef NETSNMP_FEATURE_REMOVE_FOO
 /* normal foo code */
 #else /* !NETSNMP_FEATURE_REMOVE_FOO */
 char unused_feature_foo;
 #endif /* !NETSNMP_FEATURE_REMOVE_FOO */

A few important notes:

• Always use the #ifndef version of checking. If the features.h fails to define anything, the feature will be included by default.
• Always include the define name in comments after #else and #endif clauses. Later "code-removal" scripts will depend upon it.
• The char definition is important for avoiding warnings on compilers that complain or refuse to compile empty files.