sendmail, 4th Edition

The DOL m4 Macro

Ordinarily, the $ character is interpreted by m4 as a special character when found inside its define expressions:

define(`A', `$2')
             ↑
      the $ makes $2 an m4 positional variable

There might be times, however, when you might want to put a literal $ character into a definition—perhaps when designing your own DOMAIN, FEATURE, or HACK files.

You place a literal $ into a definition with the DOL m4 macro. For example:

define(`DOWN', `R DOL(*) < @ $1 > DOL(*)    DOL(1) < @ $2 > DOL(2)')

Here, we define the m4 macro named DOWN, which takes two arguments ($1 and $2). Notice how the $ character has meaning to m4. This newly created DOWN macro can then be used in one of your .m4 files, perhaps like this:

DOWN(badhost,  outhost)

DOWN creates a rule by substituting the argument (badhost for the $1 in its definition, and outhost) for the corresponding $2. The substitution looks like this:

R DOL(*)    becomes ←   R $*
< @ $1 >    becomes ←   < @ badhost >
DOL(*)      becomes ←   $*
DOL(1)      becomes ←   $1
< @ $2 >    becomes ←   < @ outhost >
DOL(2)      becomes ←   $2

After substitution, the following new rule is the result:

R $* < @ badhost > $*       $1 < @ outhost > $2

The DOL m4 macro allowed the insertion of $ characters (such as $*) and protects you from having the literal use of $ characters being wrongly interpreted by m4.

Needless to say, you should never redefine the DOL m4 macro.