sendmail, 4th Edition

Macro Expansion Is Recursive

When text contains other macros, those other macros are also expanded. This process is recursive and continues until all macros have been expanded. For example, consider the following:

DAxxx
DByyy
DC$A.$B
DD$C.zzz

Here, the text for the macro D is $C.zzz. When the D macro is defined, it is recursively expanded like this:

$D            becomes →  $C.zzz
$C.zzz        becomes →  $A.$B.zzz
$A.$B.zzz     becomes →  xxx.$B.zzz
xxx.$B.zzz    becomes →  xxx.yyy.zzz

Notice that when sendmail recursively expands a macro, it does so one macro at a time, always expanding the leftmost macro first.

In rules, when sendmail expands a macro, it also tokenizes it. For example, placing the earlier $D in the following rule’s LHS:

R$+ @ $D      $1

causes the LHS to contain seven tokens rather than three:

R$+ @ xxx . yyy . zzz          $1

Note that the largest a recursive expansion can grow is defined at compile time with the MACBUFSIZE compile-time macro (MAX... on page 120), which defaults to 4,096 characters.