sendmail, 4th Edition

m4 and Arguments

When an m4 macro name is immediately followed by a left parenthesis, it is treated like a function call. Arguments given to it in that role are used to replace $digit expressions in the original definition. For example, suppose the m4 macro CONCAT is defined like this:

define(`CONCAT',`$1$2$3')dnl

and then later used like this:

CONCAT(`host', `.', `domain')

The result will be that the host will replace $1, the dot will replace $2, and the domain will replace $3, all jammed tightly together just as `$1$2$3' were:

host.domain

Macro arguments are used to create such techniques as FEATURE( ) and OSTYPE( ), which are described later in this chapter.