4.3 Macro arguments

When a name is seen, and it has a macro definition, it will be expanded as a macro.

If the name is followed by an opening parenthesis, the arguments will be collected before the macro is called. If too few arguments are supplied, the missing arguments are taken to be the empty string. However, some builtins are documented to behave differently for a missing optional argument than for an explicit empty string. If there are too many arguments, the excess arguments are ignored. Unquoted leading whitespace is stripped off all arguments, but whitespace generated by a macro expansion or occurring after a macro that expanded to an empty string remains intact. Whitespace includes space, tab, newline, carriage return, vertical tab, and formfeed.

define(`macro', `$1')
⇒
macro( unquoted leading space lost)
⇒unquoted leading space lost
macro(` quoted leading space kept')
⇒ quoted leading space kept
macro(
 divert `unquoted space kept after expansion')
⇒ unquoted space kept after expansion
macro(macro(`
')`whitespace from expansion kept')
⇒
⇒whitespace from expansion kept
macro(`unquoted trailing whitespace kept'
)
⇒unquoted trailing whitespace kept
⇒

Normally m4 will issue warnings if a builtin macro is called with an inappropriate number of arguments, but it can be suppressed with the --quiet command line option (or --silent, or -Q, see Invoking m4). For user defined macros, there is no check of the number of arguments given.

$ m4
index(`abc')
error→m4:stdin:1: Warning: too few arguments to builtin `index'
⇒0
index(`abc',)
⇒0
index(`abc', `b', `ignored')
error→m4:stdin:3: Warning: excess arguments to builtin `index' ignored
⇒1

$ m4 -Q
index(`abc')
⇒0
index(`abc',)
⇒0
index(`abc', `b', `ignored')
⇒1

Macros are expanded normally during argument collection, and whatever commas, quotes and parentheses that might show up in the resulting expanded text will serve to define the arguments as well. Thus, if foo expands to ‘, b, c’, the macro call

bar(a foo, d)

is a macro call with four arguments, which are ‘a ’, ‘b’, ‘c’ and ‘d’. To understand why the first argument contains whitespace, remember that unquoted leading whitespace is never part of an argument, but trailing whitespace always is.

It is possible for a macro’s definition to change during argument collection, in which case the expansion uses the definition that was in effect at the time the opening ‘(’ was seen.

define(`f', `1')
⇒
f(define(`f', `2'))
⇒1
f
⇒2

It is an error if the end of file occurs while collecting arguments.

hello world
⇒hello world
define(
^D
error→m4:stdin:2: ERROR: end of file in argument list