5. How to define new macros

Macros can be defined, redefined and deleted in several different ways. Also, it is possible to redefine a macro without losing a previous value, and bring back the original value at a later time.

5.1 Defining a macro

The normal way to define or redefine macros is to use the builtin define:

Builtin: define (name, [expansion]@c)

Defines name to expand to expansion. If expansion is not given, it is taken to be empty.

The expansion of define is void. The macro define is recognized only with parameters.

The following example defines the macro foo to expand to the text ‘Hello World.’.

define(`foo', `Hello world.')
⇒
foo
⇒Hello world.

The empty line in the output is there because the newline is not a part of the macro definition, and it is consequently copied to the output. This can be avoided by use of the macro dnl. See section Deleting whitespace in input, for details.

The first argument to define should be quoted; otherwise, if the macro is already defined, you will be defining a different macro. This example shows the problems with underquoting, since we did not want to redefine one:

define(foo, one)
⇒
define(foo, two)
⇒
one
⇒two

GNU m4 normally replaces only the topmost definition of a macro if it has several definitions from pushdef (see section Temporarily redefining macros). Some other implementations of m4 replace all definitions of a macro with define. See section Facilities in System V m4 not in GNU m4, for more details.

As a GNU extension, the first argument to define does not have to be a simple word. It can be any text string, even the empty string. A macro with a non-standard name cannot be invoked in the normal way, as the name is not recognized. It can only be referenced by the builtins indir (see section Indirect call of macros) and defn (see section Renaming macros).

Arrays and associative arrays can be simulated by using non-standard macro names.

Composite: array (index)

Composite: array_set (index, [value]@c)

Provide access to entries within an array. array reads the entry at location index, and array_set assigns value to location index.

define(`array', `defn(format(``array[%d]'', `$1'))')
⇒
define(`array_set', `define(format(``array[%d]'', `$1'), `$2')')
⇒
array_set(`4', `array element no. 4')
⇒
array_set(`17', `array element no. 17')
⇒
array(`4')
⇒array element no. 4
array(eval(`10 + 7'))
⇒array element no. 17

Change the ‘%d’ to ‘%s’ and it is an associative array.

5.2 Arguments to macros

Macros can have arguments. The nth argument is denoted by $n in the expansion text, and is replaced by the nth actual argument, when the macro is expanded. Replacement of arguments happens before rescanning, regardless of how many nesting levels of quoting appear in the expansion. Here is an example of a macro with two arguments.

Composite: exch (arg1, arg2)

Expands to arg2 followed by arg1, effectively exchanging their order.

define(`exch', `$2, $1')
⇒
exch(`arg1', `arg2')
⇒arg2, arg1

This can be used, for example, if you like the arguments to define to be reversed.

define(`exch', `$2, $1')
⇒
define(exch(``expansion text'', ``macro''))
⇒
macro
⇒expansion text

See section On Quoting Arguments to macros, for an explanation of the double quotes. (You should try and improve this example so that clients of exch do not have to double quote; or see section Answers).

As a special case, the zeroth argument, $0, is always the name of the macro being expanded.

define(`test', ``Macro name: $0'')
⇒
test
⇒Macro name: test

If you want quoted text to appear as part of the expansion text, remember that quotes can be nested in quoted strings. Thus, in

define(`foo', `This is macro `foo'.')
⇒
foo
⇒This is macro foo.

The ‘foo’ in the expansion text is not expanded, since it is a quoted string, and not a name.

GNU m4 allows the number following the ‘$’ to consist of one or more digits, allowing macros to have any number of arguments. The extension of accepting multiple digits is incompatible with POSIX, and is different than traditional implementations of m4, which only recognize one digit. Therefore, future versions of GNU M4 will phase out this feature. To portably access beyond the ninth argument, you can use the argn macro documented later (see section Recursion in m4).

POSIX also states that ‘$’ followed immediately by ‘{’ in a macro definition is implementation-defined. This version of M4 passes the literal characters ‘${’ through unchanged, but M4 2.0 will implement an optional feature similar to sh, where ‘${11}’ expands to the eleventh argument, to replace the current recognition of ‘$11’. Meanwhile, if you want to guarantee that you will get a literal ‘${’ in output when expanding a macro, even when you upgrade to M4 2.0, you can use nested quoting to your advantage:

define(`foo', `single quoted $`'{1} output')
⇒
define(`bar', ``double quoted $'`{2} output'')
⇒
foo(`a', `b')
⇒single quoted ${1} output
bar(`a', `b')
⇒double quoted ${2} output

To help you detect places in your M4 input files that might change in behavior due to the changed behavior of M4 2.0, you can use the ‘--warn-macro-sequence’ command-line option (see section Invoking m4) with the default regular expression. This will add a warning any time a macro definition includes ‘$’ followed by multiple digits, or by ‘{’. The warning is not enabled by default, because it triggers a number of warnings in Autoconf 2.61 (and Autoconf uses ‘-E’ to treat warnings as errors), and because it will still be possible to restore older behavior in M4 2.0.

$ m4 --warn-macro-sequence
define(`foo', `$001 ${1} $1')
error-->m4:stdin:1: Warning: definition of `foo' contains sequence `$001'
error-->m4:stdin:1: Warning: definition of `foo' contains sequence `${1}'
⇒
foo(`bar')
⇒bar ${1} bar

5.3 Special arguments to macros

There is a special notation for the number of actual arguments supplied, and for all the actual arguments.

The number of actual arguments in a macro call is denoted by $# in the expansion text.

Composite: nargs (…)

Expands to a count of the number of arguments supplied.

define(`nargs', `$#')
⇒
nargs
⇒0
nargs()
⇒1
nargs(`arg1', `arg2', `arg3')
⇒3
nargs(`commas can be quoted, like this')
⇒1
nargs(arg1#inside comments, commas do not separate arguments
still arg1)
⇒1
nargs((unquoted parentheses, like this, group arguments))
⇒1

Remember that ‘#’ defaults to the comment character; if you forget quotes to inhibit the comment behavior, your macro definition may not end where you expected.

dnl Attempt to define a macro to just `$#'
define(underquoted, $#)
oops)
⇒
underquoted
⇒0)
⇒oops

The notation $* can be used in the expansion text to denote all the actual arguments, unquoted, with commas in between. For example

define(`echo', `$*')
⇒
echo(arg1,    arg2, arg3 , arg4)
⇒arg1,arg2,arg3 ,arg4

Often each argument should be quoted, and the notation $@ handles that. It is just like $*, except that it quotes each argument. A simple example of that is:

define(`echo', `$@')
⇒
echo(arg1,    arg2, arg3 , arg4)
⇒arg1,arg2,arg3 ,arg4

Where did the quotes go? Of course, they were eaten, when the expanded text were reread by m4. To show the difference, try

define(`echo1', `$*')
⇒
define(`echo2', `$@')
⇒
define(`foo', `This is macro `foo'.')
⇒
echo1(foo)
⇒This is macro This is macro foo..
echo1(`foo')
⇒This is macro foo.
echo2(foo)
⇒This is macro foo.
echo2(`foo')
⇒foo

See section Tracing macro calls, if you do not understand this. As another example of the difference, remember that comments encountered in arguments are passed untouched to the macro, and that quoting disables comments.

define(`echo1', `$*')
⇒
define(`echo2', `$@')
⇒
define(`foo', `bar')
⇒
echo1(#foo'foo
foo)
⇒#foo'foo
⇒bar
echo2(#foo'foo
foo)
⇒#foobar
⇒bar'

A ‘$’ sign in the expansion text, that is not followed by anything m4 understands, is simply copied to the macro expansion, as any other text is.

define(`foo', `$$$ hello $$$')
⇒
foo
⇒$$$ hello $$$

If you want a macro to expand to something like ‘$12’, the judicious use of nested quoting can put a safe character between the $ and the next character, relying on the rescanning to remove the nested quote. This will prevent m4 from interpreting the $ sign as a reference to an argument.

define(`foo', `no nested quote: $1')
⇒
foo(`arg')
⇒no nested quote: arg
define(`foo', `nested quote around $: `$'1')
⇒
foo(`arg')
⇒nested quote around $: $1
define(`foo', `nested empty quote after $: $`'1')
⇒
foo(`arg')
⇒nested empty quote after $: $1
define(`foo', `nested quote around next character: $`1'')
⇒
foo(`arg')
⇒nested quote around next character: $1
define(`foo', `nested quote around both: `$1'')
⇒
foo(`arg')
⇒nested quote around both: arg

5.4 Deleting a macro

A macro definition can be removed with undefine:

Builtin: undefine (name…)

For each argument, remove the macro name. The macro names must necessarily be quoted, since they will be expanded otherwise.

The expansion of undefine is void. The macro undefine is recognized only with parameters.

foo bar blah
⇒foo bar blah
define(`foo', `some')define(`bar', `other')define(`blah', `text')
⇒
foo bar blah
⇒some other text
undefine(`foo')
⇒
foo bar blah
⇒foo other text
undefine(`bar', `blah')
⇒
foo bar blah
⇒foo bar blah

Undefining a macro inside that macro’s expansion is safe; the macro still expands to the definition that was in effect at the ‘(’.

define(`f', ``$0':$1')
⇒
f(f(f(undefine(`f')`hello world')))
⇒f:f:f:hello world
f(`bye')
⇒f(bye)

It is not an error for name to have no macro definition. In that case, undefine does nothing.

5.5 Renaming macros

It is possible to rename an already defined macro. To do this, you need the builtin defn:

Builtin: defn (name…)

Expands to the quoted definition of each name. If an argument is not a defined macro, the expansion for that argument is empty.

If name is a user-defined macro, the quoted definition is simply the quoted expansion text. If, instead, there is only one name and it is a builtin, the expansion is a special token, which points to the builtin’s internal definition. This token is only meaningful as the second argument to define (and pushdef), and is silently converted to an empty string in most other contexts. Combining a builtin with anything else is not supported; a warning is issued and the builtin is omitted from the final expansion.

The macro defn is recognized only with parameters.

Its normal use is best understood through an example, which shows how to rename undefine to zap:

define(`zap', defn(`undefine'))
⇒
zap(`undefine')
⇒
undefine(`zap')
⇒undefine(zap)

In this way, defn can be used to copy macro definitions, and also definitions of builtin macros. Even if the original macro is removed, the other name can still be used to access the definition.

The fact that macro definitions can be transferred also explains why you should use $0, rather than retyping a macro’s name in its definition:

define(`foo', `This is `$0'')
⇒
define(`bar', defn(`foo'))
⇒
bar
⇒This is bar

Macros used as string variables should be referred through defn, to avoid unwanted expansion of the text:

define(`string', `The macro dnl is very useful
')
⇒
string
⇒The macro 
defn(`string')
⇒The macro dnl is very useful
⇒

However, it is important to remember that m4 rescanning is purely textual. If an unbalanced end-quote string occurs in a macro definition, the rescan will see that embedded quote as the termination of the quoted string, and the remainder of the macro’s definition will be rescanned unquoted. Thus it is a good idea to avoid unbalanced end-quotes in macro definitions or arguments to macros.

define(`foo', a'a)
⇒
define(`a', `A')
⇒
define(`echo', `$@')
⇒
foo
⇒A'A
defn(`foo')
⇒aA'
echo(foo)
⇒AA'

On the other hand, it is possible to exploit the fact that defn can concatenate multiple macros prior to the rescanning phase, in order to join the definitions of macros that, in isolation, have unbalanced quotes. This is particularly useful when one has used several macros to accumulate text that M4 should rescan as a whole. In the example below, note how the use of defn on l in isolation opens a string, which is not closed until the next line; but used on l and r together results in nested quoting.

define(`l', `<[>')define(`r', `<]>')
⇒
changequote(`[', `]')
⇒
defn([l])defn([r])
])
⇒<[>]defn([r])
⇒)
defn([l], [r])
⇒<[>][<]>

Using defn to generate special tokens for builtin macros outside of expected contexts can sometimes trigger warnings. But most of the time, such tokens are silently converted to the empty string.

$ m4 -d
defn(`defn')
⇒
define(defn(`divnum'), `cannot redefine a builtin token')
error-->m4:stdin:2: Warning: define: invalid macro name ignored
⇒
divnum
⇒0
len(defn(`divnum'))
⇒0

Also note that defn with multiple arguments can only join text macros, not builtins, although a future version of GNU M4 may lift this restriction.

$ m4 -d
define(`a', `A')define(`AA', `b')
⇒
traceon(`defn', `define')
⇒
defn(`a', `divnum', `a')
error-->m4:stdin:3: Warning: cannot concatenate builtin `divnum'
error-->m4trace: -1- defn(`a', `divnum', `a') -> ``A'`A''
⇒AA
define(`mydivnum', defn(`divnum', `divnum'))mydivnum
error-->m4:stdin:4: Warning: cannot concatenate builtin `divnum'
error-->m4:stdin:4: Warning: cannot concatenate builtin `divnum'
error-->m4trace: -2- defn(`divnum', `divnum')
error-->m4trace: -1- define(`mydivnum', `')
⇒
traceoff(`defn', `define')
⇒

5.6 Temporarily redefining macros

It is possible to redefine a macro temporarily, reverting to the previous definition at a later time. This is done with the builtins pushdef and popdef:

Builtin: pushdef (name, [expansion]@c)

Builtin: popdef (name…)

Analogous to define and undefine.

These macros work in a stack-like fashion. A macro is temporarily redefined with pushdef, which replaces an existing definition of name, while saving the previous definition, before the new one is installed. If there is no previous definition, pushdef behaves exactly like define.

If a macro has several definitions (of which only one is accessible), the topmost definition can be removed with popdef. If there is no previous definition, popdef behaves like undefine.

The expansion of both pushdef and popdef is void. The macros pushdef and popdef are recognized only with parameters.

define(`foo', `Expansion one.')
⇒
foo
⇒Expansion one.
pushdef(`foo', `Expansion two.')
⇒
foo
⇒Expansion two.
pushdef(`foo', `Expansion three.')
⇒
pushdef(`foo', `Expansion four.')
⇒
popdef(`foo')
⇒
foo
⇒Expansion three.
popdef(`foo', `foo')
⇒
foo
⇒Expansion one.
popdef(`foo')
⇒
foo
⇒foo

If a macro with several definitions is redefined with define, the topmost definition is replaced with the new definition. If it is removed with undefine, all the definitions are removed, and not only the topmost one. However, POSIX allows other implementations that treat define as replacing an entire stack of definitions with a single new definition, so to be portable to other implementations, it may be worth explicitly using popdef and pushdef rather than relying on the GNU behavior of define.

define(`foo', `Expansion one.')
⇒
foo
⇒Expansion one.
pushdef(`foo', `Expansion two.')
⇒
foo
⇒Expansion two.
define(`foo', `Second expansion two.')
⇒
foo
⇒Second expansion two.
undefine(`foo')
⇒
foo
⇒foo

Local variables within macros are made with pushdef and popdef. At the start of the macro a new definition is pushed, within the macro it is manipulated and at the end it is popped, revealing the former definition.

It is possible to temporarily redefine a builtin with pushdef and defn.

5.7 Indirect call of macros

Any macro can be called indirectly with indir:

Builtin: indir (name, [args…]@c)

Results in a call to the macro name, which is passed the rest of the arguments args. If name is not defined, an error message is printed, and the expansion is void.

The macro indir is recognized only with parameters.

This can be used to call macros with computed or “invalid” names (define allows such names to be defined):

define(`$$internal$macro', `Internal macro (name `$0')')
⇒
$$internal$macro
⇒$$internal$macro
indir(`$$internal$macro')
⇒Internal macro (name $$internal$macro)

The point is, here, that larger macro packages can have private macros defined, that will not be called by accident. They can only be called through the builtin indir.

One other point to observe is that argument collection occurs before indir invokes name, so if argument collection changes the value of name, that will be reflected in the final expansion. This is different than the behavior when invoking macros directly, where the definition that was in effect before argument collection is used.

$ m4 -d
define(`f', `1')
⇒
f(define(`f', `2'))
⇒1
indir(`f', define(`f', `3'))
⇒3
indir(`f', undefine(`f'))
error-->m4:stdin:4: undefined macro `f'
⇒

When handed the result of defn (see section Renaming macros) as one of its arguments, indir defers to the invoked name for whether a token representing a builtin is recognized or flattened to the empty string.

$ m4 -d
indir(defn(`defn'), `divnum')
error-->m4:stdin:1: Warning: indir: invalid macro name ignored
⇒
indir(`define', defn(`defn'), `divnum')
error-->m4:stdin:2: Warning: define: invalid macro name ignored
⇒
indir(`define', `foo', defn(`divnum'))
⇒
foo
⇒0
indir(`divert', defn(`foo'))
error-->m4:stdin:5: empty string treated as 0 in builtin `divert'
⇒

5.8 Indirect call of builtins

Builtin macros can be called indirectly with builtin:

Builtin: builtin (name, [args…]@c)

Results in a call to the builtin name, which is passed the rest of the arguments args. If name does not name a builtin, an error message is printed, and the expansion is void.

The macro builtin is recognized only with parameters.

This can be used even if name has been given another definition that has covered the original, or been undefined so that no macro maps to the builtin.

pushdef(`define', `hidden')
⇒
undefine(`undefine')
⇒
define(`foo', `bar')
⇒hidden
foo
⇒foo
builtin(`define', `foo', defn(`divnum'))
⇒
foo
⇒0
builtin(`define', `foo', `BAR')
⇒
foo
⇒BAR
undefine(`foo')
⇒undefine(foo)
foo
⇒BAR
builtin(`undefine', `foo')
⇒
foo
⇒foo

The name argument only matches the original name of the builtin, even when the ‘--prefix-builtins’ option (or ‘-P’, see section Invoking m4) is in effect. This is different from indir, which only tracks current macro names.

$ m4 -P
m4_builtin(`divnum')
⇒0
m4_builtin(`m4_divnum')
error-->m4:stdin:2: undefined builtin `m4_divnum'
⇒
m4_indir(`divnum')
error-->m4:stdin:3: undefined macro `divnum'
⇒
m4_indir(`m4_divnum')
⇒0

Note that indir and builtin can be used to invoke builtins without arguments, even when they normally require parameters to be recognized; but it will provoke a warning, and result in a void expansion.

builtin
⇒builtin
builtin()
error-->m4:stdin:2: undefined builtin `'
⇒
builtin(`builtin')
error-->m4:stdin:3: Warning: too few arguments to builtin `builtin'
⇒
builtin(`builtin',)
error-->m4:stdin:4: undefined builtin `'
⇒
builtin(`builtin', ``'
')
error-->m4:stdin:5: undefined builtin ``'
error-->'
⇒
indir(`index')
error-->m4:stdin:7: Warning: too few arguments to builtin `index'
⇒

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