An innovation of the
m4 language, compared to some of its
predecessors (like Strachey's
GPM, for example), is the ability
to recognize macro calls without resorting to any special, prefixed
invocation character. While generally useful, this feature might
sometimes be the source of spurious, unwanted macro calls. So, GNU
m4 offers several mechanisms or techniques for inhibiting the
recognition of names as macro calls.
First of all, many builtin macros cannot meaningfully be called without arguments. As a GNU extension, for any of these macros, whenever an opening parenthesis does not immediately follow their name, the builtin macro call is not triggered. This solves the most usual cases, like for ‘include’ or ‘eval’. Later in this document, the sentence “This macro is recognized only with parameters” refers to this specific provision of GNU M4, also known as a blind builtin macro. For the builtins defined by POSIX that bear this disclaimer, POSIX specifically states that invoking those builtins without arguments is unspecified, because many other implementations simply invoke the builtin as though it were given one empty argument instead.
$ m4 eval ⇒eval eval(`1') ⇒1
There is also a command line option (--prefix-builtins, or
-P, see Invoking m4) that renames all
builtin macros with a prefix of ‘m4_’ at startup. The option has
no effect whatsoever on user defined macros. For example, with this option,
one has to write
m4_dnl and even
m4_m4exit. It also has
no effect on whether a macro requires parameters.
$ m4 -P eval ⇒eval eval(`1') ⇒eval(1) m4_eval ⇒m4_eval m4_eval(`1') ⇒1
Another alternative is to redefine problematic macros to a name less likely to cause conflicts, See Definitions.
If your version of GNU
m4 has the
compiled in, it offers far more flexibility in specifying the
syntax of macro names, both builtin or user-defined. See Changeword,
for more information on this experimental feature.
Of course, the simplest way to prevent a name from being interpreted as a call to an existing macro is to quote it. The remainder of this section studies a little more deeply how quoting affects macro invocation, and how quoting can be used to inhibit macro invocation.
Even if quoting is usually done over the whole macro name, it can also be done over only a few characters of this name (provided, of course, that the unquoted portions are not also a macro). It is also possible to quote the empty string, but this works only inside the name. For example:
`divert' ⇒divert `d'ivert ⇒divert di`ver't ⇒divert div`'ert ⇒divert
all yield the string ‘divert’. While in both:
`'divert ⇒ divert`' ⇒
divert builtin macro will be called, which expands to the
The output of macro evaluations is always rescanned. In the following
example, the input ‘x`'y’ yields the string ‘bCD’, exactly as
has been given ‘substr(ab`'cde, `1', `3')’ as input:
define(`cde', `CDE') ⇒ define(`x', `substr(ab') ⇒ define(`y', `cde, `1', `3')') ⇒ x`'y ⇒bCD
Unquoted strings on either side of a quoted string are subject to
being recognized as macro names. In the following example, quoting the
empty string allows for the second
macro to be recognized as such:
define(`macro', `m') ⇒ macro(`m')macro ⇒mmacro macro(`m')`'macro ⇒mm
Quoting may prevent recognizing as a macro name the concatenation of a macro expansion with the surrounding characters. In this example:
define(`macro', `di$1') ⇒ macro(`v')`ert' ⇒divert macro(`v')ert ⇒
the input will produce the string ‘divert’. When the quotes were
divert builtin was called instead.