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4.2 Preventing macro invocation

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 section 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, using How to define new macros.

If your version of GNU m4 has the changeword feature compiled in, it offers far more flexibility in specifying the syntax of macro names, both builtin or user-defined. See section Changing the lexical structure of words, 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`'
⇒

the divert builtin macro will be called, which expands to the empty string.

The output of macro evaluations is always rescanned. In the following example, the input ‘x`'y’ yields the string ‘bCD’, exactly as if m4 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 removed, the divert builtin was called instead.


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