16.1 Extensions in GNU M4

This version of m4 contains a few facilities that do not exist in System V m4. These extra facilities are all suppressed by using the -G command line option (see Invoking m4), unless overridden by other command line options.

• In the $n notation for macro arguments, n can contain several digits, while the System V m4 only accepts one digit. This allows macros in GNU m4 to take any number of arguments, and not only nine (see Arguments).

  This means that define(`foo', `$11') is ambiguous between implementations. To portably choose between grabbing the first parameter and appending 1 to the expansion, or grabbing the eleventh parameter, you can do the following:

  define(`a1', `A1')
  ⇒
  dnl First argument, concatenated with 1
  define(`_1', `$1')define(`first1', `_1($@)1')
  ⇒
  dnl Eleventh argument, portable
  define(`_9', `$9')define(`eleventh', `_9(shift(shift($@)))')
  ⇒
  dnl Eleventh argument, GNU style
  define(`Eleventh', `$11')
  ⇒
  first1(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
  ⇒A1
  eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
  ⇒k
  Eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
  ⇒k
  

  Also see the argn macro (see Shift).

• The divert (see Divert) macro can manage more than 9 diversions. GNU m4 treats all positive numbers as valid diversions, rather than discarding diversions greater than 9.
• Files included with include and sinclude are sought in a user specified search path, if they are not found in the working directory. The search path is specified by the -I option and the M4PATH environment variable (see Search Path).
• Arguments to undivert can be non-numeric, in which case the named file will be included uninterpreted in the output (see Undivert).
• Formatted output is supported through the format builtin, which is modeled after the C library function printf (see Format).
• Searches and text substitution through basic regular expressions are supported by the regexp (see Regexp) and patsubst (see Patsubst) builtins. Some BSD implementations use extended regular expressions instead.
• The output of shell commands can be read into m4 with esyscmd (see Esyscmd).
• There is indirect access to any builtin macro with builtin (see Builtin).
• Macros can be called indirectly through indir (see Indir).
• The name of the program, the current input file, and the current input line number are accessible through the builtins __program__, __file__, and __line__ (see Location).
• The format of the output from dumpdef and macro tracing can be controlled with debugmode (see Debug Levels).
• The destination of trace and debug output can be controlled with debugfile (see Debug Output).
• The maketemp (see Mkstemp) macro behaves like mkstemp, creating a new file with a unique name on every invocation, rather than following the insecure behavior of replacing the trailing ‘X’ characters with the m4 process id.
• POSIX only requires support for the command line options -s, -D, and -U, so all other options accepted by GNU M4 are extensions. See Invoking m4, for a description of these options.

  The debugging and tracing facilities in GNU m4 are much more extensive than in most other versions of m4.