utility is a macro processor that can be
used as a front end to any language (e.g., C, ratfor, fortran, lex, and yacc).
If no input files are given, m4
reads from the
standard input, otherwise files specified on the command line are processed in
the given order. Input files can be regular files, files in the m4 include
paths, or a single dash (‘-’), denoting standard input.
writes the processed text to the standard
output, unless told otherwise.
Macro calls have the form name(argument1[, argument2, ..., argumentN]).
There cannot be any space following the macro name and the open parenthesis
(‘(’). If the macro name is not followed by an open parenthesis it
is processed with no arguments.
Macro names consist of a leading alphabetic or underscore possibly followed by
alphanumeric or underscore characters, e.g., valid macro names match the
In arguments to macros, leading unquoted space, tab, and newline
(‘\n’) characters are ignored. To quote strings, use left and
right single quotes (e.g., ‘ this is a string with a leading
space’). You can change the quote characters with the
Most built-ins don't make any sense without arguments, and hence are not
recognized as special when not followed by an open parenthesis.
The options are as follows:
- Define the symbol name to
have some value (or
- Set trace flags. flags may
hold the following:
By default, trace is set to “eq”.
- print macro arguments.
- print macro expansion over several lines.
- print result of macro expansion.
- print filename location.
- print line number.
- quote arguments and expansion with the current
- start with all macros traced.
- number macro expansions.
- turn on all options.
- Activate GNU-m4 compatibility mode. In this mode, translit
handles simple character ranges (e.g., a-z), regular expressions mimic
emacs behavior, multiple m4wrap calls are handled as a stack, the number
of diversions is unlimited, empty names for macro definitions are allowed,
and eval understands ‘0rbase:value’ numbers.
- Add directory dirname to
the include path.
- Send trace output to
- Prefix all built-in macros with ‘m4_’. For
example, instead of writing define, use
- Output line synchronization directives, suitable for
- Turn tracing on for
- Undefine the symbol
provides the following built-in macros. They may
be redefined, losing their original meaning. Return values are null unless
- Calls a built-in by its
name, overriding possible
- Changes the start comment and end comment sequences.
Comment sequences may be up to five characters long. The default values
are the hash sign and the newline character.
With no arguments, comments are turned off. With one single argument, the
end comment sequence is set to the newline character.
- Defines the open quote and close quote sequences. Quote
sequences may be up to five characters long. The default values are the
backquote character and the quote character.
With no arguments, the default quotes are restored. With one single
argument, the close quote sequence is set to the newline character.
`Here is a quoted string'
- Decrements the argument
arg by 1. The argument
arg must be a valid numeric string.
- Define a new macro named by the first argument
name to have the value of the second
argument value. Each occurrence of
‘$n’ (where n is 0 through 9)
is replaced by the n'th argument.
‘$0’ is the name of the calling macro. Undefined arguments are
replaced by a null string. ‘$#’ is replaced by the number of
arguments; ‘$*’ is replaced by all arguments comma separated;
‘$@’ is the same as ‘$*’ but all arguments are
quoted against further expansion.
- Returns the quoted definition for each argument. This can
be used to rename macro definitions (even for built-in macros).
- There are 10 output queues (numbered 0-9). At the end of
processing m4 concatenates all the queues in
numerical order to produce the final output. Initially the output queue is
0. The divert macro allows you to select a new output queue (an invalid
argument passed to divert causes output to be discarded).
- Returns the current output queue number.
- Discard input characters up to and including the next
- Prints the names and definitions for the named items, or
for everything if no arguments are passed.
- Prints the first argument on the standard error output
- Passes its first argument to a shell and returns the
shell's standard output. Note that the shell shares its standard input and
standard error with m4.
- Computes the first argument as an arithmetic expression
using 32-bit arithmetic. Operators are the standard C ternary, arithmetic,
logical, shift, relational, bitwise, and parentheses operators. You can
specify octal, decimal, and hexadecimal numbers as in C. The optional
second argument radix specifies the radix
for the result and the optional third argument
minimum specifies the minimum number of
digits in the result.
- This is an alias for
- Returns formatstring with
escape sequences substituted with arg1
and following arguments, in a way similar to
printf(3). This built-in is
only available in GNU-m4 compatibility mode, and the only parameters
implemented are there for autoconf compatibility: left-padding flag, an
optional field width, a maximum field width, *-specified field widths, and
the %s and %c data type.
- If the macro named by the first argument is defined then
return the second argument, otherwise the third. If there is no third
argument, the value is
NULL. The word
“unix” is predefined.
- If the first argument a
matches the second argument b then
ifelse() returns the third argument
yes. If the match fails the three
arguments are discarded and the next three arguments are used until there
is zero or one arguments left, either this last argument or
NULL is returned if no other matches
- Returns the contents of the file specified in the first
argument. If the file is not found as is, look through the include path:
first the directories specified with -I on
the command line, then the environment variable
M4PATH, as a colon-separated list of
directories. Include aborts with an error message if the file cannot be
- Increments the argument by 1. The argument must be a valid
- Returns the index of the second argument in the first
argument (e.g., index(the quick brown fox jumped,
fox) returns 16). If the second argument is not found index returns
- Indirectly calls the macro whose name is passed as the
first argument, with the remaining arguments passed as first, ...
- Returns the number of characters in the first argument.
Extra arguments are ignored.
- Immediately exits with the return value specified by the
first argument, 0 if none.
- Allows you to define what happens at the final
EOF, usually for cleanup purposes
causes the macro cleanup to be invoked after all other processing is
Multiple calls to m4wrap() get inserted in
sequence at the final
- Like mkstemp.
mkstemp(3) on the first
argument, and returns the modified string. This can be used to create
unique temporary file names.
- Includes the contents of the file specified by the first
argument without any macro processing. Aborts with an error message if the
file cannot be included.
- Substitutes a regular expression in a string with a
replacement string. Usual substitution patterns apply: an ampersand
(‘&’) is replaced by the string matching the regular
expression. The string ‘\#’, where ‘#’ is a digit,
is replaced by the corresponding back-reference.
- Restores the pushdefed
definition for each argument.
- Takes the same arguments as
define, but it saves the definition on a
stack for later retrieval by popdef().
- Finds a regular expression in a string. If no further
arguments are given, it returns the first match position or -1 if no
match. If a third argument is provided, it returns the replacement string,
with sub-patterns replaced.
- Returns all but the first argument, the remaining arguments
are quoted and pushed back with commas in between. The quoting nullifies
the effect of the extra scan that will subsequently be performed.
- Similar to include, except it
ignores any errors.
- Similar to paste(), except it
ignores any errors.
- Returns a substring of the first argument starting at the
offset specified by the second argument and the length specified by the
third argument. If no third argument is present it returns the rest of the
- Passes the first argument to the shell. Nothing is
- Returns the return value from the last
- Enables tracing of macro expansions for the given
arguments, or for all macros if no argument is given.
- Disables tracing of macro expansions for the given
arguments, or for all macros if no argument is given.
- Transliterate the characters in the first argument from the
set given by the second argument to the set given by the third. You cannot
use tr(1) style
- Removes the definition for the macros specified by its
- Flushes the named output queues (or all queues if no
- A pre-defined macro for testing the OS platform.
- Returns the current file's line number.
- Returns the current file's name.
utility exits 0 on success,
and >0 if an error occurs.
But note that the m4exit
macro can modify the exit
utility is compliant with the
IEEE Std 1003.1-2008
The flags [-dgIPot
] and the
are extensions to that specification.
is not supposed to be a synonym for
, but instead to be an insecure temporary
file name creation function. It is marked by IEEE
Std 1003.1-2008 (“POSIX.1”)
as being obsolescent and should
not be used if portability is a concern.
The output format of traceon
are not specified in any standard, are
likely to change and should not be relied upon. The current format of tracing
is closely modelled on gnu-m4
, to allow
The built-ins pushdef
handle macro definitions as a stack.
interacts with the stack in an
undefined way. In this implementation, define
replaces the top-most definition only. Other implementations may erase all
definitions on the stack instead.
All built-ins do expand without arguments in many other
Many other m4
have dire size limitations with
respect to buffer sizes.
and Richard A. O'Keefe
GNU-m4 compatibility extensions by Marc Espie