macro language processor
m4 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.
m4 writes the processed text to the standard output,
unless told otherwise.
Macro calls have the form name(argument1[, argument2, ...,
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 pattern “[a-zA-Z_][a-zA-Z0-9_]*”.
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
changequote built-in macro.
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:
- 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 quotes.
- start with all macros traced.
- number macro expansions.
- turn on all options.
By default, trace is set to “eq”.
- Set warnings to be fatal. When a single
-E flag is
specified, if warnings are issued, execution continues but
m4 will exit with a non-zero exit status. When
-E flags are specified, execution will
halt upon issuing the first warning and
exit with a non-zero exit status. This behaviour matches GNU-m4 1.4.9 and
- 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 filename.
- Prefix all built-in macros with ‘m4_’. For example, instead
- Output line synchronization directives, suitable for
- Turn tracing on for macro.
- Undefine the symbol name.
m4 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.
`Here is a quoted string'
With no arguments, the default quotes are restored. With one
single argument, the close quote sequence is set to the newline
- 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 newline.
- 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 stream.
- 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
- 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
- If the first argument a matches the second argument
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 were
- 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 included.
- Increments the argument by 1. The argument must be a valid numeric
- 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 -1.
- Indirectly calls the macro whose name is passed as the first argument,
with the remaining arguments passed as first, ... arguments.
- Returns the number of characters in the first argument. Extra arguments
- Immediately exits with the return value specified by the first argument, 0
- Allows you to define what happens at the final
EOF, usually for cleanup purposes (e.g.,
m4wrap("cleanup(tempfile)") causes the
macro cleanup to be invoked after all other processing is done).
Multiple calls to
inserted in sequence at the final
- Invokes 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
- 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
- Restores the
pushdefed definition for each
- Takes the same arguments as
define, but it saves
the definition on a stack for later retrieval by
- 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
- 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
- Similar to
paste(), except it ignores any
- 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 string.
- Passes the first argument to the shell. Nothing is returned.
- 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 abbreviations.
- Removes the definition for the macros specified by its arguments.
- Flushes the named output queues (or all queues if no arguments).
- A pre-defined macro for testing the OS platform.
- Returns the current file's line number.
- Returns the current file's name.
m4 utility exits 0 on success,
and >0 if an error occurs.
But note that the
m4exit macro can modify
the exit status, as can the
m4 utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
The flags [
-dEgIPot] and the macros
extensions to that specification.
maketemp is not supposed to be a synonym
mkstemp, 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
dumpdef 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
autoconf to work.
popdef handle macro definitions as a stack. However,
define 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
All built-ins do expand without arguments in many other
m4 have dire size limitations
with respect to buffer sizes.