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m4 - macro language processor


Exit Status


m4 [-d flags] [-t name] [-gs] [-D name [= value]] [-U name] [-I dirname] []


The 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). The m4 utility reads from the standard input and writes the processed text to the standard output.

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 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 do not make any sense without arguments, and hence are not recognized as special when not followed by an open parenthesis.

The options are as follows:

-s Emit #line directives for cpp(1).
-D name [= value]
Define the symbol name to have some value (or NULL).
-U name
Undefine the symbol name.
-I dirname
Add directory dirname to the include path.
-d flags
Set trace flags. The flags argument may hold the following:

a print macro arguments
c print macro expansion over several lines
e print result of macro expansion
f print filename location
l print line number
q quote arguments and expansion with the current quotes
t start with all macros traced
x number macro expansions
V turn on all options

By default, trace is set to eq.

-t macro
Turn tracing on for macro.
-g Activate GNU-m4 compatibility mode. In this mode, changequote with two empty parameters deactivates quotes, translit handles simple character ranges (e.g., a-z), regular expressions mimic emacs(1) behavior, and the number of diversions is unlimited.


The m4 utility provides the following built-in macros. They may be redefined, losing their original meaning. Return values are null unless otherwise stated.
builtin Calls a built-in by its name, overriding possible redefinitions.
changecom Changes the start and end comment sequences. The default is the pound sign (‘#’) and the newline character. With no arguments, the comment sequence is reset to the default, in GNU m4 mode, comments are turned off. The maximum length for a comment marker is five characters.
changequote Defines the quote symbols to be the first and second arguments. The symbols may be up to five characters long. If no arguments are given it restores the default open and close single quotes.
decr Decrements the argument by 1. The argument must be a valid numeric string.
define Define a new macro named by the first argument to have the value of the second argument. 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.
defn Returns the quoted definition for each argument. This can be used to rename macro definitions (even for built-in macros).
divert 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).
divnum Returns the current output queue number.
dnl Discards input characters up to and including the next newline.
dumpdef Prints the names and definitions for the named items, or for everything if no arguments are passed.
errprint Prints the first argument on the standard error output stream.
esyscmd 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.
eval 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 second argument (if any) specifies the radix for the result, and the third argument (if any) specifies the minimum number of digits in the result.
expr This is an alias for eval.
ifdef 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.
ifelse If the first argument matches the second argument then ifelse returns the third argument. 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 found.
include 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. Aborts with an error message if the file cannot be included.
incr Increments the argument by 1. The argument must be a valid numeric string.
index 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.
indir Indirectly calls the macro whose name is passed as the first arguments, with the remaining arguments passed as first, etc. arguments.
len Returns the number of characters in the first argument. Extra arguments are ignored.
m4exit Immediately exits with the return value specified by the first argument, 0 if none.
m4wrap 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).
maketemp Translates the string "XXXXX" in the first argument with the current process ID leaving other characters alone. This can be used to create unique temporary file names.
paste 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.
patsubst 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.
popdef Restores the pushdef ed definition for each argument.
pushdef Takes the same arguments as define, but it saves the definition on a stack for later retrieval by popdef.
regexp 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.
shift 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.
sinclude Similar to include, except it ignores any errors.
spaste Similar to paste, except it ignores any errors.
substr 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.
syscmd Passes the first argument to the shell. Nothing is returned.
sysval Returns the return value from the last syscmd.
traceon Enables tracing of macro expansions for the given arguments, or for all macros if no argument is given.
traceoff Disables tracing of macro expansions for the given arguments, or for all macros if no argument is given.
translit 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.
undefine Removes the definition for the macros specified by its arguments.
undivert Flushes the named output queues (or all queues if no arguments).
unix A pre-defined macro for testing the OS platform.
__line__ Returns the current file’s line number.
__file__ Returns the current file’s name.


.Ex -std

The m4exit macro may be used to change the exit status from the input file.


The m4 utility follows the -susv2, along with a few extensions taken from GNU-m4. Flags -I -, -d , and -t are non-standard.

The output format of tracing and 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 modeled on GNU-m4, to allow autoconf to work.

For portability, one should not use the macros builtin, esyscmd, expr, indir, paste, patsubst, regexp, spaste, unix, __line__, and __file__.

All built-ins do expand without arguments in many other m4 implementations.

Many other m4 implementations have dire size limitations with respect to buffer sizes.


The m4 utility conforms to -p1003.1-2001.


An m4 command appeared in PWB UNIX.


.An -nosplit
.An Ozan Yigit Aq
oz@sis.yorku.ca and
.An Richard A. O’Keefe Aq ok@goanna.cs.rmit.OZ.AU . GNU-m4 compatibility extensions by
.An Marc Espie Aq espie@cvs.openbsd.org .


The m4 utility does not recognize multibyte characters.

July 3, 2004 M4 (1)
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