M4(1) BSD General Commands Manual M4(1)
m4 -- macro language processor
m4 [-gPs] [-Dname[=value]] [-d flags] [-I dirname] [-o filename] [-t macro] [-Uname]
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). 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
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
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 spe-
cial when not followed by an open parenthesis.
The options are as follows:
Define the symbol name to have some value (or NULL).
Set trace flags. flags 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".
-g 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.
-P Prefix all built-in macros with 'm4_'. For example, instead of writing define, use
-s Output line synchronization directives, suitable for cpp(1).
Turn tracing on for macro.
-Uname Undefine the symbol name.
m4 provides the following built-in macros. They may be redefined, losing their original
meaning. Return values are null unless otherwise stated.
Calls a built-in by its name, overriding possible redefinitions.
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 new-
# This is a comment
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
`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 character.
decr(arg) Decrements the argument arg by 1. The argument arg must be a valid numeric
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. '$0' is the name of the calling macro. Unde-
fined arguments are replaced by a null string. '$#' 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).
divert(num) There are 10 output queues (numbered 0-9). At the end of processing m4 con-
catenates all the queues in numerical order to produce the final output. Ini-
tially the output queue is 0. The divert macro allows you to select a new out-
put queue (an invalid argument passed to divert causes output to be discarded).
divnum Returns the current output queue number.
dnl 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 with m4.
eval(expr) Computes the first argument as an arithmetic expression using 32-bit arith-
metic. Operators are the standard C ternary, arithmetic, logical, shift, rela-
tional, 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 num-
ber of digits in the result.
expr(expr) This is an alias for eval.
format(formatstring, arg1, ...)
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.
ifdef(name, yes, no)
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(a, b, yes, ...)
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 were found.
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 speci-
fied 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.
incr(arg) Increments the argument by 1. The argument must be a valid numeric string.
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(macro, arg1, ...)
Indirectly calls the macro whose name is passed as the first argument, with the
remaining arguments passed as first, ... arguments.
len(arg) Returns the number of characters in the first argument. Extra arguments are
Immediately exits with the return value specified by the first argument, 0 if
Allows you to define what happens at the final EOF, usually for cleanup pur-
poses (e.g., m4wrap("cleanup(tempfile)") causes the macro cleanup to be invoked
after all other processing is done).
Multiple calls to m4wrap() get inserted in sequence at the final EOF.
Invokes mkstemp(3) on the first argument, and returns the modified string.
This can be used to create unique temporary file names.
paste(file) 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(string, regexp, replacement)
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().
regexp(string, regexp, replacement)
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.
substr(string, offset, length)
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(cmd) Passes the first argument to the shell. Nothing is returned.
sysval Returns the return value from the last syscmd.
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.
translit(string, mapfrom, mapto)
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 arguments.
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.
The m4 utility is compliant with the IEEE Std 1003.1-2008 (``POSIX.1'') specification.
The flags [-dgIot] and the macros builtin, esyscmd, expr, format, indir, paste, patsubst,
regexp, spaste, unix, __line__, and __file__ are extensions to that specification.
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 modelled on
gnu-m4, to allow autoconf to work.
The built-ins pushdef and 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 stack
All built-ins do expand without arguments in many other m4.
Many other m4 have dire size limitations with respect to buffer sizes.
Ozan Yigit <email@example.com> and Richard A. O'Keefe <firstname.lastname@example.org.OZ.AU>.
GNU-m4 compatibility extensions by Marc Espie <email@example.com>.
BSD October 14, 2009 BSD