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] [file ...]
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 speci-
fied 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, ..., argumentN]).
There cannot be any space following the macro name and the open parenthe-
sis ('('). 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 fol-
lowed 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:
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 han-
dles 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
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 m4_define.
-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 redefini-
Changes the start comment and end comment sequences. Com-
ment sequences may be up to five characters long. The
default values are the hash sign and the newline character.
# This is a comment
With no arguments, comments are turned off. With one single
argument, the end comment sequence is set to the newline
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
decr(arg) 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 argu-
ment. '$0' is the name of the calling macro. Undefined
arguments are replaced by a null string. '$#' is replaced
by the number of arguments; '$*' is replaced by all argu-
ments comma separated; '$@' 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
divert(num) 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
divnum Returns the current output queue number.
dnl Discard input characters up to and including the next new-
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.
eval(expr) 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 argu-
ment (if any) specifies the minimum number of digits in the
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
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 argu-
ments 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 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.
incr(arg) Increments the argument by 1. The argument must be a valid
Returns the index of the second argument in the first argu-
ment (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, ...
len(arg) 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 (e.g., m4wrap("cleanup(tempfile)")
causes the macro cleanup to be invoked after all other pro-
cessing is done).
Multiple calls to m4wrap() get inserted in sequence at the
Invokes mkstemp(3) on the first argument, and returns the
modified string. This can be used to create unique tempo-
rary 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 replace-
ment string. Usual substitution patterns apply: an amper-
sand ('&') 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 defini-
tion on a stack for later retrieval by popdef().
regexp(string, regexp, replacement)
Finds a regular expression in a string. If no further argu-
ments 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 spec-
ified 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
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 argu-
ments, 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 abbreviations.
Removes the definition for the macros specified by its argu-
Flushes the named output queues (or all queues if no argu-
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 exits 0 on success, and >0 if an error occurs.
But note that the m4exit macro can modify the exit status.
The m4 utility is mostly compliant with the specification.
The flags [-dgIot] and the macros builtin, esyscmd, expr, format, indir,
paste, patsubst, regexp, spaste, unix, __line__, and __file__ are exten-
sions to that specification.
maketemp is not supposed to be a synonym for mkstemp, but instead to be
an insecure temporary file name creation function. The change causes no
known compatibility issues.
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 cur-
rent format of tracing is closely modelled on gnu-m4, to allow autoconf
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 instead.
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 <firstname.lastname@example.org> and Richard A. O'Keefe
GNU-m4 compatibility extensions by Marc Espie <email@example.com>.
BSD October 21, 2010 BSD
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