M4
Section: POSIX Programmer's Manual (1P)
Updated: 2017
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This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
m4 --- macro processor
SYNOPSIS
m4 [-s] [-D name[=val]]... [-U name]... file...
DESCRIPTION
The
m4
utility is a macro processor that shall read one or more text files,
process them according to their included macro statements, and write
the results to standard output.
OPTIONS
The
m4
utility shall conform to the Base Definitions volume of POSIX.1-2017,
Section 12.2,
Utility Syntax Guidelines,
except that the order of the
-D
and
-U
options shall be significant, and options can be interspersed with
operands.
The following options shall be supported:
- -s
-
Enable line synchronization output for the
c99
preprocessor phase (that is,
#line
directives).
- -D name[=val]
-
Define
name
to
val
or to null if =val
is omitted.
- -U name
-
Undefine
name.
OPERANDS
The following operand shall be supported:
- file
-
A pathname of a text file to be processed. If no
file
is given, or if it is
'-',
the standard input shall be read.
STDIN
The standard input shall be a text file that is used if no
file
operand is given, or if it is
'-'.
INPUT FILES
The input file named by the
file
operand shall be a text file.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
m4:
- LANG
-
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1-2017,
Section 8.2, Internationalization Variables
for the precedence of internationalization variables used to determine
the values of locale categories.)
- LC_ALL
-
If set to a non-empty string value, override the values of all the
other internationalization variables.
- LC_CTYPE
-
Determine the locale for the interpretation of sequences of bytes of
text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments and input files).
- LC_MESSAGES
-
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
- NLSPATH
-
Determine the location of message catalogs for the processing of
LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The standard output shall be the same as the input files, after being
processed for macro expansion.
STDERR
The standard error shall be used to display strings with the
errprint
macro, macro tracing enabled by the
traceon
macro, the defined text for macros written by the
dumpdef
macro, or for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
The
m4
utility shall compare each token from the input against the set of
built-in and user-defined macros. If the token matches the name of a
macro, then the token shall be replaced by the macro's defining text, if
any, and rescanned for matching macro names. Once no portion of the
token matches the name of a macro, it shall be written to standard
output. Macros may have arguments, in which case the arguments shall
be substituted into the defining text before it is rescanned.
Macro calls have the form:
-
name(arg1, arg2, ..., argn)
Macro names shall consist of letters, digits, and underscores, where
the first character is not a digit. Tokens not of this form shall not
be treated as macros.
The application shall ensure that the
<left-parenthesis>
immediately follows the name of the macro. If a token matching the name
of a macro is not followed by a
<left-parenthesis>,
it is handled as a use of that macro without arguments.
If a macro name is followed by a
<left-parenthesis>,
its arguments are the
<comma>-separated
tokens between the
<left-parenthesis>
and the matching
<right-parenthesis>.
Unquoted white-space characters preceding each argument shall be
ignored. All other characters, including trailing white-space characters,
are retained.
<comma>
characters enclosed between
<left-parenthesis>
and
<right-parenthesis>
characters do not delimit arguments.
Arguments are positionally defined and referenced. The string
"$1"
in the defining text shall be replaced by the first argument. Systems
shall support at least nine arguments; only the first nine can be
referenced, using the strings
"$1"
to
"$9",
inclusive. The string
"$0"
is replaced with the name of the macro. The string
"$#"
is replaced by the number of arguments as a string. The string
"$*"
is replaced by a list of all of the arguments, separated by
<comma>
characters. The string
"$@"
is replaced by a list of all of the arguments separated by
<comma>
characters, and each argument is quoted using the current left and right
quoting strings. The string
"${"
produces unspecified behavior.
If fewer arguments are supplied than are in the macro definition, the
omitted arguments are taken to be null. It is not an error if more
arguments are supplied than are in the macro definition.
No special meaning is given to any characters enclosed between matching
left and right quoting strings, but the quoting strings are themselves
discarded. By default, the left quoting string consists of a grave accent
(backquote) and the right quoting string consists of an acute accent
(single-quote); see also the
changequote
macro.
Comments are written but not scanned for matching macro names; by
default, the begin-comment string consists of the
<number-sign>
character and the end-comment string consists of a
<newline>.
See also the
changecom
and
dnl
macros.
The
m4
utility shall make available the following built-in macros. They can be
redefined, but once this is done the original meaning is lost. Their
values shall be null unless otherwise stated. In the descriptions
below, the term
defining text
refers to the value of the macro: the second argument to the
define
macro, among other things. Except for the first argument to the
eval
macro, all numeric arguments to built-in macros shall be interpreted as
decimal values. The string values produced as the defining text of the
decr,
divnum,
incr,
index,
len,
and
sysval
built-in macros shall be in the form of a decimal-constant as defined
in the C language.
- changecom
-
The
changecom
macro shall set the begin-comment and end-comment strings. With no
arguments, the comment mechanism shall be disabled. With a single non-null
argument, that argument shall become the begin-comment and the
<newline>
shall become the end-comment string. With two non-null arguments,
the first argument shall become the begin-comment string and the second
argument shall become the end-comment string. The behavior is unspecified
if either argument is provided but null. Systems shall support comment
strings of at least five characters.
- changequote
-
The
changequote
macro shall set the begin-quote and end-quote strings. With no
arguments, the quote strings shall be set to the default values (that
is, `'). The behavior is unspecified if there is a single argument
or either argument is null. With two non-null arguments, the first
argument shall become the begin-quote string and the second argument
shall become the end-quote string. Systems shall support quote strings
of at least five characters.
- decr
-
The defining text of the
decr
macro shall be its first argument decremented by 1. It shall be an
error to specify an argument containing any non-numeric characters.
The behavior is unspecified if
decr
is not immediately followed by a
<left-parenthesis>.
- define
-
The second argument shall become the defining text of the macro
whose name is the first argument. It is unspecified whether the
define
macro deletes all prior definitions of the macro named by its first
argument or preserves all but the current definition of the macro.
The behavior is unspecified if
define
is not immediately followed by a
<left-parenthesis>.
- defn
-
The defining text of the
defn
macro shall be the quoted definition (using the current quoting
strings) of its arguments. The behavior is unspecified if
defn
is not immediately followed by a
<left-parenthesis>.
- divert
-
The
m4
utility maintains nine temporary buffers, numbered 1 to 9, inclusive.
When the last of the input has been processed, any output that has been
placed in these buffers shall be written to standard output in
buffer-numerical order. The
divert
macro shall divert future output to the buffer specified by its
argument. Specifying no argument or an argument of 0 shall resume the
normal output process. Output diverted to a stream with a negative
number shall be discarded. Behavior is implementation-defined if
a stream number larger than 9 is specified. It shall be an error to
specify an argument containing any non-numeric characters.
- divnum
-
The defining text of the
divnum
macro shall be the number of the current output stream as a string.
- dnl
-
The
dnl
macro shall cause
m4
to discard all input characters up to and including the next
<newline>.
- dumpdef
-
The
dumpdef
macro shall write the defined text to standard error for each of the
macros specified as arguments, or, if no arguments are specified, for
all macros.
- errprint
-
The
errprint
macro shall write its arguments to standard error. The behavior
is unspecified if
errprint
is not immediately followed by a
<left-parenthesis>.
- eval
-
The
eval
macro shall evaluate its first argument as an arithmetic expression,
using signed integer arithmetic with at least 32-bit precision. At least
the following C-language operators shall be supported, with precedence,
associativity, and behavior as described in
Section 1.1.2.1, Arithmetic Precision and Operations:
-
-
()
unary +
unary -
~
!
binary *
/
%
binary +
binary -
<<
>>
<
<=
>
>=
==
!=
binary &
ha
|
&&
||
Systems shall support octal and hexadecimal numbers as in the ISO C standard.
The second argument, if specified, shall set the radix for the result;
if the argument is blank or unspecified, the default is 10. Behavior is
unspecified if the radix falls outside the range 2 to 36, inclusive. The
third argument, if specified, sets the minimum number of digits in the
result. Behavior is unspecified if the third argument is less than
zero. It shall be an error to specify the second or third argument
containing any non-numeric characters. The behavior is unspecified if
eval
is not immediately followed by a
<left-parenthesis>.
- ifdef
-
If the first argument to the
ifdef
macro is defined, the defining text shall be the second argument.
Otherwise, the defining text shall be the third argument, if specified,
or the null string, if not. The behavior is unspecified if
ifdef
is not immediately followed by a
<left-parenthesis>.
- ifelse
-
The
ifelse
macro takes three or more arguments. If the first two arguments compare
as equal strings (after macro expansion of both arguments), the
defining text shall be the third argument. If the first two arguments
do not compare as equal strings and there are three arguments, the
defining text shall be null. If the first two arguments do not compare
as equal strings and there are four or five arguments, the defining
text shall be the fourth argument. If the first two arguments do not
compare as equal strings and there are six or more arguments, the first
three arguments shall be discarded and processing shall restart with
the remaining arguments. The behavior is unspecified if
ifelse
is not immediately followed by a
<left-parenthesis>.
- include
-
The defining text for the
include
macro shall be the contents of the file named by the first argument. It
shall be an error if the file cannot be read. The behavior is unspecified if
include
is not immediately followed by a
<left-parenthesis>.
- incr
-
The defining text of the
incr
macro shall be its first argument incremented by 1. It shall be an
error to specify an argument containing any non-numeric characters.
The behavior is unspecified if
incr
is not immediately followed by a
<left-parenthesis>.
- index
-
The defining text of the
index
macro shall be the first character position (as a string) in the first
argument where a string matching the second argument begins (zero
origin), or -1 if the second argument does not occur.
The behavior is unspecified if
index
is not immediately followed by a
<left-parenthesis>.
- len
-
The defining text of the
len
macro shall be the length (as a string) of the first argument.
The behavior is unspecified if
len
is not immediately followed by a
<left-parenthesis>.
- m4exit
-
Exit from the
m4
utility. If the first argument is specified, it shall be the exit
code. If no argument is specified, the exit code shall be zero. It
shall be an error to specify an argument containing any non-numeric
characters. If the first argument is zero or no argument is specified,
and an error has previously occurred (for example, a
file
operand that could not be opened), it is unspecified whether the exit
status is zero or non-zero.
- m4wrap
-
The first argument shall be processed when EOF is reached. If the
m4wrap
macro is used multiple times, the arguments specified shall be
processed in the order in which the
m4wrap
macros were processed. The behavior is unspecified if
m4wrap
is not immediately followed by a
<left-parenthesis>.
- maketemp
-
The defining text shall be the first argument, with any trailing
'X'
characters replaced with the current process ID as a string.
The behavior is unspecified if
maketemp
is not immediately followed by a
<left-parenthesis>.
- mkstemp
-
The defining text shall be as if it were the resulting pathname after
a successful call to the
mkstemp()
function defined in the System Interfaces volume of POSIX.1-2017 called with the first argument to the
macro invocation. If a file is created, that file shall be closed.
If a file could not be created, the
m4
utility shall write a diagnostic message to standard error and shall
continue processing input but its final exit status shall be non-zero;
the defining text of the macro shall be the empty string. The behavior
is unspecified if
mkstemp
is not immediately followed by a
<left-parenthesis>.
- popdef
-
The
popdef
macro shall delete the current definition of its arguments, replacing
that definition with the previous one. If there is no previous
definition, the macro is undefined. The behavior is unspecified if
popdef
is not immediately followed by a
<left-parenthesis>.
- pushdef
-
The
pushdef
macro shall be equivalent to the
define
macro with the exception that it shall preserve any current definition
for future retrieval using the
popdef
macro. The behavior is unspecified if
pushdef
is not immediately followed by a
<left-parenthesis>.
- shift
-
The defining text for the
shift
macro shall be a comma-separated list of its arguments except the first
one. Each argument shall be quoted using the current quoting strings.
The behavior is unspecified if
shift
is not immediately followed by a
<left-parenthesis>.
- sinclude
-
The
sinclude
macro shall be equivalent to the
include
macro, except that it shall not be an error if the file is inaccessible.
The behavior is unspecified if
sinclude
is not immediately followed by a
<left-parenthesis>.
- substr
-
The defining text for the
substr
macro shall be the substring of the first argument beginning at the
zero-offset character position specified by the second argument. The
third argument, if specified, shall be the number of characters to
select; if not specified, the characters from the starting point to the
end of the first argument shall become the defining text. It shall not
be an error to specify a starting point beyond the end of the first
argument and the defining text shall be null. It shall be an error to
specify an argument containing any non-numeric characters.
The behavior is unspecified if
substr
is not immediately followed by a
<left-parenthesis>.
- syscmd
-
The
syscmd
macro shall interpret its first argument as a shell command line. The
defining text shall be the string result of that command. The string
result shall not be rescanned for macros while setting the defining
text. No output redirection shall be performed by the
m4
utility. The exit status value from the command can be retrieved using
the
sysval
macro. The behavior is unspecified if
syscmd
is not immediately followed by a
<left-parenthesis>.
- sysval
-
The defining text of the
sysval
macro shall be the exit value of the utility last invoked by the
syscmd
macro (as a string).
- traceon
-
The
traceon
macro shall enable tracing for the macros specified as arguments, or,
if no arguments are specified, for all macros. The trace output shall
be written to standard error in an unspecified format.
- traceoff
-
The
traceoff
macro shall disable tracing for the macros specified as arguments, or,
if no arguments are specified, for all macros.
- translit
-
The defining text of the
translit
macro shall be the first argument with every character that occurs in
the second argument replaced with the corresponding character from the
third argument. If no replacement character is specified for some
source character because the second argument is longer than the third
argument, that character shall be deleted from the first argument in
translit's
defining text. The behavior is unspecified if the
'-'
character appears within the second or third argument anywhere besides
the first or last character. The behavior is unspecified if the same
character appears more than once in the second argument. The behavior
is unspecified if
translit
is not immediately followed by a
<left-parenthesis>.
- undefine
-
The
undefine
macro shall delete all definitions (including those preserved using the
pushdef
macro) of the macros named by its arguments. The behavior is unspecified if
undefine
is not immediately followed by a
<left-parenthesis>.
- undivert
-
The
undivert
macro shall cause immediate output of any text in temporary buffers
named as arguments, or all temporary buffers if no arguments are
specified. Buffers can be undiverted into other temporary buffers.
Undiverting shall discard the contents of the temporary buffer. The
behavior is unspecified if an argument contains any non-numeric
characters.
EXIT STATUS
The following exit values shall be returned:
- 0
-
Successful completion.
- >0
-
An error occurred
If the
m4exit
macro is used, the exit value can be specified by the input file.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The
defn
macro is useful for renaming macros, especially built-ins.
Since
eval
defers to the ISO C standard, some operations have undefined behavior. In some
implementations, division or remainder by zero cause a fatal signal,
even if the division occurs on the short-circuited branch of
"&&"
or
"||".
Any operation that overflows in signed arithmetic produces undefined
behavior. Likewise, using the
shift
operators with a shift amount that is not positive and smaller
than the precision is undefined, as is shifting a negative number to
the right. Historically, not all implementations obeyed C-language
precedence rules:
'~'
and
'!'
were lower than
'==';
'=='
and
'!='
were not lower than
'<';
and
'|'
was not lower than
'ha';
the liberal use of
"()"
can force the desired precedence even with these non-compliant
implementations. Furthermore, some traditional implementations treated
'ha'
as an exponentiation operator, although most implementations now use
"**"
as an extension for this purpose.
When a macro has been multiply defined via the
pushdef
macro, it is unspecified whether the
define
macro will alter only the most recent definition (as though by
popdef
and
pushdef),
or replace the entire stack of definitions with a single definition
(as though by
undefine
and
pushdef).
An application desiring particular behavior for the
define
macro in this case can redefine it accordingly.
Applications should use the
mkstemp
macro instead of the obsolescent
maketemp
macro for creating temporary files.
EXAMPLES
If the file
m4src
contains the lines:
-
The value of `VER' is "VER".
ifdef(`VER', ``VER'' is defined to be VER., VER is not defined.)
ifelse(VER, 1, ``VER'' is `VER'.)
ifelse(VER, 2, ``VER'' is `VER'., ``VER'' is not 2.)
end
then the command
-
m4 m4src
or the command:
-
m4 -U VER m4src
produces the output:
-
The value of VER is "VER".
VER is not defined.
VER is not 2.
end
The command:
-
m4 -D VER m4src
produces the output:
-
The value of VER is "".
VER is defined to be .
VER is not 2.
end
The command:
-
m4 -D VER=1 m4src
produces the output:
-
The value of VER is "1".
VER is defined to be 1.
VER is 1.
VER is not 2.
end
The command:
-
m4 -D VER=2 m4src
produces the output:
-
The value of VER is "2".
VER is defined to be 2.
VER is 2.
end
RATIONALE
Historic System V-based behavior treated
"${"
in a macro definition as two literal characters. However, this sequence
is left unspecified so that implementations may offer extensions
such as
"${11}"
meaning the eleventh positional parameter. Macros can still be defined with
appropriate uses of nested quoting to result in a literal
"${"
in the output after rescanning removes the nested quotes.
In the
translit
built-in, historic System V-based behavior treated
'-'
as a literal; GNU behavior treats it as a range. This version of
the standard allows either behavior.
FUTURE DIRECTIONS
None.
SEE ALSO
c99
The Base Definitions volume of POSIX.1-2017,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .