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13.10 m1 - Simple Macro Processor

Contributed by Jon Bentley

The m1 program is a "little brother" to the m4 macro processor found on UNIX systems. It was originally published in the article m1: A Mini Macro Processor , in Computer Language , June 1990, Volume 7, Number 6, pages 47-61. This program was brought to my attention by Ozan Yigit. Jon Bentley kindly sent me his current version of the program, as well as an early draft of his article (I was having trouble getting a copy of the published one). A PostScript version of this paper is included with the example programs, available from O'Reilly's FTP server (see the Preface). I wrote these introductory notes, and the program notes below. [A.R.]

A macro processor copies its input to its output, while performing several jobs. The tasks are:

  1. Define and expand macros. Macros have two parts, a name and a body. All occurrences of a macro's name are replaced with the macro's body.

  2. Include files. Special include directives in a data file are replaced with the contents of the named file. Includes can usually be nested, with one included file including another. Included files are processed for macros.

  3. Conditional text inclusion and exclusion. Different parts of the text can be included in the final output, often based upon whether a macro is or isn't defined.

  4. Depending on the macro processor, comment lines can appear that will be removed from the final output.

If you're a C or C++ programmer, you're already familiar with the built-in preprocessor in those languages. UNIX systems have a general-purpose macro processor called m4 . This is a powerful program, but somewhat difficult to master, since macro definitions are processed for expansion at definition time, instead of at expansion time. m1 is considerably simpler than m4 , making it much easier to learn and to use.

Here is Jon's first cut at a very simple macro processor. All it does is define and expand macros. We can call it m0a . In this and the following programs, the "at" symbol (@) distinguishes lines that are directives, and also indicates the presence of macros that should be expanded.

/^@define[ \t]/ {
	name = $2
	$1 = $2 = ""; sub(/^[ \t]+/, "")
	symtab[name] = $0
	next
}
{
	for (i in symtab)
		gsub("@" i "@", symtab[i])
	print
}

This version looks for lines beginning with "@define." This keyword is $1 and the macro name is taken to be $2. The rest of the line becomes the body of the macro. The next input line is then fetched using next . The second rule simply loops through all the defined macros, performing a global substitution of each macro with its body in the input line, and then printing the line. Think about the tradeoffs in this version of simplicity versus program execution time.

The next version ( m0b ) adds file inclusion:

function dofile(fname) {
	while (getline <fname > 0) {
		if (/^@define[ \t]/) {		# @define name value
			name = $2
			$1 = $2 = ""; sub(/^[ \t]+/, "")
			symtab[name] = $0
		} else if (/^@include[ \t]/)	# @include filename
			dofile($2)
		else {				# Anywhere in line @name@
			for (i in symtab)
				gsub("@" i "@", symtab[i])
			print
		}
	}
	close(fname)
}
BEGIN {
	if (ARGC == 2)
		dofile(ARGV[1])
	else
		dofile("/dev/stdin")
}

Note the way dofile() is called recursively to handle nested include files.

With all of that introduction out of the way, here is the full-blown m1 program.

#! /bin/awk -f
# NAME
#
# m1
#
# USAGE
#
# awk -f m1.awk [file...]
#
# DESCRIPTION
#
# M1 copies its input file(s) to its output unchanged except as modified by
# certain "macro expressions."  The following lines define macros for
# subsequent processing:
#
#     @comment Any text
#     @@                     same as @comment
#     @define name value
#     @default name value    set if name undefined
#     @include filename
#     @if varname            include subsequent text if varname != 0
#     @unless varname        include subsequent text if varname == 0
#     @fi                    terminate @if or @unless
#     @ignore DELIM          ignore input until line that begins with DELIM
#     @stderr stuff          send diagnostics to standard error
#
# A definition may extend across many lines by ending each line with
# a backslash, thus quoting the following newline.
#
# Any occurrence of @name@ in the input is replaced in the output by
# the corresponding value.
#
# @name at beginning of line is treated the same as @name@.
#
# BUGS
#
# M1 is three steps lower than m4.  You'll probably miss something
# you have learned to expect.
#
# AUTHOR
#
# Jon L. Bentley, jlb@research.bell-labs.com
#

function error(s) {
	print "m1 error: " s | "cat 1>&2"; exit 1
}

function dofile(fname,  savefile, savebuffer, newstring) {
	if (fname in activefiles)
		error("recursively reading file: " fname)
	activefiles[fname] = 1
	savefile = file; file = fname
	savebuffer = buffer; buffer = ""
	while (readline() != EOF) {
		if (index($0, "@") == 0) {
			print $0
		} else if (/^@define[ \t]/) {
			dodef()
		} else if (/^@default[ \t]/) {
			if (!($2 in symtab))
				dodef()
		} else if (/^@include[ \t]/) {
			if (NF != 2) error("bad include line")
			dofile(dosubs($2))
		} else if (/^@if[ \t]/) {
			if (NF != 2) error("bad if line")
			if (!($2 in symtab) || symtab[$2] == 0)
				gobble()
		} else if (/^@unless[ \t]/) {
			if (NF != 2) error("bad unless line")
			if (($2 in symtab) && symtab[$2] != 0)
				gobble()
		} else if (/^@fi([ \t]?|$)/) { # Could do error checking here
		} else if (/^@stderr[ \t]?/) { 
			print substr($0, 9) | "cat 1>&2"
		} else if (/^@(comment|@)[ \t]?/) {
		} else if (/^@ignore[ \t]/) { # Dump input until $2
			delim = $2
			l = length(delim)
			while (readline() != EOF)
				if (substr($0, 1, l) == delim)
					break
		} else {
			newstring = dosubs($0)
			if ($0 == newstring || index(newstring, "@") == 0)
				print newstring
			else
				buffer = newstring "\n" buffer
		}
	}
	close(fname)
	delete activefiles[fname]
	file = savefile
	buffer = savebuffer
}

# Put next input line into global string "buffer"
# Return "EOF" or "" (null string)

function readline(  i, status) {
	status = ""
	if (buffer != "") {
		i = index(buffer, "\n")
		$0 = substr(buffer, 1, i-1)
		buffer = substr(buffer, i+1)
	} else {
		# Hume: special case for non v10: if (file == "/dev/stdin")
		if (getline <file <= 0)
			status = EOF
	}
	# Hack: allow @Mname at start of line w/o closing @
	if ($0 ~ /^@[A-Z][a-zA-Z0-9]*[ \t]*$/)
		sub(/[ \t]*$/, "@")
	return status
}

function gobble(  ifdepth) {
	ifdepth = 1
	while (readline() != EOF) {
		if (/^@(if|unless)[ \t]/)
			ifdepth++
		if (/^@fi[ \t]?/ && --ifdepth <= 0)
			break
	}
}

function dosubs(s,  l, r, i, m) {
	if (index(s, "@") == 0)
		return s
	l = ""	# Left of current pos; ready for output
	r = s	# Right of current; unexamined at this time
	while ((i = index(r, "@")) != 0) {
		l = l substr(r, 1, i-1)
		r = substr(r, i+1)	# Currently scanning @
		i = index(r, "@")
		if (i == 0) {
			l = l "@"
			break
		}
		m = substr(r, 1, i-1)
		r = substr(r, i+1)
		if (m in symtab) {
			r = symtab[m] r
		} else {
			l = l "@" m
			r = "@" r
		}
	}
	return l r
}

function dodef(fname,  str, x) {
	name = $2
	sub(/^[ \t]*[^ \t]+[ \t]+[^ \t]+[ \t]*/, "")  # OLD BUG: last * was +
	str = $0
	while (str ~ /\\$/) {
		if (readline() == EOF)
			error("EOF inside definition")
		x = $0
		sub(/^[ \t]+/, "", x)
		str = substr(str, 1, length(str)-1) "\n" x
	}
	symtab[name] = str
}

BEGIN {	EOF = "EOF"
	if (ARGC == 1)
		dofile("/dev/stdin")
	else if (ARGC >= 2) {
		for (i = 1; i < ARGC; i++)
			dofile(ARGV[i])
	} else
		error("usage: m1 [fname...]")
}

13.10.1 Program Notes for m1

The program is nicely modular, with an error() function similar to the one presented in Chapter 11, A Flock of awks , and each task cleanly divided into separate functions.

The main program occurs in the BEGIN procedure at the bottom. It simply processes either standard input, if there are no arguments, or all of the files named on the command line.

The high-level processing happens in the dofile() function, which reads one line at a time, and decides what to do with each line. The activefiles array keeps track of open files. The variable fname indicates the current file to read data from. When an "@include" directive is seen, dofile() simply calls itself recursively on the new file, as in m0b . Interestingly, the included filename is first processed for macros. Read this function carefully - there are some nice tricks here.

The readline() function manages the "pushback." After expanding a macro, macro processors examine the newly created text for any additional macro names. Only after all expanded text has been processed and sent to the output does the program get a fresh line of input.

The dosubs() function actually performs the macro substitution. It processes the line left-to-right, replacing macro names with their bodies. The rescanning of the new line is left to the higher-level logic that is jointly managed by readline() and dofile() . This version is considerably more efficient than the brute-force approach used in the m0 programs.

Finally, the dodef() function handles the defining of macros. It saves the macro name from $2, and then uses sub() to remove the first two fields. The new value of $0 now contains just (the first line of) the macro body. The Computer Language article explains that sub() is used on purpose, in order to preserve whitespace in the macro body. Simply assigning the empty string to $1 and $2 would rebuild the record, but with all occurrences of whitespace collapsed into single occurrences of the value of OFS (a single blank). The function then proceeds to gather the rest of the macro body, indicated by lines that end with a "\". This is an additional improvement over m0 : macro bodies can be more than one line long.

The rest of the program is concerned with conditional inclusion or exclusion of text; this part is straightforward. What's nice is that these conditionals can be nested inside each other.

m1 is a very nice start at a macro processor. You might want to think about how you could expand upon it; for instance, by allowing conditionals to have an "@else" clause; processing the command line for macro definitions; "undefining" macros, and the other sorts of things that macro processors usually do.

Some other extensions suggested by Jon Bentley are:

  1. Add "@shell DELIM shell line here," which would read input lines up to "DELIM," and send the expanded output through a pipe to the given shell command.

  2. Add commands "@longdef" and "@longend." These commands would define macros with long bodies, i.e., those that extend over more than one line, simplifying the logic in dodoef() .

  3. Add "@append MacName MoreText," like ".am" in troff . This macro in troff appends text to an already defined macro. In m1 , this would allow you to add on to the body of an already defined macro.

  4. Avoid the V10 /dev/stdin special file. The Bell Labs UNIX systems[1] have a special file actually named /dev/stdin , that gives you access to standard input. It occurs to me that the use of "-" would do the trick, quite portably. This is also not a real issue if you use gawk or the Bell Labs awk, which interpret the special file name /dev/stdin internally (see Chapter 11).

    [1] And some other UNIX systems, as well.

As a final note, Jon often makes use of awk in two of his books, Programming Pearls , and More Programming Pearls - Confessions of a Coder (both published by Addison-Wesley). These books are both excellent reading.


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