mp4h - Macro Processor for HTML Documents

This documentation describes mp4h version 1.3.1.

Introduction

The mp4h software is a macro-processor specifically designed to deal with HTML documents. It allows powerful programming constructs, with a syntax familiar to HTML authors.

This software is based on Meta-HTML, written by Brian J. Fox, Even if both syntaxes look similar, source code is completely different. Indeed, a subset of Meta-HTML was used as a part of a more complex program, WML (Website Meta Language) written by Ralf S. Engelschall and which I maintain since January 1999. For licensing reasons, it was hard to hack Meta-HTML and so I decided to write my own macro-processor.

Instead of rewriting it from scratch, I preferred using another macro-processor engine. I chose GNU m4, written by Rene Seindal, because of its numerous advantages : this software is stable, robust and very well documented. This version of mp4h is derived from GNU m4 version 1.4n, which is a development version.

The mp4h software is not an HTML editor; its unique goal is to provide an easy way to define its own macros inside HTML documents. There is no plan to add functionalities to automagically produce valid HTML documents, if you want to clean up your code or validate it, simply use a post-processor like tidy.

Command line options

Optional arguments are enclosed within square brackets. All option synonyms have a similar syntax, so when a long option accepts an argument, short option do too.

Syntax call is

mp4h [options] [filename [filename] ...]

Options are described below. If no filename is specified, or if its name is -, then characters are read on standard input.

Operation modes

Preprocessor features

Parser features

NUMBER is a combination of

In version 1.3.1, default value is 3114=2+8+32+1024+2048.

Limits control

Debugging

Flags are any of:

Description

The mp4h software is a macro-processor, which means that keywords are replaced by other text. This chapter describes all primitives. As mp4h has been specially designed for HTML documents, its syntax is very similar to HTML, with tags and attributes. One important feature has no equivalent in HTML: comments until end of line. All text following three colons is discarded until end of line, like

;;;  This is a comment

Function Macros

Note: All examples in this documentation are processed through mp4h with expansion flags set to zero (see a description of possible expansion flags at the end of document), it is why simple tags contain a trailing slash. But mp4h can output plain HTML files with other expansion flags.

The definition of new tags is the most common task provided by mp4h. As with HTML, macro names are case insensitive, unless -c option is used to change this default behaviour. In this documentation, only lowercase letters are used. There are two kinds of tags: simple and complex. A simple tag has the following form:

<name [attributes] />

<name [attributes]>
body
</name>

Since version 0.9.1, mp4h knows XHTML syntax too, so your input file may conform to HTML or XHTML syntax. In this manual, we adopt the latter, which is why simple tags have a trailing slash in attributes. If you want to produce HTML files with this input file, you may either choose an adequate --expansion flag or use a post-processor like tidy.

When a simple tag is defined by mp4h, it can be parsed even if the trailing slash is omitted, because mp4h knows that this tag is simple. But it is a good practice to always append a trailing slash to simple tags.

  In macro descriptions below, a slash indicates a simple tag, and a V letter that attributes are read verbatim (without expansion) (see the chapter on macro expansion for further details).

This function lets you define your own tags. First argument is the command name. Replacement text is the function body.

<define-tag foo>bar</define-tag>
<foo/>

bar

Even if spaces have usually few incidence on HTML syntax, it is important to note that

<define-tag foo>bar</define-tag>

<define-tag foo>
bar
</define-tag>

• whitespace=delete
  Some spaces are suppressed in replacement text, in particular any leading or trailing spaces, and newlines not enclosed within angle brackets.
• endtag=required
  Define a complex tag

  Source
  <define-tag foo>bar</define-tag> <foo/>
  Output
  bar

  Source
  <define-tag bar endtag=required>;;; body is: %body</define-tag> <bar>Here it is</bar>
  Output
  body is: Here it is

• attributes=verbatim
  By default attributes are expanded before text is replaced. If this attribute is used, attributes are inserted into replacement text without expansion.

  Source
  <define-tag foo>quux</define-tag> <define-tag bar attributes=verbatim endtag=required> Body: %Ubody Attributes: %Uattributes </define-tag> <bar txt="<foo/>">Here we go</bar>
  Output
  Body: Here we go Attributes: txt=<foo/>

This command is similar to the previous one, except that no operation is performed if this command was already defined.

Copy a function. This command is useful to save a macro definition before redefining it.

<define-tag foo>one</define-tag>
<let bar=foo />
<define-tag foo>two</define-tag>
<foo/><bar/>

twoone

Delete a command definition.

<define-tag foo>one</define-tag>
<undef foo />
<foo/>

<foo />

Add text to a predefined macro. This mechanism allows modifications of existing macros without having to worry about its type, whether it is complex or not.

<let foo=add />
<set-hook foo position=before>
Before</set-hook>
<set-hook foo position=after>
After</set-hook>
<foo 1 2 3 4 />

Before10
After

Print current hooks of a macro.

Text inserted with position=before:<get-hook foo position=before />!
Text inserted with position=after:<get-hook foo position=after />!

Text inserted with position=before:
Before!
Text inserted with position=after:
After!

Like %attributes, except that attr=value pairs are printed with double quotes surrounding attribute values, and a leading space is added if some text is printed.

<define-tag foo>;;;
%attributes
<img<attributes-quote %attributes />/>
</define-tag>
<foo id="logo" src="logo.gif" name="Logo" alt="Our logo" />
<foo/>

id=logo src=logo.gif name=Logo alt=Our logo
<img id="logo" src="logo.gif" name="Logo" alt="Our logo"/>


<img/>

Extract from %attributes the attr=value pairs for names matching any of name1, name2....

<define-tag img whitespace=delete>
<img* <attributes-extract name,src,alt %attributes />  />
</define-tag>
<img id="logo" src="logo.gif" name="Logo" alt="Our logo"  />

<img src=logo.gif name=Logo alt=Our logo    />

Remove from %attributes the attr=value pairs for names matching any of name1, name2....

<define-tag img whitespace=delete>
<img* <attributes-quote <attributes-remove name,src,alt %attributes />/> />
</define-tag>
<img id="logo" src="logo.gif" name="Logo" alt="Our logo"  />

<img  id="logo"   />

Note: The two previous functions are special, because unlike all other macros, their expansion do not form a group. This is necessary to parse the resulting list of attributes.

In those two functions, names of attributes may be regular expressions. Main goal of these primitives is to help writing macros accepting any kind of attributes without having to declare them. A canonical example is

<define-tag href whitespace=delete>
<preserve url name />
<set-var <attributes-extract url,name %attributes />/>
<a <attributes-quote <attributes-remove url,name %attributes />/>
   href="<get-var url />"><get-var name /></a>
<restore  url name />
</define-tag>
<href class=web url="http://www.foo.com" name="Welcome"  />

<a  class="web"
   href="http://www.foo.com">Welcome</a>

But we want now to add an image attribute. So we may write

<define-tag href whitespace=delete>
<preserve url name image />
<set-var <attributes-extract url,name,image %attributes />/>
<a <attributes-quote <attributes-remove url,name,image %attributes />/>
   href="<get-var url />">
<if <get-var image />
   <img <attributes-quote <attributes-remove url,name,image %attributes />/>
      src="<get-var image />" alt="<get-var name />" border=0  />
  <get-var name />
/>
</a>
<restore  url name image />
</define-tag>
<href class=web url="http://www.foo.com" name="Welcome" image="foo.png"/>

<a  class="web"
   href="http://www.foo.com"><img  class="web"
      src="foo.png" alt="Welcome" border=0      /></a>

We need a mechanism to tell mp4h that some attributes refer to specific HTML tags. A solution is to prepend attribute with tag name, e.g.

<define-tag href whitespace=delete>
<preserve url name image />
<set-var <attributes-extract url,name,image %attributes />/>
<a <attributes-quote <attributes-extract a:.* %attributes />/>
   href="<get-var url />">
<if <get-var image />
   <img <attributes-quote <attributes-extract img:.* %attributes />/>
      src="<get-var image />" alt="<get-var name />"  />
  <get-var name />
/>
</a>
<restore  url name image />
</define-tag>
<href a:class=web img:id=logo img:border=1
      url="http://www.foo.com" name="Welcome" image="foo.png" />

<a  a:class="web"
   href="http://www.foo.com"><img  img:id="logo" img:border="1"
      src="foo.png" alt="Welcome"      /></a>

This example shows that regular expressions may be used within attributes names, but it is still incomplete, because we want to remove prefix from attributes. One solution is with subst-in-string, but there is a more elegant one:

<define-tag href whitespace=delete>
<preserve url name image />
<set-var <attributes-extract url,name,image %attributes />/>
<a <attributes-quote <attributes-extract :a:(.*) %attributes />/>
   href="<get-var url />">
<if <get-var image />
   <img <attributes-quote <attributes-extract :img:(.*) %attributes />/>
      src="<get-var image />" alt="<get-var name />"  />
  <get-var name />
/>
</a>
<restore  url name image />
</define-tag>
<href :a:class=web :img:id=logo :img:border=1
      url="http://www.foo.com" name="Welcome" image="foo.png" />

<a  class="web"
   href="http://www.foo.com"><img  id="logo" border="1"
      src="foo.png" alt="Welcome"      /></a>

When there are subexpressions within regular expressions, they are printed instead of the whole expression. Note also that I put a colon before the prefix in order not to mix them with XML namespaces.

Entities

Entities are macros in the same way as tags, but they do not take any arguments. Whereas tags are normally used to mark up text, entities contain already marked up text. Also note that unlike tags, entities are by default case sensitive.

An entity has the following form:

&entity;

This function lets you define your own entities. First argument is the entity name. Replacement text is the function body.

<define-entity foo>bar</define-entity>
&foo;

bar

Variables

Variables are a special case of simple tags, because they do not accept attributes. In fact their use is different, because variables contain text whereas macros act like operators. A nice feature concerning variables is their manipulation as arrays. Indeed variables can be considered like newline separated lists, which will allow powerful manipulation functions as we will see below.

This command sets variables.

As above but attributes are read verbatim.

This command assigns a variable the value of the body of the command. This is particularly useful when variable values contain newlines and/or quotes.

Note that the variable can not be indexed with this command. Note also, that this command behaves as set-var-verbatim: The body is not expanded until the variable is shown with get-var.

Show variable contents. If a numeric value within square brackets is appended to a variable name, it represents the index of an array. The first index of arrays is 0 by convention.

<set-var version="0.10.1" />
This is version <get-var version />
<set-var-x name="osversion">Operating system is
"<include command="uname" /><include command="uname -r" />"</set-var-x>
<get-var osversion />

This is version 0.10.1

Operating system is
"Linux
4.15.0-55-generic
"

<set-var foo="0
1
2
3" />
<get-var foo[2] foo[0] foo />

200
1
2
3

As above but attributes are not expanded.

<define-tag foo>0.10.1</define-tag>
<set-var version="<foo/>" />;;;
Here is version <get-var version />
<set-var-verbatim version="<foo/>" />;;;
Here is version <get-var version />
<set-var-verbatim version="<foo/>" />;;;
Here is version <get-var-once version />

Here is version 0.10.1
Here is version 0.10.1
Here is version <foo/>

All variables are global, there is no variable or macro scope. For this reason a stack is used to preserve variables. When this command is invoked, arguments are names of variables, whose values are put at the top of the stack and variables are reset to an empty string.

This is the opposite: arguments are names of variables, which are set to the value found at the top of the stack, and stack is popped down.

Note: The preserve tag pushes its last argument first, whereas restore first pops its first argument.

<define-tag foo whitespace=delete>
<preserve src name text />
<set-var %attributes />
Inside: src=<get-var src /> name=<get-var name /> text=<get-var text />
<restore  src name text />
</define-tag>
<set-var src=foo.png text="Hello, World!" />
Before: src=<get-var src /> name=<get-var name /> text=<get-var text />
<foo src=bar name=quux />
After: src=<get-var src /> name=<get-var name /> text=<get-var text />

Before: src=foo.png name= text=Hello, World!
Inside: src=bar name=quux text=
After: src=foo.png name= text=Hello, World!

Undefine variables.

Returns true when this variable exists.

Increment the variable whose name is the first argument. Default increment is one.

• by=value
  Change increment amount.

<set-var i=10 />
<get-var i />
<increment i /><get-var i />
<increment i by="-3" /><get-var i />

10
11
8

Decrement the variable whose name is the first argument. Default decrement is one.

• by=value
  Change decrement amount.

<set-var i=10 />
<get-var i />
<decrement i /><get-var i />
<decrement i by="3" /><get-var i />

10
9
6

Copy a variable into another.

<set-var i=10 />
<copy-var i j />
<get-var j />

10

If this variable is not defined or is defined to an empty string, then it is set to the second argument.

<unset-var title />
<defvar title "Title" /><get-var title />
<defvar title "New title" /><get-var title />

Title
Title

Show information on symbols. If it is a variable name, the STRING word is printed as well as the number of lines contained within this variable. If it is a macro name, one of the following messages is printed: PRIM COMPLEX, PRIM TAG, USER COMPLEX or USER TAG

<set-var x="0\n1\n2\n3\n4" />
<define-tag foo>bar</define-tag>
<define-tag bar endtag=required>quux</define-tag>
<symbol-info x />
<symbol-info symbol-info />
<symbol-info define-tag />
<symbol-info foo />
<symbol-info bar />

STRING
5
PRIM TAG
PRIM COMPLEX
USER TAG
USER COMPLEX

String Functions

Prints the length of the string.

<set-var foo="0
1
2
3" />;;;
<string-length <get-var foo /> />
<set-var foo="0 1 2 3" />;;;
<set-var l=<string-length <get-var foo /> /> />;;;
<get-var l />

7
7

Convert to lowercase letters.

<downcase "Does it work?" />

does it work?

Convert to uppercase letters.

<upcase "Does it work?" />

DOES IT WORK?

Convert to a title, with a capital letter at the beginning of every word.

<capitalize "Does it work?" />

Does It Work?

Extracts a substring from a string. First argument is original string, second and third are respectively start and end indexes. By convention first character has a null index.

<set-var foo="abcdefghijk" />
<substring <get-var foo /> 4 />
<substring <get-var foo /> 4 6 />

efghijk
ef

Returns true if first two arguments are equal.

1:<string-eq "aAbBcC" "aabbcc" />
2:<string-eq "aAbBcC" "aAbBcC" />

1:
2:true

• caseless=true
  Comparison is case insensitive.

1:<string-eq "aAbBcC" "aabbcc" caseless=true />
2:<string-eq "aAbBcC" "aAbBcC" caseless=true />

1:true
2:true

Returns true if the first two arguments are not equal.

1:<string-neq "aAbBcC" "aabbcc" />
2:<string-neq "aAbBcC" "aAbBcC" />

1:true
2:

• caseless=true
  Comparison is case insensitive.

1:<string-neq "aAbBcC" "aabbcc" caseless=true />
2:<string-neq "aAbBcC" "aAbBcC" caseless=true />

1:
2:

Compares two strings and returns one of the values less, greater or equal depending on this comparison.

1:<string-compare "aAbBcC" "aabbcc" />
2:<string-compare "aAbBcC" "aAbBcC" />

1:less
2:equal

• caseless=true
  Comparison is case insensitive.

1:<string-compare "aAbBcC" "aabbcc" caseless=true />

1:equal

Prints an array containing indexes where the character appear in the string.

• caseless=true
  Comparison is case insensitive.

1:<char-offsets "abcdAbCdaBcD" a />
2:<char-offsets "abcdAbCdaBcD" a caseless=true />

1:0
8
2:0
4
8

Prints according to a given format. Currently only the %s flag character is recognized, and $ extension is supported to change order of arguments.

1:<printf "foo %s bar %s" baz 10 />
2:<printf "foo %2$s bar %1$s" baz 10 />

1:foo baz bar 10
2:foo 10 bar baz

Regular Expressions

Regular expression support is provided by the PCRE (Perl Compatible Regular Expressions) library package, which is open source software, copyright by the University of Cambridge. This is a very nice piece of software, latest versions are available at ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.

Before version 1.0.6, POSIX regular expressions were implemented. For this reason, the following macros recognize two attributes, caseless=true and singleline=true|false. But Perl allows a much better control on regular expressions with so called modifiers, which are assed to the new reflags attribute. It may contain one or more modifiers:

Note: Attribute singleline=true is a synonym for the s modifier, whereas singleline=false is a synonym for the m modifier. This behaviour was different up to mp4h 1.0.6.

Replace a regular expression in a string by a replacement text.

<set-var foo="abcdefghijk" />
<subst-in-string <get-var foo /> "[c-e]" />
<subst-in-string <get-var foo /> "([c-e])" "\\1 " />

abfghijk
abc d e fghijk

<set-var foo="abcdefghijk\nabcdefghijk\nabcdefghijk" />
<subst-in-string <get-var foo /> ".$" "" />
<subst-in-string <get-var foo /> ".$" "" singleline=false />
<subst-in-string <get-var foo /> "
   ([a-c]) | [0-9]
     " ":\\1:" reflags=x />

abcdefghijk
abcdefghijk
abcdefghij
abcdefghij
abcdefghij
abcdefghij
:a::b::c:defghijk
:a::b::c:defghijk
:a::b::c:defghijk

Performs substitutions inside variable content.

• action=report
  Prints true if string contains regexp.
• action=extract
  Prints the expression matching regexp in string.
• action=delete
  Prints the string without the expression matching regexp in string.
• action=startpos
  Prints the first char of the expression matching regexp in string. If there is no match, returns -1.
• action=endpos
  Prints the last char of the expression matching regexp in string. If there is no match, returns -1.
• action=length
  Prints the length of the expression matching regexp in string.

1:<match "abcdefghijk" "[c-e]+" />
2:<match "abcdefghijk" "[c-e]+" action=extract />
3:<match "abcdefghijk" "[c-e]+" action=delete />
4:<match "abcdefghijk" "[c-e]+" action=startpos />
5:<match "abcdefghijk" "[c-e]+" action=endpos />
6:<match "abcdefghijk" "[c-e]+" action=length />

1:true
2:cde
3:abfghijk
4:2
5:5
6:3

Arrays

With mp4h one can easily deal with string arrays. Variables can be treated as a single value or as a newline separated list of strings. Thus after defining

<set-var digits="0
1
2
3" />

<get-var digits />
<get-var digits[2] />

0
1
2
3
2

Returns an array size which is the number of lines present in the variable.

<array-size digits />

4

Add a value (or more if this value contains newlines) at the end of an array.

<array-push digits "10\n11\n12" />
<get-var digits />

0
1
2
3
10
11
12

Remove the toplevel value of an array and returns this string.

Prints the last entry of an array.

<array-topvalue digits />

12

Add a value at the end of an array if this value is not already present in this variable.

<array-add-unique digits 2 />
<get-var digits />

0
1
2
3
10
11
12

• caseless=true
  Comparison is case insensitive.

Concatenates all arrays into the first one.

<set-var foo="foo" />
<set-var bar="bar" />
<array-concat foo bar /><get-var foo />

foo
bar

If value is contained in array, returns its index otherwise returns -1.

<array-member digits 11 />

5

• caseless=true
  Comparison is case insensitive.

Shifts an array. If offset is negative, indexes below 0 are lost. If offset is positive, first indexes are filled with empty strings.

<array-shift digits 2 />
Now: <get-var digits />
<array-shift digits -4 />
And: <get-var digits />

Now: 

0
1
2
3
10
11
12

And: 2
3
10
11
12

• start=start
  Change origin of shifts (default is 0).

  Source
  <array-shift digits -2 start=2 /><get-var digits />
  Output
  2 3 12

Sort lines of an array in place. Default is to sort lines alphabetically.

<sort digits /><get-var digits />

12
2
3

• caseless=true
  Comparison is case insensitive.
• numeric=true
  Sort lines numerically

  Source
  <sort digits numeric=true /><get-var digits />
  Output
  2 3 12

• sortorder=reverse
  Reverse sort order

  Source
  <sort digits numeric=true sortorder=reverse />;;; <get-var digits />
  Output
  12 3 2

Numerical operators

These operators perform basic arithmetic operations. When all operands are integers result is an integer too, otherwise it is a float. These operators are self-explanatory.

<add 1 2 3 4 5 6 />
<add 1 2 3 4 5 6. />

21
21.000000

<define-tag factorial whitespace=delete>
<ifeq %0 1 1 <multiply %0 "<factorial <substract %0 1 /> />" /> />
</define-tag>
<factorial 6 />

720

Unlike functions listed above the modulo function cannot handle more than 2 arguments, and these arguments must be integers.

<modulo 345 7 />

2

Those functions compare two numbers and returns true when this comparison is true. If one argument is not a number, comparison is false.

Returns true if first argument is greater than second.

Returns true if first argument is lower than second.

Returns true if arguments are equal.

Returns true if arguments are not equal.

Relational operators

Returns true if string is empty, otherwise returns an empty string.

Returns the last argument if all arguments are non empty.

Returns the first non empty argument.

Flow functions

This function groups multiple statements into a single one. Some examples will be seen below with conditional operations.

A less intuitive but very helpful use of this macro is to preserve newlines when whitespace=delete is specified.

<define-tag text1>
Text on
3 lines without
whitespace=delete
</define-tag>
<define-tag text2 whitespace=delete>
Text on
3 lines with
whitespace=delete
</define-tag>
<define-tag text3 whitespace=delete>
<group "Text on
3 lines with
whitespace=delete" />
</define-tag>
<text1/>
<text2/>
<text3/>

Text on
3 lines without
whitespace=delete

Text on3 lines withwhitespace=delete
Text on
3 lines with
whitespace=delete

Note that newlines are suppressed in text2 and result is certainly unwanted.

Like group, but this tag is complex.

• separator=string
  By default arguments are put aside. This attribute define a separator inserted between arguments.

Does the opposite job to group, its argument is no more treated as a single object when processed by another command.

Prints its arguments without expansion. They will never be expanded unless the expand tag is used to cancel this noexpand tag.

Cancels the noexpand tag.

<subst-in-string "=LT=define-tag foo>bar=LT=/define-tag>" "=LT=" "<" />
<foo/>
<subst-in-string "=LT=define-tag foo>quux=LT=/define-tag>" "=LT="
   "<noexpand "<" />" />
<foo/>

bar
<define-tag foo>quux</define-tag>
bar

If string is non empty, second argument is evaluated otherwise third argument is evaluated.

<define-tag test whitespace=delete>
<if %0 "yes" "no" />
</define-tag>
<test "string" />
<test "" />

yes
no

If first two arguments are identical strings, third argument is evaluated otherwise fourth argument is evaluated.

If first two arguments are not identical strings, third argument is evaluated otherwise fourth argument is evaluated.

When argument is not empty, its body is evaluated.

While condition is true, body function is evaluated.

<set-var i=10 />
<while <gt <get-var i /> 0 />>;;;
  <get-var i /> <decrement i />;;;
</while>

10 9 8 7 6 5 4 3 2 1 

This macro is similar to the foreach Perl's macro: a variable loops over array values and function body is evaluated for each value. first argument is a generic variable name, and second is the name of an array.

<set-var x="1\n2\n3\n4\n5\n6" />
<foreach i x><get-var i /> </foreach>

1 2 3 4 5 6 

• start=start
  Skips first indexes.

  Source
  <set-var x="1\n2\n3\n4\n5\n6" /> <foreach i x start=3><get-var i /> </foreach>
  Output
  4 5 6

• end=end
  Stops after index has reached that value.

  Source
  <set-var x="1\n2\n3\n4\n5\n6" /> <foreach i x end=3><get-var i /> </foreach>
  Output
  1 2 3

• step=step
  Change index increment (default is 1). If step is negative, array is treated in reverse order.

  Source
  <set-var x="1\n2\n3\n4\n5\n6" /> <foreach i x step=2><get-var i /> </foreach> <foreach i x step=-2><get-var i /> </foreach>
  Output
  1 3 5 6 4 2

This command performs multiple conditions with a single instruction.

<set-var i=0 />
<define-tag test>
<var-case
  x=1   <group <increment i /> x<get-var i /> />
  x=2   <group <decrement i /> x<get-var i /> />
  y=1   <group <increment i /> y<get-var i /> />
  y=2   <group <decrement i /> y<get-var i /> />
/>
</define-tag>
<set-var x=1 y=2 /><test/>
<set-var x=0 y=2 /><test/>

x1y0


y-1

Breaks the innermost while loop.

<set-var i=10 />
<while <gt <get-var i /> 0 />>;;;
  <get-var i /> <decrement i />;;;
  <ifeq <get-var i /> 5 <break/> />;;;
</while>

10 9 8 7 6 

This command immediately exits from the innermost macro. A message may also be inserted. But this macro changes token parsing so its use may become very hazardous in some situations.

• up=number
  This attribute determines how much levels have to be exited. By default only one level is skipped. With a null value, all current macros are exited from. A negative value do the same, and stops processing current file.

Prints a warning on standard error.

Immediately exits program.

• message=string
  Prints a message to the standard error.
• status=rc
  Selects the code returned by the program (-1 by default).

This is a special command: its content is stored and will be expanded after end of input.

File functions

Returns a newline separated list of files contained in a given directory.

<directory-contents . matching=".*\\.mp4h$" />

mp4h.mp4h

Resolves all symbolic links, extra ``/'' characters and references to /./ and /../ in pathname, and expands into the resulting absolute pathname. All but the last component of pathname must exist when real-path is called.

This tag is particularly useful when comparing if file or directory names are identical.

<real-path pathname=<__file__/> />

/work/mp4h-1.3.1/doc/mp4h.mp4h

Returns true if file exists.

Returns an array of information on this file. These information are: size, type, ctime, mtime, atime, owner and group.

<get-file-properties <__file__/> />

68629
FILE
1569279618
1569279618
1569279618
root
root

Insert the contents of a file in the file system - if the file attribute is given -, or the output from executing a system command - if the command attribute is given - into the input stream. For backwards compatibility, if neither the file nor the command attributes are given, the first argument is taken as a file to include.

• file=filename
  

  The given file is read and inserted into the input stream. This attribute cannot be combined with the command attribute.

  Files are first searched in current directory, then in directories specified on command-line with the -I option, next in directories listed in the MP4HLIB environment variable (it used to be MP4HPATH for versions prior to 1.3), and last under the compile-time location (/usr/local/lib/mp4h/1.3.1:/usr/local/share/mp4h by default).

• command=command-line
  

  The given command line is executed on the operating system, and the output of it is inserted in the input stream. This attribute cannot be combined with the file attribute.

  The given command line is executed using the popen(3) standard C library routine. The command is executed using the standard system shell, which on Posix compliant systems is sh(1).

• alt=action
  If file is not found, this alternate action is handled. If this atribute is not set and file is not found, then an error is raised. This attribute has no effect when the command attribute is specified.
• verbatim=true
  File content is included without expansion. This is similar to using the m4 undivert macro with a filename as argument.

<include command="uname -a" />

Linux e2a60a28d4ec 4.15.0-55-generic #60-Ubuntu SMP Tue Jul 2 18:22:20 UTC 2019 x86_64 GNU/Linux

Load definitions from a package file.

This tag does nothing, its body is simply discarded.

Change comment characters.

By default, all characters between <@[ and ]@> pairs are read without parsing. When called without argument, this macro inhibates this feature. When called with two arguments, it redefines begin and end delimiters. Begin delimiter must begin with a left-angle bracket, and end delimiter must end with a right-angle bracket.

• display=visible
  Delimiters are also written into output.

Diversion functions

Diversions are a way of temporarily saving output. The output of mp4h can at any time be diverted to a temporary file, and be reinserted into the output stream, undiverted, again at a later time.

Numbered diversions are counted from 0 upwards, diversion number 0 being the normal output stream. The number of simultaneous diversions is limited mainly by the memory used to describe them, because mp4h tries to keep diversions in memory. However, there is a limit to the overall memory usable by all diversions taken altogether. When this maximum is about to be exceeded, a temporary file is opened to receive the contents of the biggest diversion still in memory, freeing this memory for other diversions. So, it is theoretically possible that the number of diversions be limited by the number of available file descriptors.

Output is diverted using this tag, where diversion-number is the diversion to be used. If the divnum attribute is left out, diversion-number is assumed to be zero. If output is diverted to a non-existent diversion, it is simply discarded. This can be used to suppress unwanted output. See the example below.

When all mp4h input will have been processed, all existing diversions are automatically undiverted, in numerical order.

Several calls of divert with the same argument do not overwrite the previous diverted text, but append to it.

<divert divnum="-1"/>
This is sent nowhere...
<divert/>
This is output.

This is sent nowhere...

This is output.

This tag explicitly undiverts diverted text saved in the diversion with the specified number. If the divnum attribute is not given, all diversions are undiverted, in numerical order.

When diverted text is undiverted, it is not reread by mp4h, but rather copied directly to the current output. It is therefore not an error to undivert into a diversion.

Unlike m4, the mp4h undivert tag does not allow a file name as argument. The same can be accomplished with the include tag with the verbatim="true" attribute.

<divert divnum="1"/>
This text is diverted.
<divert/>
This text is not diverted.
<undivert divnum="1"/>

This text is diverted.

This text is not diverted.

This tag expands to the number of the current diversion.

Initial <divnum/>
<divert divnum="1"/>
Diversion one: <divnum/>
<divert divnum="2"/>
Diversion two: <divnum/>
<divert/>

Initial 0

Diversion one: 1

Diversion two: 2

Debugging functions

When constructs become complex it could be hard to debug them. Functions listed below are very useful when you could not figure what is wrong. These functions are not perfect yet and must be improved in future releases.

Prints the replacement text of a user defined macro. For instance, the macro used to generate all examples of this documentation is

<function-def example />

<set-var-verbatim verb-body=%ubody />
<subst-in-var verb-body "&" "&amp;" />
<subst-in-var verb-body "<" "&lt;" />
<subst-in-var verb-body ">" "&gt;" />
<set-var body=%body />
<subst-in-var body "<(/?[a-z]*)[*]" "<\\1" />
<subst-in-var body "&" "&amp;" />
<subst-in-var body "<" "&lt;" />
<subst-in-var body ">" "&gt;" />
<subst-in-var body "<three-colon/>[^;\n]*\n[ \t]*" "" />
<subst-in-var body "<three-colon/>$" "" reflags=m />
<subst-in-var body "^\n*(.*?)\n*$" "\\1" reflags=s />
<table border=2 cellpadding=0 cellspacing=0 width="80%" summary="">
    <tr><th bgcolor="#ccccff" align=left>&nbsp;&nbsp;&nbsp;Source</th></tr>
    <tr><td bgcolor="#ccff99" width="80%"><dnl/>
      <pre><get-var-once verb-body /></pre><dnl/>
    </td></tr>
    <tr><th bgcolor="#ccccff" align=left>&nbsp;&nbsp;&nbsp;Output</th></tr>
    <tr><td bgcolor="#ff99cc" width="80%"><dnl/>
      <pre><get-var-once body /></pre><dnl/>
    </td></tr>
</table>

This command acts like the -d flag but can be ynamically changed.

Selects a file where debugging messages are diverted. If this filename is empty, debugging messages are sent back to standard error, and if it is set to - these messages are discarded.

Note: There is no way to print these debugging messages into the document being processed.

Declare these macros traced, i.e. information about these macros will be printed if -d flag or debugmode macro are used.

These macros are no more traced.

Miscellaneous

Without argument this macro prints current input filename. With an argument, this macro sets the string returned by future invocation of this macro.

Without argument this macro prints current number line in input file. With an argument, this macro sets the number returned by future invocation of this macro.

This is <__file__/>, line <__line__/>.

This is ./mp4h.mp4h, line 2201.

If you closely look at source code you will see that this number is wrong. Indeed the number line is the end of the entire block containing this instruction.

Prints the version of mp4h.

Discard all characters until newline is reached. This macro ensures that following string is a comment and does not depend of the value of comment characters.

<dnl/>This is a comment
foo
<dnl/>This is a comment
bar

foo
bar

Prints local time according to the epoch passed on argument. If there is no argument, current local time is printed.

• time
  An epoch time specification.
• format
  A format specification as used with the strftime(3) C library routine.

<date/>
<set-var info=<get-file-properties <__file__/> /> />
<date <get-var info[2] /> />
<date time="<get-var info[2] />" format="%Y-%m-%d %H:%M:%S" />

Mon Sep 23 23:01:22 2019

Mon Sep 23 23:00:18 2019
2019-09-23 23:00:18

Prints the time spent since last call to this macro. The printed value is the number of clock ticks, and so is dependent of your CPU.

The number of clock ticks since the beginning of generation of
this documentation by &mp4h; is:
<timer/>

The number of clock ticks since the beginning of generation of
this documentation by <b>mp4h</b> is:
user 6
sys 0

Set locale-specific variables. By default, the portable "C" locale is selected. As locales have different names on different platforms, you must refer to your system documentation to find which values are adapted to your system.

Change the output format of floats by setting the number of digits after the decimal point. Default is to print numbers in the "%6.f" format.

<add 1.2 3.4 />
<mp4h-output-radix 2 />
<add 1.2 3.4 />

4.600000

4.60

External packages

It is possible to include external files with the include command. Files are first searched in current directory, then in directories specified on command-line with the -I option, next in directories listed in the MP4HLIB environment variable (it used to be MP4HPATH for versions prior to 1.3), and last under the compile-time location (/usr/local/lib/mp4h/1.3.1:/usr/local/share/mp4h by default).

Another way to include packages is with the use command. There are two differences between use and include: first, package name has no suffix; and more important, a package cannot be loaded more than once.

Macro expansion

This part describes internal mechanism of macro expansion. It must be as precise and exhaustive as possible so contact me if you have any suggestion.

Basics

Let us begin with some examples:

<define-tag foo>
This is a simple tag
</define-tag>
<define-tag bar endtag=required>
This is a complex tag
</define-tag>
<foo/>
<bar>Body function</bar>

This is a simple tag


This is a complex tag

User defined macros may have attributes like HTML tags. To handle these attributes in replacement text, following conventions have been adopted (mostly derived from Meta-HTML):

• Sequence %name is replaced by the command name.
• Attributes are numbered from 0. In replacement text, %0 is replaced by first argument, %1 by the 2nd, etc. As there is no limitation on the number of arguments, %20 is the 21st argument and not the third followed by the 0 letter.

  Source
  <define-tag href> <a href="%0">%1</a> </define-tag> <href http://www.gimp.org "The Gimp" />
  Output
  <a href="http://www.gimp.org">The Gimp</a>

• Sequence %# prints number of attributes.
• Sequence %% is replaced by %, which is useful in nested definitions.

  Source
  <define-tag outer>;;; outer, # attributes: %# <define-tag inner1>;;; inner1, # attributes: %#;;; </define-tag>;;; <define-tag inner2>;;; inner2, # attributes: %%#;;; </define-tag>;;; <inner1 %attributes and some others /> <inner2 %attributes and some others /> </define-tag> <outer list attributes />
  Output
  outer, # attributes: 2 inner1, # attributes: 2 inner2, # attributes: 5

• Sequence %attributes is replaced by the space separated list of attributes.

  Source
  <define-tag mail1> <set-var %attributes /> <get-var name /> <get-var mail /> </define-tag> <set-var name="" mail="" /> <mail1 name="Dr. Foo" mail="hello@foo.com" />
  Output
  Dr. Foo hello@foo.com

• Sequence %body is replaced by the body of a complex macro.

  Source
  <define-tag mail2 endtag=required whitespace=delete> <set-var %attributes /> <a href="mailto:<get-var mail />">%body</a> </define-tag> <mail2 mail="hello@foo.com"> <img src="photo.png" alt="Dr. Foo" border=0 /> </mail2>
  Output
  <a href="mailto:hello@foo.com"> <img src="photo.png" alt="Dr. Foo" border=0 /> </a>

• The two forms above accept modifiers. When %Aattributes or %Abody is used, a newline separated list of attributes is printed.

  Source
  <define-tag show-attributes whitespace=delete> <set-var list="%Aattributes" i=0 /> <foreach attr list> <group "%<get-var i />: <get-var attr />" /> <increment i /> </foreach> </define-tag> <show-attributes name="Dr. Foo" mail="hello@foo.com" />
  Output
  %0: name=Dr. Foo%1: mail=hello@foo.com

• Another alternate form is obtained by replacing A by U, in which case text is replaced but will not be expanded. This does make sense only when macro has been defined with attributes=verbatim, otherwise attributes are expanded before replacement.

  Source
  <define-tag show1> Before expansion: %Uattributes After expansion: %attributes </define-tag> <define-tag show2 attributes=verbatim> Before expansion: %Uattributes After expansion: %attributes </define-tag> <define-tag bar>and here %attributes</define-tag> <show1 <bar we go /> /> <show2 <bar we go /> />
  Output
  Before expansion: and here we go After expansion: and here we go Before expansion: <bar we go /> After expansion: and here we go

• Modifiers A and U can be combined.

Note: Input expansion is completely different in Meta-HTML and in mp4h. With Meta-HTML it is sometimes necessary to use other constructs like %xbody and %qbody. In order to improve compatibity with Meta-HTML, these constructs are recognized and are interpreted like %body. Another feature provided for compatibility reason is the fact that for simple tags %body and %attributes are equivalent. These features are in the current mp4h version but may disappear in future releases.

Attributes

Attributes are separated by spaces, tabulations or newlines, and each attribute must be a valid mp4h entity. For instance with the definitions above, <bar> can not be an attribute since it must be finished by </bar>. But this is valid:

<foo <foo/> />

<foo <foo name=src url=ici /> />

In these examples, the foo tag has only one argument.

Under certain circumstances it is necessary to group multiple statements into a single one. This can be done with double quotes or with the group primitive, e.g.

<foo "This is the 1st attribute"
           <group and the second /> />

Note: Unlike HTML single quotes can not replace doube quotes for this purpose.

If double quotes appear in an argument, they must be escaped by a backslash \.

  <set-var text="Text with double quotes \" inside" />
  <get-var text />

  
  Text with double quotes " inside

Macro evaluation

Macros are characterized by

• name
• container status (simple or complex)
• if attributes are expanded or not
• function type (primitive or user defined macro)
• for primitives, address of corresponding code in memory and for user defined macros the replacement text

Characters are read on input until a left angle bracket is found. Then macro name is read. After that attributes are read, verbatim or not depending on how this macro as been defined. And if this macro is complex, its body is read verbatim. When this is finished, some special sequences in replacement text are replaced (like %body, %attributes, %0, %1, etc.) and resulting text is put on input stack in order to be rescanned.

Note: By default attributes are evaluated before any replacement.

Consider the following example, to change text in typewriter font:

<define-tag text-tt endtag=required whitespace=delete>
<tt>%body</tt>
</define-tag>

This definition has a major drawback:

<text-tt>This is an <text-tt>example</text-tt></text-tt>

<tt>This is an <tt>example</tt></tt>

First idea is to use an auxiliary variable to know whether we still are inside such an environment:

<set-var _text:tt=0 />
<define-tag text-tt endtag=required whitespace=delete>
<increment _text:tt />
<ifeq <get-var _text:tt /> 1 "<tt*>" />
%body
<ifeq <get-var _text:tt /> 1 "</tt*>" />
<decrement _text:tt />
</define-tag>

(the presence of asterisks in HTML tags is explained in next section).

<text-tt>This is an <text-tt>example</text-tt></text-tt>

<tt>This is an example</tt>

But if we use simple tags, as in the example below, our definition does not seem to work. It is because attributes are expanded before they are put into replacement text.

<define-tag opt><text-tt>%attributes</text-tt></define-tag>
<opt "This is an <opt example />" />

<tt>This is an <tt>example</tt></tt>

If we want to prevent this problem we have to forbid attributes expansion with

<define-tag opt attributes=verbatim>;;;
<text-tt>%attributes</text-tt>;;;
</define-tag>
<opt "This is an <opt example />" />

<tt>This is an example</tt>

Expansion flags

When you want to embed some server-side scripting language in your pages, you face up some weird problems, like in

<a href=<%= $url %>>Hello</a>

The question is how do mp4h know that this input has some extra delimiters? The answer is that mp4h should not try to handle some special delimiters, because it cannot handle all of them (there are ASP, ePerl, PHP,... and some of them are customizable). Now, remember that mp4h is a macro-processor, not an XML parser. So we must focus on macros,and format our input file so that it can be parsed without any problem. Previous example may be written

<a href="<%= $url %>">Hello</a>

because quotes prevent inner right-angle bracket from closing the a tag.

Another common problem is when we need to print only a begin or an end tag alone. For instance it is very desirable to define its own headers and footers with

<define-tag header>
<html*>
<head>
... put here some information ....
</head>
<body* bgcolor="#ffffff" text="#000000">
</define-tag>
<define-tag footer>
</body*>
</html*>
</define-tag>

Asterisks mark these tags as pseudo-simple tags, which means that they are complex HTML tags, but used as simple tags within mp4h because tags would not be well nested otherwise.

This asterisk is called ``trailing star'', it appears at the end of the tag name.

Sometimes HTML tags are not parsable, as in this javascript code:

  ...
    document.write('<*img src="foo.gif"');
    if (text) document.write(' alt="'+text+'"');
    document.write('>');
  ...

The ``leading star'' is an asterisk between left-angle bracket and tag name, which prevents this tag from being parsed.

That said we can now understand what the --expansion flag is for. It controls how expansion is performed by mp4h. It is followed by an integer, which is a bit sum of the following values

Run

  mp4h -h

Author

Denis Barbier

Mp4h has its own homepage.

Thanks

Sincere thanks to Brian J. Fox for writing Meta-HTML and Rene Seindal for maintaining this wonderful macro parser called GNU m4.