\input cwebmac \parskip 0pt plus 1pt \def\RA{\char'31 } % right arrow \def\hang{\hangindent 4em\ignorespaces} \font\eightrm=cmr8 \font\ninerm=cmr9 \font\ninett=cmtt9 \font\eighttt=cmtt8 \def\Pascal{Pascal} \font\quoterm=cmssq8 \font\quoteit=cmssqi8 \def\pb{\.{|...|}} \def\v{\.{\char'174}} % vertical (|) in typewriter font \def\lpile{\def\cr{\hfill\endline}\matrix} % I only use \lpile by itself \abovedisplayskip=.5\abovedisplayskip \belowdisplayskip=.5\belowdisplayskip \abovedisplayshortskip=.5\abovedisplayshortskip \belowdisplayshortskip=.5\belowdisplayshortskip \advance\pageheight by \baselineskip % the manual just got a bit longer \advance\fullpageheight by \baselineskip \setpage \outer\def\section #1.{\penalty-50\vskip 6pt plus 3pt minus 3pt \noindent{\bf #1.}\quad\ignorespaces} \def\lheader{\mainfont\the\pageno\hfill\sc\runninghead\hfill} \def\rheader{\hfill\sc\runninghead\hfill\mainfont\the\pageno} \def\runninghead{{\tentt CWEB} USER MANUAL (VERSION 2.8)} % This verbatim mode assumes that ! marks are !! in the text being copied. \def\verbatim{\begingroup \def\do##1{\catcode`##1=12 } \dospecials \parskip 0pt \parindent 0pt \let\!=! \catcode`\ =13 \catcode`\^^M=13 \tt \catcode`\!=0 \verbatimdefs \verbatimgobble} {\catcode`\^^M=13{\catcode`\ =13\gdef\verbatimdefs{\def^^M{\ \par}\let =\ }} % \gdef\verbatimgobble#1^^M{}} \null\vfill \noindent Copyright \copyright\ 1987, 1990, 1992 Silvio Levy and Donald E. Knuth \bigskip\noindent Permission is granted to make and distribute verbatim copies of this document provided that the copyright notice and this permission notice are preserved on all copies. \smallskip\noindent Permission is granted to copy and distribute modified versions of this document under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. \pageno=0 \titletrue\eject \centerline{\titlefont The {\ttitlefont CWEB} System of Structured Documentation} \vskip 15pt plus 3pt minus 3pt \noindent This document describes a version of Don Knuth's \.{WEB} system, adapted to \Cee\ by Silvio Levy. Knuth's original Pascal programs have been entirely rewritten in \Cee; many changes were made to take advantage of features offered by \Cee\ but non-existent in \Pascal. Readers who are familiar with Knuth's memo ``The \.{WEB} System of Structured Documentation'' will be able to skim this material rapidly, because \.{CWEB} is essentially a simplified subset of \.{WEB}. \.{CWEB} does not need \.{WEB}'s features for macro definition and string handling, because \Cee\ and its preprocessor already take care of macros and strings. Similarly, the \.{WEB} conventions of denoting octal and hexadecimal constants by \.{@'77} and \.{@"3f} are replaced by \Cee's conventions \.{\v077\v} and \.{\v0x3f\v}, respectively. All other features of \.{WEB} have been retained, and a few new features have been added. \section Introduction. The philosophy behind \.{WEB} is that an experienced system programmer, who wants to provide the best possible documentation of software products, needs two things simultaneously: a language like \TeX\ for formatting, and a language like \Cee\ for programming. Neither type of language can provide the best documentation by itself. But when both are appropriately combined, we obtain a system that is much more useful than either language separately. The structure of a software program may be thought of as a ``web'' that is made up of many interconnected pieces. To document such a program, we want to explain each individual part of the web and how it relates to its neighbors. The typographic tools provided by \TeX\ give us an opportunity to explain the local structure of each part by making that structure visible, and the programming tools provided by \Cee\ make it possible for us to specify the algorithms formally and unambiguously. By combining the two, we can develop a style of programming that maximizes our ability to perceive the structure of a complex piece of software, and at the same time the documented programs can be mechanically translated into a working software system that matches the documentation. The \.{WEB} system consists of two programs named \.{WEAVE} and \.{TANGLE}. More precisely, \.{CWEB} calls them \.{CWEAVE} and \.{CTANGLE}. When writing a \.{CWEB} program the user keeps the \Cee\ code and the documentation in the same file, called the \.{WEB} file and generally named \.{something.w}. The command `\.{cweave something}' creates an output file \.{something.tex}, which can then be fed to \TeX, yielding a ``pretty printed'' version of \.{something.w} that correctly handles typographic details like page layout and the use of indentation, italics, boldface, and mathematical symbols. The typeset output also includes extensive cross-index information that is gathered automatically. Similarly, if you run the command `\.{ctangle something}' you will get a \Cee\ file \.{something.c}, with can then be compiled to yield executable code. Besides providing a documentation tool, \.{WEB} enhances the \Cee\ language by providing the ability to permute pieces of the program text, so that a large system can be understood entirely in terms of small sections and their local interrelationships. The \.{TANGLE} program is so named because it takes a given web and moves the sections from their web structure into the order required by \Cee; the advantage of programming in \.{WEB} is that the algorithms can be expressed in ``untangled'' form, with each section explained separately. The \.{WEAVE} program is so named because it takes a given web and intertwines the \TeX\ and \Cee\ portions contained in each section, then it knits the whole fabric into a structured document. (Get it? Wow.) Perhaps there is some deep connection here with the fact that the German word for ``weave'' is ``{\it web\/}'', and the corresponding Latin imperative is ``{\it texe\/}''! A user of \.{CWEB} should be fairly familiar with the \Cee\ programming language. A minimal amount of acquaintance with \TeX\ is also desirable, but in fact it can be acquired as one uses \.{WEB}, since straight text can be typeset in \TeX\ with virtually no knowledge of that language. To someone familiar with both \Cee\ and \TeX\ the amount of effort necessary to learn the commands of \.{WEB} is small. \section The language. Two kinds of material go into \.{WEB} files: \TeX\ text and \Cee\ text. A programmer writing in \.{WEB} should be thinking both of the documentation and of the \Cee\ program being created; i.e., the programmer should be instinctively aware of the different actions that \.{WEAVE} and \.{TANGLE} will perform on the \.{WEB} file. \TeX\ text is essentially copied without change by \.{WEAVE}, and it is entirely deleted by \.{TANGLE}; the \TeX\ text is ``pure documentation.'' \Cee\ text, on the other hand, is formatted by \.{WEAVE} and it is shuffled around by \.{TANGLE}, according to rules that will become clear later. For now the important point to keep in mind is that there are two kinds of text. Writing \.{WEB} programs is something like writing \TeX\ documents, but with an additional ``\Cee\ mode'' that is added to \TeX's horizontal mode, vertical mode, and math mode. A \.{WEB} file is built up from units called {\sl sections\/} that are more or less self-contained. Each section has three parts: \yskip\item{1)} A \TeX\ part, containing explanatory material about what is going on in the section. \item{2)} A middle part, containing macro definitions that serve as abbreviations for \Cee\ constructions that would be less comprehensible if written out in full each time. They are turned by \.{TANGLE} into preprocessor macro definitions. \item{3)} A \Cee\ part, containing a piece of the program that \.{TANGLE} will produce. This \Cee\ code should ideally be about a dozen lines long, so that it is easily comprehensible as a unit and so that its structure is readily perceived. \yskip\noindent The three parts of each section must appear in this order; i.e., the \TeX\ commentary must come first, then the middle part, and finally the \Cee\ code. Any of the parts may be empty. A section begins with either of the symbols `\.{@\ }' or `\.{@*}', where `\.{\ }' denotes a blank space. A section ends at the beginning of the next section (i.e., at the next `\.{@\ }' or `\.{@*}'), or at the end of the file, whichever comes first. The \.{WEB} file may also contain material that is not part of any section at all, namely the text (if any) that occurs before the first section. Such text is said to be ``in limbo''; it is ignored by \.{TANGLE} and copied essentially verbatim by \.{WEAVE}, so its function is to provide any additional formatting instructions that may be desired in the \TeX\ output. Indeed, it is customary to begin a \.{WEB} file with \TeX\ code in limbo that loads special fonts, defines special macros, changes the page sizes, and/or produces a title page. Sections are numbered consecutively, starting with 1. These numbers appear at the beginning of each section of the \TeX\ documentation output by \.{WEAVE}, and they appear as bracketed comments at the beginning and end of the code generated by that section in the \Cee\ program output by \.{TANGLE}. Fortunately, you never mention these numbers yourself when you are writing in \.{WEB}. You just say `\.{@\ }' or `\.{@*}' at the beginning of each new section, and the numbers are supplied automatically by \.{WEAVE} and \.{TANGLE}. As far as you are concerned, a section has a {\sl name\/} instead of a number; its name is specified by writing `\.{@<}' followed by \TeX\ text followed by `\.{@>}'. When \.{WEAVE} outputs a section name, it replaces the `\.{@<}' and `\.{@>}' by angle brackets and inserts the section number in small type. Thus, when you read the output of \.{WEAVE} it is easy to locate any section that is referred to in another section. For expository purposes, a section name should be a good description of the contents of that section; i.e., it should stand for the abstraction represented by the section. Then the section can be ``plugged into'' one or more other sections in such a way that unimportant details of its inner workings are suppressed. A section name therefore ought to be long enough to convey the necessary meaning. Unfortunately, it is laborious to type such long names over and over again, and it is also difficult to specify a long name twice in exactly the same way so that \.{WEAVE} and \.{TANGLE} will be able to match the names to the sections. To ameliorate this difficulty, \.{WEAVE} and \.{TANGLE} let you abbreviate a section name, so long as the full name appears somewhere in the \.{WEB} file; you can type simply `\.{@<$\alpha$...@>}', where $\alpha$ is any string that is a prefix of exactly one section name appearing in the file. For example, `\.{@}' can be abbreviated to `\.{@}' if no other section name begins with the five letters `\.{Clear}'. Elsewhere you might use the abbreviation `\.{@}', and so on. Section names must otherwise match character for character, except that consecutive characters of white space (spaces, tab marks, newlines, and/or form feeds) are treated as equivalent to a single space, and such spaces are deleted at the beginning and end of the name. Thus, `\.{@< Clear { }the arrays @>}' will also match the name in the previous example. Spaces following the ellipsis in abbeviations are ignored as well, but not those before, so that `\.{@}' would not match `\.{@}'. We have said that a section begins with `\.{@\ }' or `\.{@*}', but we didn't say how it gets divided up into a \TeX\ part, a middle part, and a \Cee\ part. The middle part begins with the first appearance of `\.{@d}' or `\.{@f}' in the section, and the \Cee\ part begins with the first appearance of `\.{@c}' or `\.{@
=}'. In the latter case you are saying, in effect, that the section name stands for the \Cee\ text that follows. Alternatively, if the \Cee\ part begins with `\.{@c}' instead of a section name, the current section is said to be {\sl unnamed}. The construct `\.{@
}' can appear any number of times in the \Cee\ part of a section: subsequent appearences indicate that the named section is being ``used'' rather than ``defined'', that is, that the \Cee\ code for the named section, presumably defined elsewhere, should be spliced in at this point in the \Cee\ program. Indeed, the main idea of \.{TANGLE} is to make a \Cee\ program out of individual sections, named and unnamed. The exact way in which this is done is the following: First all the macro definitions indicated by `\.{@d}' are turned into \Cee\ preprocessor macro definitions and copied at the beginning. Then the \Cee\ parts of unnamed sections are copied down, in order; this constitutes the initial approximation $T_0$ to the text of the program. (There should be at least one unnamed section, otherwise there will be no program.) Then all section names that appear in the initial text $T_0$ are replaced by the \Cee\ parts of the corresponding sections, and this substitution process continues until no section names remain. All comments are removed, because the \Cee\ program is intended only for the eyes of the \Cee\ compiler. If the same name has been given to more than one section, the \Cee\ text for that name is obtained by putting together all of the \Cee\ parts in the corresponding sections. This feature is useful, for example, in a section named `Global variables', since one can then declare global variables in whatever sections those variables are introduced. When several sections have the same name, \.{WEAVE} assigns the first section number as the number corresponding to that name, and it inserts a note at the bottom of that section telling the reader to `See also sections so-and-so'; this footnote gives the numbers of all the other sections having the same name as the present one. The \Cee\ text corresponding to a section is usually formatted by \.{WEAVE} so that the output has an equivalence sign in place of the equals sign in the \.{WEB} file; i.e., the output says `$\langle\,$section name$\,\rangle\equiv\null$\Cee\ text'. However, in the case of the second and subsequent appearances of a section with the same name, this `$\equiv$' sign is replaced by `$\mathrel+\equiv$', as an indication that the following \Cee\ text is being appended to the \Cee\ text of another section. As \.{TANGLE} enters and leaves sections, it inserts preprocessor \.{\#line} commands into the \Cee\ output file. This means that when the compiler gives you error messages, or when you debug your program, the messages refer to line numbers in the \.{WEB} file, and not in the \Cee\ file. In most cases you can therefore forget about the \Cee\ file altogether. The general idea of \.{WEAVE} is to make a \.{.tex} file from the \.{WEB} file in the following way: The first line of the \.{.tex} file tells \TeX\ to input a file with macros that define \.{CWEB}'s documentation conventions. The next lines of the file will be copied from whatever \TeX\ text is in limbo before the first section. Then comes the output for each section in turn, possibly interspersed with end-of-page marks. Finally, \.{WEAVE} will generate a cross-reference index that lists each section number in which each \Cee\ identifier appears, and it will also generate an alphabetized list of the section names, as well as a table of contents that shows the page and section numbers for each ``starred'' section. What is a ``starred'' section, you ask? A section that begins with `\.{@*}' instead of `\.{@\ }' is slightly special in that it denotes a new major group of sections. The `\.{@*}' should be followed by the title of this group, followed by a period. Such sections will always start on a new page in the \TeX\ output, and the group title will appear as a running headline on all subsequent pages until the next starred section. The title will also appear in the table of contents, and in boldface type at the beginning of its section. Caution: Do not use \TeX\ control sequences in such titles, unless you know that the \.{cwebmac} macros will do the right thing with them. The reason is that these titles are converted to uppercase when they appear as running heads, and they are converted to boldface when they appear at the beginning of their sections, and they are also written out to a table-of-contents file used for temporary storage while \TeX\ is working; whatever control sequences you use must be meaningful in all three of these modes. The \TeX\ output produced by \.{WEAVE} for each section consists of the following: First comes the section number (e.g., `\.{\\M123.}' at the beginning of section 123, except that `\.{\\N}' appears in place of `\.{\\M}' at the beginning of a starred section). Then comes the \TeX\ part of the section, copied almost verbatim except as noted below. Then comes the middle part and the \Cee\ part, formatted so that there will be a little extra space between them if both are nonempty. The middle and \Cee\ parts are obtained by inserting a bunch of funny-looking \TeX\ macros into the \Cee\ program; these macros handle typographic details about fonts and proper math spacing, as well as line breaks and indentation. When you are typing \TeX\ text, you will probably want to make frequent reference to variables and other quantities in your \Cee\ code, and you will want those variables to have the same typographic treatment when they appear in your text as when they appear in your program. Therefore the \.{WEB} language allows you to get the effect of \Cee\ editing within \TeX\ text, if you place `\.|' marks before and after the \Cee\ material. For example, suppose you want to say something like this: $$\hbox{ If \\{pa} is declared as `\&{int} ${}{*}\\{pa}$', the assignment $\\{pa}\K{\AND}\|a[\T{0}]$ makes \\{pa} point to the zeroth element of \|a.}$$ The \TeX\ text would look like this in your \.{WEB} file: $$\lpile{\.{If |pa| is declared as `|int *pa|', the}\cr \.{assignment |pa=\&a[0]| makes |pa| point to the zeroth element of |a|.}\cr}$$ And \.{WEAVE} translates this into something you are glad you didn't have to type: $$\lpile{\.{If \\\\\{pa\} is declared as `\\\&\{int\} \$\{\}\{*\}\\\\\{pa\}\$',}\cr \.{the assignment \$\\\\\{pa\}\\K\{\\AND\}\\|a[\\T\{0\}]\$}\cr \.{makes \\\\\{pa\} point to the zeroth element of \\|a.}\cr}$$ Incidentally, the cross-reference index that \.{WEAVE} would make, in the presence of a comment like this, would include the current section number as one of the index entries for \\{pa}, even though \\{pa} might not appear in the \Cee\ part of this section. Thus, the index covers references to identifiers in the explanatory comments as well as in the program itself; you will soon learn to appreciate this feature. However, the identifiers \&{int} and \|a\ would not be indexed, because \.{WEAVE} does not make index entries for reserved words or single-letter identifiers. Such identifiers are felt to be so ubiquitous that it would be pointless to mention every place where they occur. Although a section begins with \TeX\ text and ends with \Cee\ text, we have noted that the dividing line isn't sharp, since \Cee\ text can be included in \TeX\ text if it is enclosed in `\pb'. Conversely, \TeX\ text also appears frequently within \Cee\ text, because everything in comments (i.e., between \.{/*} and \.{*/}) is treated as \TeX\ text. Furthermore, a section name, like a comment, is expected to be found in \Cee\ text, but the name itself consists of \TeX\ text; thus, a \.{WEB} file typically involves constructions like `\.{if} \.{(x==0)} \.{@}' where we go back and forth between \Cee\ and \TeX\ conventions in a natural way. To include a section name in \TeX\ text, that is, in the \TeX\ part of a section or in a \Cee\ comment, you can enclose it in `\pb'. In this case the section name is being ``cited'', rather than defined or used. \section Macros. The control code \.{@d} followed by $$\\{identifier}\.{ }\hbox{\Cee\ text}\qquad\hbox{or by}\qquad \\{identifier}\.(\\{par}_1,\ldots,\\{par}_n\.{) }\hbox{\Cee\ text}$$ (where there is no blank between the \\{identifier} and the parentheses in the second case) is transformed by \.{TANGLE} into a preprocessor command, starting with \.{\#define}, which is printed at the top of the \Cee\ output file as explained earlier. A `\.{@d}' macro definition can go on for several lines, and the newlines don't have to be protected by backslashes, since \.{TANGLE} itself inserts the backslashes. If for any reason you need a \.{\#define} command at a specific spot in your \Cee\ file, you can treat it as \Cee\ code, instead of as a \.{WEB} macro; but then you do have to protect newlines yourself. \section Strings and constants. If you want a string to appear in the \Cee\ file, delimited by pairs of \.' or \." marks as usual, you can type it exactly so in the \.{WEB} file, except that the character `\.@' should be typed `\.{@@}' (it becomes a control code, the only one that can appear in strings; see below). Strings should end on the same line as they begin, unless there's a backslash at the end of lines within them. \TeX\ and \Cee\ have different ways to refer to octal and hex constants, because \TeX\ is oriented to technical writing while \Cee\ is oriented to computer processing. In \TeX\ you make a constant octal or hexadecimal by prepending \.' or \.", respectively, to it; in \Cee\ the constant should be preceded by \.0 or \.{0x}. In \.{WEB} it seems reasonable to let each convention hold in its respective realm; so in \Cee\ text you get $40_8$ by typing `\.{040}', which \.{TANGLE} faithfully copies into the \Cee\ file (for the compiler's benefit) and which \.{WEAVE} prints as $\T{\~40}$. Similarly, \.{WEAVE} prints the hexadecimal \Cee\ constant `\.{0x20}' as \T{\^20}. The use of italic font for octal digits and typewriter font for hexadecimal digits makes the meaning of such constants clearer in a document. For consistency, then, you should type `\.{|040|}' or `\.{|0x20|}' in the \TeX\ part of the section. \section Control codes. A \.{WEB} {\sl control code\/} is a two-character combination of which the first is `\.@'. We've already seen the meaning of several control codes; here is a complete list of all of them. The letters $L$, $T$, $C$, $\\{Se}$, $\\{Co}$, and/or $S$ following each code indicate whether or not that code is allowable in limbo, in \TeX\ text, in \Cee\ text, in section names, in comments, and/or in strings. A bar over such a letter means that the control code terminates the present part of the \.{WEB} file; for example, $\overline L$ means that this control code ends the limbo material before the first section. \gdef\@#1[#2] {\penalty-100\yskip\hangindent 2em\noindent\.{@#1\unskip \spacefactor1000{ }}$[#2]$\quad} \def\more{\hangindent 2em \hangafter0} \def\oP{\overline C} \def\oT{\overline T\mskip1mu} \@@ [\\{Co},L,\\{Se},C,S,T] A double \.@ denotes the single character `\.@'. This is the only control code that is legal in limbo, in comments, and in strings. Note that you must use this convention if you are giving an internet email address in a \.{WEB} file (e.g., \.{levy@@math.berkeley.edu}). \@\ [\overline L,\oP,\oT] This denotes the beginning of a new (unstarred) section. A tab mark or form feed or end-of-line character is equivalent to a space when it follows an \.@ sign (and in most other cases). \@* [\overline L,\oP,\oT] This denotes the beginning of a new starred section, i.e., a section that begins a new major group. The title of the new group should appear after the \.{@*}, followed by a period. As explained above, \TeX\ control sequences should be avoided in such titles unless they are quite simple. When \.{WEAVE} and \.{TANGLE} read a \.{@*}, they print an asterisk on the terminal followed by the current section number, so that the user can see some indication of progress. The very first section should be starred. % This should be outdented, since it applies to more several items. \penalty-100\yskip\noindent The middle part of each section consists of any number of macro definitions (beginning with \.{@d}) and format definitions (beginning with \.{@f} or \.{@s}), intermixed in any order. \@d [\oP,\oT] Macro definitions begin with \.{@d} (or \.{@D}), followed by an identifier and optional parameters and \Cee\ text as explained earlier. \@f [\oP,\oT] Format definitions begin with \.{@f} (or \.{@F}); they cause \.{WEAVE} to treat identifiers in a special way when they appear in \Cee\ text. The general form of a format definition is `\.{@f} \|l \|r', followed by an optional comment enclosed between \.{/*} and \.{*/}, where \|l and \|r are identifiers; \.{WEAVE} will subsequently treat identifier \|l as it currently treats \|r. This feature allows a \.{WEB} programmer to invent new reserved words and/or to unreserve some of \Cee's reserved identifiers. If \|r is the special identifier `\\{TeX}', identifier \|l will be formatted as a \TeX\ control sequence; for example, `\.{@f foo TeX}' in the \.{WEB} file will cause identifier \\{foo} to be output as \.{\\foo} by \.{WEAVE}. The programmer should define \.{\\foo} to have whatever custom format is desired, assuming \TeX\ math mode. (Each underline character is converted to \.{x} when making the \TeX\ control sequence; thus \\{foo\_bar} becomes \.{\\fooxbar}.) \more \.{WEAVE} knows that identifiers being defined with a \&{typedef} should become reserved words; thus you don't need format definitions very often. \@s [\oP,\oT] Same as \.{@f}, but \.{WEAVE} does not show the format definition in the output. This is used mostly in \.{@i} files. \@c [\oP,\oT] The \Cee\ part of an unnamed section begins with \.{@c} (or \.{@C}). This causes \.{TANGLE} to append the following \Cee\ code to the initial program text $T_0$ as explained above. The \.{WEAVE} processor does not cause a `\.{@c}' to appear explicitly in the \TeX\ output, so if you are creating a \.{WEB} file based on a \TeX-printed \.{WEB} documentation you have to remember to insert \.{@c} in the appropriate places of the unnamed sections. \@p [\oP,\oT] The \Cee\ part of an unnamed section may alternatively begin with \.{@p} (or \.{@P}), for compatibility with other \.{WEB} systems. \.{CWEB} treats \.{@c} and \.{@p} identically. \more Because of the rules by which every section is broken into three parts, the control codes `\.{@d}', `\.{@f}', `\.{@c}', and `\.{@p}' are not allowed to occur once the \Cee\ part of a section has begun. \@< [C,\oT] A section name (or unambiguous prefix, as discussed above) is delimited by \.{@<} and \.{@>}, and consists of \TeX\ text; but the whole construct \.{@<...@>} is conceptually a \Cee\ element. (In this sense a section name is like a \Cee\ comment.) The \TeX\ text should not contain any \.{WEB} control codes except \.{@@}, unless these control codes appear in \Cee\ text that is delimited by \pb. An occurrence of a section name may indicate that the section is being defined, used, or cited. \more A \.{@<} appearing in the \TeX\ part (or the middle part) of a section inaugurates the \Cee\ part of the section, and defines the following section name to stand for the subsequent \Cee\ code. The closing \.{@>} should be followed by \.{=} or \.{+=}. \more In the \Cee\ part of a section, \.{@<...@>} indicates that the named section is being used (its \Cee\ definition is spliced in by \.{TANGLE}, as explained above). As an error-detection measure, \.{TANGLE} and \.{WEAVE} complain if such a section name is followed by \.=, because most likely this is meant as the definition of a new section, and so should be preceded by \.{@\ }. If you really want to say $\langle\,$foo$\,\rangle=\\{bar}$, where $\langle\,$foo$\,\rangle$ is being used and not defined, put a newline before the \.=. \more Finally, \.{|@<...@>|} appearing in a \TeX\ context (in the \TeX\ part of a section, or in a comment) means that the named section is being referred to. Such an occurrence is ignored by \.{TANGLE}. Note that even here we think of the section name as being a \Cee\ element, hence the \pb. There is one \TeX\ context where \.{|@<...@>|} is not allowed: inside another section name. \@( [C,\oT] A section name can begin with \.{@(} instead of \.{@<}. Everything works exactly as before, except that \.{@(foo@>} denotes a special section name all of whose \Cee\ code is written by \.{TANGLE} to file \.{foo}. In this way you can get multiple-file output from a single \.{WEB} file. (The \.{@d} definitions are not output to such files, only to the master \.{.c} file.) \@' [C] This control code is dangerous because it has quite different meanings in \.{CWEB} and the original \.{WEB}. In \.{CWEB} it produces the decimal constant corresponding to the ASCII code for a string of length~1 (e.g., \.{@'a'} is \.{TANGLE}d into \.{97} and \.{@'\\t'} into \.9). You might want to use this if you need to work in ASCII on a non-ASCII machine; but in most cases the \Cee\ conventions of \.{} are adequate for character-set-independent programming. \@\& [C] The \.{@\&} operation causes whatever is on its left to be adjacent to whatever is on its right, in the \Cee\ output. No spaces or line breaks will separate these two items. \penalty-100\yskip \noindent The next several control codes introduce ``control texts'', which end with the next `\.{@>}'. The closing `\.{@>}' must be on the same line of the \.{WEB} file where the control text began. Furthermore, no \.{WEB} control codes are allowed in a control text, not even \.{@@}. (If you need an \.{@} sign you can get around this restriction by typing `\.{\{\\AT\}}'.) \@\^ [C,T] The control text that follows, up to the next `\.{@>}', will be entered into the index together with the identifiers of the \Cee\ program; this text will appear in roman type. For example, to put the phrase ``system dependencies'' into the index, type `\.{@\^system dependencies@>}' in each section that you want to index as system dependent. \@. [C,T] The control text that follows will be entered into the index in \.{typewriter} \.{type}. \@: [C,T] The control text that follows will be entered into the index in a format controlled by the \TeX\ macro `\.{\\9}', which you should define as desired. \@t [C] The control text that follows will be put into a \TeX\ \.{\\hbox} and formatted along with the neighboring \Cee\ program. This text is ignored by \.{TANGLE}, but it can be used for various purposes within \.{WEAVE}. For example, you can make comments that mix \Cee\ and classical mathematics, as in `$\\{size}<2^{15}$', by typing `\.{|size < @t\$2\^\{15\}\$@>|}'. \@= [C] The control text that follows will be passed verbatim to the \Cee\ program. \@q [C,T] The control text that follows will be totally ignored; it's a comment for readers of the \.{WEB} file only. \@! [C,T] The section number in an index entry will be underlined if `\.{@!}' immediately precedes the identifier or control text being indexed. This convention is used to distinguish the sections where an identifier is defined, or where it is explained in some special way, from the sections where it is used. A~reserved word or an identifier of length one will not be indexed except for underlined entries. An `\.{@!}' is implicitly inserted by \.{WEAVE} when an identifier is being defined or declared in \Cee\ code; for example, the definition $$\hbox{\&{int} \\{array}[\\{max\_dim}], \\{count}${}=\\{old\_count};$}$$ makes the names \\{array} and \\{count} get an underlined entry in the index. Statement labels, function definitions like $\\{main}(\\{argc},\39\\{argv})$, and \&{typedef} definitions also imply underlining. A function definition doesn't define its arguments; the arguments will, however, be defined (i.e., their index entries will be underlined), if their types are declared before the body of the function in the usual way (e.g., `\.{int}~\\{argc}; \.{char}~${**}\\{argv}$; $\{\,\ldots\,\}$'). \yskip\noindent The next eight control codes (namely `\.{@,}', `\.{@/}', `\.{@|}', `\.{@\#}', `\.{@+}', `\.{@;}', `\.{@[}', and `\.{@]}') have no effect on the \Cee\ program output by \.{TANGLE}; they merely help to improve the readability of the \TeX-formatted \Cee\ that is output by \.{WEAVE}, in unusual circumstances. \.{WEAVE}'s built-in formatting method is fairly good when dealing with syntactically correct \Cee\ text, but it is incapable of handling all possible cases, because it must deal with fragments of text involving macros and section names; these fragments do not necessarily obey \Cee's syntax. Although \.{WEB} allows you to override the automatic formatting, your best strategy is not to worry about such things until you have seen what \.{WEAVE} produces automatically, since you will probably need to make only a few corrections when you are touching up your documentation. \@, [C] This control code inserts a thin space in \.{WEAVE}'s output; it is ignored by \.{TANGLE}. Sometimes you need this extra space if you are using macros in an unusual way, e.g., if two identifiers are adjacent. \@/ [C] This control code causes a line break to occur within a \Cee\ program formatted by \.{WEAVE}; it is ignored by \.{TANGLE}. Line breaks are chosen automatically by \TeX\ according to a scheme that works 99\%\ of the time, but sometimes you will prefer to force a line break so that the program is segmented according to logical rather than visual criteria. \@| [C] This control code specifies an optional line break in the midst of an expression. For example, if you have a long expression on the right-hand side of an assignment statement, you can use `\.{@|}' to specify breakpoints more logical than the ones that \TeX\ might choose on visual grounds. \@\# [C] This control code forces a line break, like \.{@/} does, and it also causes a little extra white space to appear between the lines at this break. You might use it, for example, between groups of macro definitions that are logically separate but within the same section. \.{CWEB} automatically inserts this extra space between functions, between external declarations and functions, and between declarations and statements within a function. \@+ [C] This control code cancels a line break that might otherwise be inserted by \.{WEAVE}, e.g., before the word `\&{else}', if you want to put a short if--else construction on a single line. It is ignored by \.{TANGLE}. If you say `\.{\{@+}' at the beginning of a compound statement that is the body of a function, the first declaration or statement of the function will appear on the same line as the left brace, and it will be indented by the same amount as the second declaration or statement on the next line. \@; [C] This control code is treated like a semicolon, for formatting purposes, except that it is invisible. You can use it, for example, after a section name or macro when the \Cee\ text represented by that section or macro is a compound statement or ends with a semicolon. Consider constructions like $$\lpile{\.{if (condition) macro @;}\cr \.{else break;}\cr}$$ where \\{macro} is defined to be a compound statement (enclosed in braces). This is a well-known infelicity of \Cee\ syntax. \@{[} [C] Place this before a macro argument that isn't otherwise formatted correctly. \@] [C] Place this after a macro argument that isn't otherwise formatted correctly. \yskip\noindent The next two entries describe ``meta'' control codes: they govern the input that \.{WEB} sees. \@{x @y @z}[\\{change\_file}] \.{WEAVE} and \.{TANGLE} are designed to work with two input files, called \\{web\_file} and \\{change\_file}, where \\{change\_file} contains data that overrides selected portions of \\{web\_file}. The resulting merged text is actually what has been called the \.{WEB} file elsewhere in this report. \more Here's how it works: The change file consists of zero or more ``changes,'' where a change has the form `\.{@x}$\langle$old lines$\rangle$\.{@y}$\langle$% new lines$\rangle$\.{@z}'. The special control codes \.{@x}, \.{@y}, \.{@z}, which are allowed only in change files, must appear at the beginning of a line; the remainder of such a line is ignored. The $\langle$old lines$\rangle$ represent material that exactly matches consecutive lines of the \\{web\_file}; the $\langle$new lines$\rangle$ represent zero or more lines that are supposed to replace the old. Whenever the first ``old line'' of a change is found to match a line in the \\{web\_file}, all the other lines in that change must match too. \more Between changes, before the first change, and after the last change, the change file can have any number of lines that do not begin with `\.{@x}', `\.{@y}', or~`\.{@z}'. Such lines are bypassed and not used for matching purposes. \more This dual-input feature is useful when working with a master \.{WEB} file that has been received from elsewhere (e.g., \.{tangle.w} or \.{weave.w} or \.{tex.web}), when changes are desirable to customize the program for your local computer system. You will be able to debug your system-dependent changes without clobbering the master web file; and once your changes are working, you will be able to incorporate them readily into new releases of the master web file that you might receive from time to time. \@i [\\{web\_file}] Furthermore the \\{web\_file} itself can be a combination of several files. When either \.{CWEAVE} or \.{CTANGLE} is reading a file and encounters the control code \.{@i} at the beginning of a line, it interrupts normal reading and start looking at the file named after the \.{@i}, much as the \Cee\ preprocessor does when it encounters an \.{\#include} line. After the included file has been entirely read, the program goes back to the next line of the original file. The file name following \.{@i} can be surrounded by \." characters, but such delimiters are optional. Include files can nest. \more Change files can have lines starting with \.{@i}. In this way you can replace one included file with another. It is not possible to replace individual lines of an included file using `\.{@x}', etc.; conceptually, the replacement mechanism described above does its work first, and its output is then checked for \.{@i} lines. \section Additional features and caveats. 1. In certain installations of \.{CWEB} that {\def\\#1#2{`{\tentex\char'#1#2}'}% have an extended character set, the characters \\13, \\01, \\31, \\32, \\34, \\35, \\36, \\37, \\04, \\20, and \\21} can be typed as abbreviations for `\.{++}', `\.{--}', `\.{->}', `\.{!=}', `\.{<=}', `\.{>=}', `\.{==}', `\.{\v\v}', `\.{\&\&}', `\.{<<}', and `\.{>>}', respectively. 2. If you have an extended character set, all of the characters listed in Appendix~C of {\sl The \TeX book\/} can be used in strings. But you should stick to standard ASCII characters if you want to write programs that will be useful to all the poor souls out there who don't have extended character sets. 3. The \TeX\ file output by \.{WEAVE} is broken into lines having at most 80 characters each. The algorithm that does this line breaking is unaware of \TeX's convention about comments following `\.\%' signs on a line. When \TeX\ text is being copied, the existing line breaks are copied as well, so there is no problem with `\.\%' signs unless the original \.{WEB} file contains a line more than eighty characters long or a line with \Cee\ text in \pb\ that expands to more than eighty characters long. Such lines should not have `\.\%' signs. 4. \Cee\ text is translated by a ``bottom up'' procedure that identifies each token as a ``part of speech'' and combines parts of speech into larger and larger phrases as much as possible according to a special grammar that is explained in the documentation of \.{WEAVE}. It is easy to learn the translation scheme for simple constructions like single identifiers and short expressions, just by looking at a few examples of what \.{WEAVE} does, but the general mechanism is somewhat complex because it must handle much more than \Cee\ itself. Furthermore the output contains embedded codes that cause \TeX\ to indent and break lines as necessary, depending on the fonts used and the desired page width. For best results it is wise to avoid enclosing long \Cee\ texts in \pb, since the indentation and line breaking codes are omitted when the \pb\ text is translated from \Cee\ to \TeX. Stick to simple expressions or statements. If a \Cee\ preprocessor command is enclosed in \pb, the \.\# that introduces it must be at the beginning of a line, or \.{WEAVE} won't print it correctly. 5. Comments are not permitted in \pb\ text. After a `\.|' signals the change from \TeX\ text to \Cee\ text, the next `\.|' that is not part of a string or control text or section name ends the \Cee\ text. 6. A comment must have properly nested occurrences of left and right braces, otherwise \.{WEAVE} will complain. But it does try to balance the braces, so that \TeX\ won't foul up too much. 7. When you're debugging a program and decide to omit some of your \Cee\ code, do NOT simply ``comment it out.'' Such comments are not in the spirit of \.{WEB} documentation; they will appear to readers as if they were explanations of the uncommented-out instructions. Furthermore, comments of a program must be valid \TeX\ text; hence \.{WEAVE} will get confused if you enclose \Cee\ statements in \.{/*...*/} instead of in \.{/*|...|*/}. If you must comment out \Cee\ code, you can surround it with preprocessor commands like \.{\#if 0==1} and \.{\#endif}. 8. The \.{@f} feature allows you to define one identifier to act like another, and these format definitions are carried out sequentially. In general, a given identifier has only one printed format throughout the entire document, and this format is used even before the \.{@f} that defines it. The reason is that \.{WEAVE} operates in two passes; it processes \.{@f}'s and cross-references on the first pass and it does the output on the second. (However, identifiers that implicitly get a boldface format, thanks to a \.{typedef} declaration, don't obey this rule: they are printed differently before and after the relevant \.{typedef}. This is unfortunate, but hard to fix. You can get around the restriction by using \.{@s}, before or after the \.{typedef}.) 9. Sometimes it is desirable to insert spacing into \Cee\ code that is more general than the thin space provided by `\.{@,}'. The \.{@t} feature can be used for this purpose; e.g., `\.{@t\\hskip 1in@>}' will leave one inch of blank space. Furthermore, `\.{@t\\4@>}' can be used to backspace by one unit of indentation, since the control sequence \.{\\4} is defined in \.{cwebmac} to be such a backspace. (This control sequence is used, for example, at the beginning of lines that contain labeled statements, so that the label will stick out a little at the left.) You can also use `\.{@t\}\\3\{-5@>}' to force a break in the middle of an expression. \section Running the programs. The \UNIX\ command line for \.{CTANGLE} is $$\.{ctangle [options] web\_file[.w] [change\_file[.ch] [out\_file]]}$$ and the same conventions apply to \.{CWEAVE}. If no change file is specified, the change file is null. The extensions \.{.w} and \.{.ch} are appended only if the given file names contain no dot. If the web file defined in this way cannot be found, the extension \.{.web} will be tried. For example, `\.{cweave} \.{cob}' will try to read \.{cob.w}; failing that, it will try \.{cob.web} before giving up. If no output file name is specified, the name of the \Cee\ file output by \.{CTANGLE} is obtained by appending the extension \.{.c}; the name of the \TeX\ file output by \.{CWEAVE} gets the extension \.{.tex}. Programmers who like terseness might choose to set up their operating shell so that `\.{wv}' expands to `\.{cweave -bhp}'; this will suppress most terminal output from \.{CWEAVE} except for error messages. Options are introduced either by a \.- sign, to turn an option off, or by a \.+ sign to turn one on. For example, `\.{-fb}' turns off options \.f and \.b; `\.{+s}' turns on option \.s. Options can be specified before the file names, after the file names, or both. The following options are currently implemented: \yskip \def\option#1 {\textindent{\.#1}\hangindent2\parindent} \option b Print a banner line at the beginning of execution. (On by default.) \option f Force line breaks after each \Cee\ statement formatted by \.{WEAVE}. (On by default; \.{-f} saves paper but looks less \Cee-like to some people.) (Has no effect on \.{TANGLE}.) \option h Print a happy message at the conclusion of a successful run. (On by default.) \option p Give progress reports as the program runs. (On by default.) \option s Show statistics about memory usage after the program runs to completion. (Off by default.) This feature works only if the programs have been compiled with the \.{-DSTAT} switch. If you have large \.{WEB} files or sections, you may need to see how close you come to exceeding the capacity of \.{TANGLE} and/or \.{WEAVE}. \option x Include indexes and a table of contents in the \TeX\ file output by \.{WEAVE}. (On by default.) (Has no effect on \.{TANGLE}.) \section Further details about formatting. You may not like the way \.{WEAVE} handles certain situations. If you're desperate, you can customize \.{WEAVE} by changing its grammar. This means changing the source code, a task that you might find amusing. A table of grammar rules appears in the \.{WEAVE} source listing, and you can make a separate copy of that table by copying the file \.{prod.w} found in the \.{cweb} sources and saying `\.{cweave}~\.{-x}~\.{prod}'. If you compile \.{WEAVE} with the \.{-DDEBUG} option on the command line, you will be able to see exactly how \.{WEAVE} is parsing your \Cee\ code by preceding it with the line `\.{@ @c @2}'. (The control code `\.{@2}' turns on a ``peeping'' mode, and `\.{@0}' turns it off.) For example, if you run \.{WEAVE} on the file \medskip \begingroup \verbatim @ @c @2 main (argc,argv) char **argv; { for (;argc>0;argc--) printf("%s\n",argv[argc-1]); } !endgroup \endgroup \medskip\noindent you get the following gibberish on your screen: \medskip \begingroup \verbatim [...] 4:*exp ( +exp+ )... 8:*exp +exp+ int... 5:*+exp+ int +unorbinop+... [...] 45: +fn_decl+*+{+ -stmt- +}- 40:*+fn_decl+ -stmt- 37:*+function- [...] !endgroup \endgroup \medskip The first line says that grammar rule 4 has just been applied, and \.{WEAVE} currently has in its memory a sequence of chunks of \TeX\ code (called ``scraps'') that are respectively of type \\{exp} (for expression), open-parenthesis, \\{exp} again, close-parenthesis, and further scraps that haven't yet been considered by the parser. (The \.+ and \.- signs stipulate that \TeX\ should be in or out of math mode at the scrap boundaries. The \.* shows the parser's current position.) Then rule 8 is applied, and the sequence $(\,exp\,)$ becomes an \\{exp} and so on. In the end the whole \Cee\ text has become one big scrap of type \\{function}. Sometimes things don't work as smoothly, and you get a bunch of lines lumped together. This means that \.{WEAVE} could not digest something in your \Cee\ code. For instance, suppose `\.{@}' had appeared instead of `\.{char **argv;}' in the program above. Then \.{WEAVE} would have been somewhat mystified, since it thinks that section names are just \\{exp}s. Thus it would tell \TeX\ to format `\X2:Argument declarations\X' on the same line as `$\\{main}(\\{argc},\39\\{argv}{}$)'. In this case you should help \.{WEAVE} by putting `\.{@/}' after `\.{main(argc,argv)}'. \.{CWEAVE} automatically inserts a bit of extra space between declarations and the first apparent statement of a block. One way to defeat this is $$\vbox{\halign{#\hfil\cr \.{int x;@+@t@>@;@/}\cr \.{@@;@\#}\cr}}$$ the `\.{@\#}' will put extra space after `$\langle\,$Other locals$\,\rangle$'. \section Acknowledgments. The authors wish to thank all who contributed suggestions and criticism to the development of \.{CWEB}. We are especially grateful to Norman Ramsey, Joachim Schnitter, Klaus Guntermann, and Nelson Beebe, who contributed code. Ramsey also has made literate programming accessible to users of yet other languages by means of his \.{SPIDER} system [see {\sl Communications of the ACM\/ \bf32} (1989), 1051--1055]. \section Appendices. The basic ideas of \.{WEB} can be understood most easily by looking at examples of ``real'' programs. Appendix~A shows the \.{CWEB} input that generated sections 17--18 of the \.{common.w} file, which contains routines common to \.{CWEAVE} and \.{CTANGLE}. Appendix~B shows the corresponding \Cee\ code output by \.{CTANGLE}. Appendix~C shows the corresponding \TeX\ code output by \.{CWEAVE}, and Appendix~D shows how that output looks when printed out. Appendix E is the file that sets \TeX\ up to accept the output of \.{CWEAVE}, and Appendix~F discusses how to use some of those macros to vary the output formats. \vfil\eject \def\runninghead{APPENDIX A --- {\tentt WEB} FILE FORMAT} \section Appendix A. The following is an excerpt of the file \.{common.w}, which contains routines shared by \.{CWEAVE} and \.{CTANGLE}. Note that some of the lines are indented to show the program structure. The indentation is ignored by \.{WEAVE} and \.{TANGLE}, but users find that \.{WEB} files are quite readable if they have some such indentation. The reader should first compare Appendix~A to Appendix~B; then the same material should be compared to Appendices~C and~D. \vskip 6pt \begingroup \def\tt{\eighttt} \baselineskip9pt \verbatim @ Procedure |prime_the_change_buffer| sets |change_buffer| in preparation for the next matching operation. Since blank lines in the change file are not used for matching, we have |(change_limit==change_buffer && !!changing)| if and only if the change file is exhausted. This procedure is called only when |changing| is 1; hence error messages will be reported correctly. @= prime_the_change_buffer() change_limit=change_buffer; /* this value is used if the change file ends */ @; @; @; @ While looking for a line that begins with \.{@@x} in the change file, we allow lines that begin with \.{@@}, as long as they don't begin with \.{@@y}, \.{@@z} or \.{@@i} (which would probably mean that the change file is fouled up). @= while(1) { change_line++; if (!!input_ln(change_file)) return; if (limit @ Here we are looking at lines following the \.{@@x}. @= change_line++; if (!!input_ln(change_file)) { err_print("!! Change file ended after @@x"); @.Change file ended...@> return; } while (limit==buffer); @ @= change_limit=change_buffer-buffer+limit; strncpy(change_buffer,buffer,limit-buffer+1); !endgroup \endgroup \vfill\eject \def\runninghead{APPENDIX B --- TRANSLATION BY {\tentt TANGLE}} \section Appendix B. Here's the portion of the \Cee\ code generated by \.{TANGLE} that corresponds to Appendix~A. Notice that sections~13, 14 and~15 have been tangled into section~12. \vskip6pt \begingroup \def\tt{\eighttt} \baselineskip9pt \verbatim /*:9*//*12:*/ #line 240 "common.w" prime_the_change_buffer() change_limit= change_buffer; /*13:*/ #line 253 "common.w" while(1){ change_line++; if(!!input_ln(change_file))return; if(limit}', the backslash character gets in the way, and this entry wouldn't appear in the index with the T's. The solution is to use the `\.{@:}' feature, declaring a macro that simply removes a sort key as follows: $$\.{\\def\\9\#1\{\}}$$ Now you can say, e.g., `\.{@:TeX\}\{\\TeX@>}' in your \.{WEB} file; \.{WEAVE} puts it into the index alphabetically, based on the sort key, and produces the macro call `\.{\\9\{TeX\}\{\\TeX\}}' which will ensure that the sort key isn't printed. A similar idea can be used to insert hidden material into section names so that they are alphabetized in whatever way you might wish. Some people call these tricks ``special refinements''; others call them ``kludges''. \point 12. The control sequence \.{\\secno} is set to the number of the section being typeset. \point 13. If you want to list only the sections that have changed, together with the index, put the command `\.{\\let\\maybe=\\iffalse}' in the limbo section before the first section of your \.{WEB} file. It's customary to make this the first change in your change file. \point 14. To get output in languages other than English, redefine the macros \.{\\A}, \.{\\ET}, \.{\\Q}, \.{\\U}, \.{\\ch}, \.{\\fin}, and \.{\\con}. \.{CWEAVE} itself need not be changed. \point 15. All accents and special text symbols of plain \TeX\ format will work in \.{CWEB} documents just as they are described in Chapter~9 of {\sl The \TeX book}, with one exception. The dot accent (normally \.{\\.}) must be typed \.{\\:} instead. \vfill\end