% wc: An example of CWEB by Silvio Levy and Donald E. Knuth\nocon % omit table of contents\datethis % print date on listing\def\SPARC{SPARC\-\kern.1em station}@* An example of {\tt CWEB}.  This example, based on a program byKlaus Guntermann and Joachim Schrod [{\sl TUGboat\/ \bf7} (1986),134--137] presents the ``word count'' program from \UNIX/, rewritten in\.{CWEB} to demonstrate literate programming in \CEE/.  The level ofdetail in this document is intentionally high, for didactic purposes;many of the things spelled out here don't need to be explained inother programs.The purpose of \.{wc} is to count lines, words, and/or characters in alist of files. The number of lines in a file is the number of newlinecharacters it contains. The number of characters is the file length in bytes.A ``word'' is a maximal sequence of consecutive characters other thannewline, space, or tab, containing at least one visible ASCII code.(We assume that the standard ASCII code is in use.)This version of \.{wc} has a nonstandard ``silent'' option (\.{-s}),which suppresses printing except for the grand totals over all files.@ Most \.{CWEB} programs share a common structure.  It's probably agood idea to state the overall structure explicitly at the outset,even though the various parts could all be introduced in unnamedsections of the code if we wanted to add them piecemeal.Here, then, is an overview of the file \.{wc.c} that is definedby this \.{CWEB} program \.{wc.w}:@c@<Header files to include@>@/@<Global variables@>@/@<Functions@>@/@<The main program@>@ We must include the standard I/O definitions, since we want to sendformatted output to |stdout| and |stderr|.@<Header files...@>=#include <stdio.h>@  The |status| variable will tell the operating system if the run wassuccessful or not, and |prog_name| is used in case there's an error message tobe printed.@d OK 0 /* |status| code for successful run */@d usage_error 1 /* |status| code for improper syntax */@d cannot_open_file 2 /* |status| code for file access error */@<Global variables@>=int status=OK; /* exit status of command, initially |OK| */char *prog_name; /* who we are */@ Now we come to the general layout of the |main| function. @<The main...@>=main (argc,argv)    int argc; /* the number of arguments on the \UNIX/ command line */    char **argv; /* the arguments themselves, an array of strings */{  @<Variables local to |main|@>@;  prog_name=argv[0];  @<Set up option selection@>;  @<Process all the files@>;  @<Print the grand totals if there were multiple files @>;  exit(status);}@ If the first argument begins with a `\.{-}', the user is choosingthe desired counts and specifying the order in which they should bedisplayed.  Each selection is given by the initial character(lines, words, or characters).  For example, `\.{-cl}' would causejust the number of characters and the number of lines to be printed,in that order. The default, if no special argument is given, is `\.{-lwc}'.We do not process this string now; we simply remember where it is.It will be used to control the formatting at output time.If the `\.{-}' is immediately followed by `\.{s}', only summary totalsare printed.@<Var...@>=int file_count; /* how many files there are */char *which; /* which counts to print */int silent=0; /* nonzero if the silent option was selected */@ @<Set up o...@>=which="lwc"; /* if no option is given, print all three values */if (argc>1 && *argv[1] == '-') {  argv[1]++;  if (*argv[1]=='s') silent=1,argv[1]++;  if (*argv[1]) which=argv[1];  argc--; argv++;}file_count=argc-1;@ Now we scan the remaining arguments and try to open a file, ifpossible.  The file is processed and its statistics are given.We use a |do|~\dots~|while| loop because we should read from thestandard input if no file name is given.@<Process...@>=argc--;do@+{  @<If a file is given, try to open |*(++argv)|; |continue| if unsuccessful@>;  @<Initialize pointers and counters@>;  @<Scan file@>;  @<Write statistics for file@>;  @<Close file@>;  @<Update grand totals@>; /* even if there is only one file */}@+while (--argc>0);@ Here's the code to open the file.  A special trick allows us tohandle input from |stdin| when no name is given.Recall that the file descriptor to |stdin| is~0; that's what weuse as the default initial value.@<Variabl...@>=int fd=0; /* file descriptor, initialized to |stdin| */@ @d READ_ONLY 0 /* read access code for system |open| routine */@<If a file...@>=if (file_count>0 && (fd=open(*(++argv),READ_ONLY))<0) {  fprintf (stderr, "%s: cannot open file %s\n", prog_name, *argv);@.cannot open file@>  status|=cannot_open_file;  file_count--;  continue;}@ @<Close file@>=close(fd);@ We will do some homemade buffering in order to speed things up: Characterswill be read into the |buffer| array before we process them.To do this we set up appropriate pointers and counters.@d buf_size BUFSIZ /* \.{stdio.h}'s |BUFSIZ| is chosen for efficiency*/@<Var...@>=char buffer[buf_size]; /* we read the input into this array */register char *ptr; /* the first unprocessed character in |buffer| */register char *buf_end; /* the first unused position in |buffer| */register int c; /* current character, or number of characters just read */int in_word; /* are we within a word? */long word_count, line_count, char_count; /* number of words, lines,     and characters found in the file so far */@ @<Init...@>=ptr=buf_end=buffer; line_count=word_count=char_count=0; in_word=0;@ The grand totals must be initialized to zero at the beginning of theprogram. If we made these variables local to |main|, we would have todo this initialization explicitly; however, \CEE/'s globals are automaticallyzeroed. (Or rather, ``statically zeroed.'') (Get it?)@^Joke@>@<Global var...@>=long tot_word_count, tot_line_count, tot_char_count; /* total number of words, lines, and chars */@ The present section, which does the counting that is \.{wc}'s {\it raisond'\^etre}, was actually one of the simplest to write. We look at eachcharacter and change state if it begins or ends a word.@<Scan...@>=while (1) {  @<Fill |buffer| if it is empty; |break| at end of file@>;  c=*ptr++;  if (c>' ' && c<0177) { /* visible ASCII codes */    if (!in_word) {word_count++; in_word=1;}    continue;  }  if (c=='\n') line_count++;  else if (c!=' ' && c!='\t') continue;  in_word=0; /* |c| is newline, space, or tab */}@ Buffered I/O allows us to count the number of characters almost for free.@<Fill |buff...@>=if (ptr>=buf_end) {  ptr=buffer; c=read(fd,ptr,buf_size);  if (c<=0) break;  char_count+=c; buf_end=buffer+c;}@ It's convenient to output the statistics by defining a new function|wc_print|; then the same function can be used for the totals.Additionally we must decide here if we know the name of the filewe have processed or if it was just |stdin|.@<Write...@>=if (!silent) {  wc_print(which, char_count, word_count, line_count);  if (file_count) printf (" %s\n", *argv); /* not |stdin| */  else printf ("\n"); /* |stdin| */}@ @<Upda...@>=tot_line_count+=line_count;tot_word_count+=word_count;tot_char_count+=char_count;@ We might as well improve a bit on \UNIX/'s \.{wc} by displaying thenumber of files too.@<Print the...@>=if (file_count>1 || silent) {  wc_print(which, tot_char_count, tot_word_count, tot_line_count);  if (!file_count) printf("\n");  else printf(" total in %d file%s\n",file_count,file_count>1?"s":"");}@ Here now is the function that prints the values according to thespecified options.  The calling routine is supposed to supply anewline. If an invalid option character is found we informthe user about proper usage of the command. Counts are printed in8-digit fields so that they will line up in columns.@d print_count(n) printf("%8ld",n)@<Fun...@>=wc_print(which, char_count, word_count, line_count)char *which; /* which counts to print */long char_count, word_count, line_count; /* given totals */{  while (*which)     switch (*which++) {    case 'l': print_count(line_count); break;    case 'w': print_count(word_count); break;    case 'c': print_count(char_count); break;    default: if ((status & usage_error)==0) {        fprintf (stderr, "\nUsage: %s [-lwc] [filename ...]\n", prog_name);@.Usage: ...@>        status|=usage_error;      }    }}@ Incidentally, a test of this program against the system \.{wc}command on a \SPARC\ showed that the ``official'' \.{wc} was slightlyslower. Furthermore, although that \.{wc} gave an appropriate errormessage for the options `\.{-abc}', it made no complaints about theoptions `\.{-labc}'! Dare we suggest that the system routine might have beenbetter if its programmer had used a more literate approach?@* Index.Here is a list of the identifiers used, and where they appear. Underlinedentries indicate the place of definition. Error messages are also shown.