The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Exercise 6.06 on page 145
Implement a simple version of the #define processor (i.e., no arguments) suitable for use with C programs, based on the routines of this section. You may also find getch and ungetch helpful. .
Solution by Barrett Drawdy
/* * K&R2 exercise 6-6. By: Barrett Drawdy * "Implement a simple version of the #define processor (i.e., no arguments) * suitable for use with C programs, based on the routines of this section. * You may also find getch and ungetch helpful." */ #include <stdio.h> #include <stdlib.h> #include <string.h> #include <ctype.h> #define HASHSIZE 101 #define MAXWORD 1000 /* nlist: From K&R2 page 144. Points to a search text and * a replacement text */ struct nlist { struct nlist *next; char *name; char *defn; }; static struct nlist *hashtab[HASHSIZE]; struct nlist *lookup(char *); struct nlist *install(char *, char *); int process(void); int getch(void); void ungetch(int); int preproc(void); int backslash(void); int comment(void); int literal(void); int readword(void); int main(void) { int c; const int done = 0; /* just a dummy for the infinite loop */ int status = 1; /* 1 at line begin, 2 for failure, 0 otherwise */ while(!done) { while(isspace(c = getch())) { putchar(c); if(c == '\n') status = 1; } if(c == '#' && status == 1) status = preproc(); else if(c == '\\') status = backslash(); else if(c == '/') status = comment(); else if(c == '\"') status = literal(); else if(c == EOF) return 0; else if(!isalpha(c) && c != '_') { putchar(c); status = 0; } else { ungetch(c); status = readword(); } if(status == 2) return 1; } return 0; } /* end main() */ /* * preproc: Called when a '#' is found at the beginning of a line. * If a #define statement is found, it stores add the search and replacement * text to the hash table. Otherwise it just prints what it reads. * The line a #define statement is found on is replaced with a blank line. */ int preproc(void) { int c; char name[MAXWORD+1], defn[MAXWORD+1]; char *n, *d; /* Read what comes after the '#' */ for(n = name; isalpha(c = getch()) && n - name < MAXWORD; ++n) *n = c; *n = '\0'; /* If it's a #define... */ if(strcmp(name, "define") == 0) { /* ignore whitespace */ while(isspace(c)) { if(c == '\n') { /* a #define followed by a '\n' is discarded */ putchar(c); return 1; } c = getch(); } /* Read search text */ for(n = name; (isalnum(c) || c == '_') && n - name < MAXWORD; ++n) { *n = c; c = getch(); } *n = '\0'; /* Ignore whitespace */ while(isspace(c)) { if(c == '\n') *defn = '\0'; /* If there is no replacement text. */ c = getch(); } /* Read replacement text. */ for(d = defn; (isalnum(c) || c == '_') && d - defn < MAXWORD; ++d) { *d = c; c = getch(); } *d = '\0'; /* Add to hash table. */ if(install(name, defn) == NULL) return 2; } else { /* not a #define statement */ putchar('#'); printf("%s", name); } /* finish line reading/printing line */ while(c != '\n') { if(c == EOF) return EOF; putchar(c); c = getch(); } putchar(c); return 1; } /* end preproc() */ /* * backslash: Makes sure that if a /, *, or " is preceded by a \ it is * just printed, not used in a comment or string literal. */ int backslash(void) { int c, slash = 1; putchar('\\'); while((c = getch()) == '\\') { slash = !slash; putchar(c); } if(slash) putchar(c); else ungetch(c); return 0; } /* end backslash() */ /* * comment: Prints comments without changes. */ int comment(void) { int c; int after_star = 0; putchar('/'); if((c = getch()) == '*') { /* comment begin */ putchar(c); c = getch(); while(c != EOF) { if(c == '\\') { backslash(); after_star = 0; } else if(c == '*') { after_star = 1; putchar(c); } else if(c == '/' && after_star) { putchar(c); return 0; } else { after_star = 0; putchar(c); } c = getch(); } if(c == EOF) return EOF; putchar(c); return 0; } else { ungetch(c); return 0; } } /* end comment() */ /* * literal: Prints string literals without changes. */ int literal(void) { int c; putchar('\"'); c = getch(); while(c != '\"' && c != EOF) { if(c == '\\') backslash(); else putchar(c); c = getch(); } if(c == EOF) return EOF; putchar(c); return 0; } /* end literal() */ /* * readwood: Reads an identifier and looks for it in the hash table. If * it's found, it's replaced with the replacement text. Otherwise, it is * printed as is. */ int readword(void) { int c; char word[MAXWORD]; char *w; struct nlist *node; c = getch(); for(w = word; (isalnum(c) || c == '_') && c != EOF; ++w) { *w = c; c = getch(); } *w = '\0'; node = lookup(word); if(node == NULL) printf("%s", word); else printf("%s", node->defn); if(c == EOF) return EOF; ungetch(c); return 0; } /* end readword() */ /*************************************************************************** * From K&R2 * ***************************************************************************/ unsigned hash(char *); char *strdup(char *); /* lookup: From K&R2 page 145. Look for s in hashtab. */ struct nlist *lookup(char *s) { struct nlist *np; for(np = hashtab[hash(s)]; np != NULL; np = np->next) if(strcmp(s, np->name) == 0) return np; return NULL; } /* install: From K&R2 page 145. Put (name, defn) in hashtab. */ struct nlist *install(char *name, char *defn) { struct nlist *np; unsigned hashval; if((np = lookup(name)) == NULL) { np = (struct nlist *) malloc(sizeof(*np)); if(np == NULL || (np->name = strdup(name)) == NULL) return NULL; hashval = hash(name); np->next = hashtab[hashval]; hashtab[hashval] = np; } else free((void *) np->defn); if((np->defn = strdup(defn)) == NULL) return NULL; return np; } /* hash: From K&R2 page 144. Form hash value for string s. */ unsigned hash(char *s) { unsigned hashval; for(hashval = 0; *s != '\0'; ++s) hashval = *s + 31 * hashval; return hashval % HASHSIZE; } /* strdup: From K&R2 page 143. Makes a duplicate of s. */ char *strdup(char *s) { char *p; p = (char *) malloc(strlen(s) + 1); if(p != NULL) strcpy(p, s); return p; } #define BUFSIZE 100 char buf[BUFSIZE]; /* buffer for ungetch() */ int bufp = 0; /* next free position in buf */ int getch(void) { /* get a (possibly pushed back) character */ return (bufp > 0) ? buf[--bufp] : getchar(); } void ungetch(int c) { /* push character back on input */ if(bufp >= BUFSIZE) printf("ungetch: too many characters\n"); else buf[bufp++] = c; return; }
