root/grep.c

DEFINITIONS

1. grep
2. main
3. match
4. matchhere
5. matchstar

   1 // Simple grep.  Only supports ^ . * $ operators.
   3 #include "types.h"
   4 #include "stat.h"
   5 #include "user.h"
   7 char buf[1024];
   8 int match(char*, char*);
  10 void
grep(char *pattern, int fd)
  12 {
  13   int n, m;
  14   char *p, *q;
m = 0;
  17   while((n = read(fd, buf+m, sizeof(buf)-m-1)) > 0){
m += n;
buf[m] = '\0';
p = buf;
  21     while((q = strchr(p, '\n')) != 0){
  22       *q = 0;
  23       if(match(pattern, p)){
  24         *q = '\n';
write(1, p, q+1 - p);
  26       }
p = q+1;
  28     }
  29     if(p == buf)
m = 0;
  31     if(m > 0){
m -= p - buf;
memmove(buf, p, m);
  34     }
  35   }
  36 }
  38 int
  39 main(int argc, char *argv[])
  40 {
  41   int fd, i;
  42   char *pattern;
  44   if(argc <= 1){
printf(2, "usage: grep pattern [file ...]\n");
exit();
  47   }
pattern = argv[1];
  50   if(argc <= 2){
grep(pattern, 0);
exit();
  53   }
  55   for(i = 2; i < argc; i++){
  56     if((fd = open(argv[i], 0)) < 0){
printf(1, "grep: cannot open %s\n", argv[i]);
exit();
  59     }
grep(pattern, fd);
close(fd);
  62   }
exit();
  64 }
  66 // Regexp matcher from Kernighan & Pike,
  67 // The Practice of Programming, Chapter 9.
  69 int matchhere(char*, char*);
  70 int matchstar(int, char*, char*);
  72 int
match(char *re, char *text)
  74 {
  75   if(re[0] == '^')
  76     return matchhere(re+1, text);
  77   do{  // must look at empty string
  78     if(matchhere(re, text))
  79       return 1;
  80   }while(*text++ != '\0');
  81   return 0;
  82 }
  84 // matchhere: search for re at beginning of text
  85 int matchhere(char *re, char *text)
  86 {
  87   if(re[0] == '\0')
  88     return 1;
  89   if(re[1] == '*')
  90     return matchstar(re[0], re+2, text);
  91   if(re[0] == '$' && re[1] == '\0')
  92     return *text == '\0';
  93   if(*text!='\0' && (re[0]=='.' || re[0]==*text))
  94     return matchhere(re+1, text+1);
  95   return 0;
  96 }
  98 // matchstar: search for c*re at beginning of text
  99 int matchstar(int c, char *re, char *text)
 100 {
 101   do{  // a * matches zero or more instances
 102     if(matchhere(re, text))
 103       return 1;
 104   }while(*text!='\0' && (*text++==c || c=='.'));
 105   return 0;
 106 }