root/user/grep.c

DEFINITIONS

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

   1 // Simple grep.  Only supports ^ . * $ operators.
   2 
   3 #include "kernel/types.h"
   4 #include "kernel/stat.h"
   5 #include "kernel/fcntl.h"
   6 #include "user/user.h"
   7 
   8 char buf[1024];
   9 int match(char*, char*);
  10 
  11 void
  12 grep(char *pattern, int fd)
  13 {
  14   int n, m;
  15   char *p, *q;
  16 
  17   m = 0;
  18   while((n = read(fd, buf+m, sizeof(buf)-m-1)) > 0){
  19     m += n;
  20     buf[m] = '\0';
  21     p = buf;
  22     while((q = strchr(p, '\n')) != 0){
  23       *q = 0;
  24       if(match(pattern, p)){
  25         *q = '\n';
  26         write(1, p, q+1 - p);
  27       }
  28       p = q+1;
  29     }
  30     if(m > 0){
  31       m -= p - buf;
  32       memmove(buf, p, m);
  33     }
  34   }
  35 }
  36 
  37 int
  38 main(int argc, char *argv[])
  39 {
  40   int fd, i;
  41   char *pattern;
  42 
  43   if(argc <= 1){
  44     fprintf(2, "usage: grep pattern [file ...]\n");
  45     exit(1);
  46   }
  47   pattern = argv[1];
  48 
  49   if(argc <= 2){
  50     grep(pattern, 0);
  51     exit(0);
  52   }
  53 
  54   for(i = 2; i < argc; i++){
  55     if((fd = open(argv[i], O_RDONLY)) < 0){
  56       printf("grep: cannot open %s\n", argv[i]);
  57       exit(1);
  58     }
  59     grep(pattern, fd);
  60     close(fd);
  61   }
  62   exit(0);
  63 }
  64 
  65 // Regexp matcher from Kernighan & Pike,
  66 // The Practice of Programming, Chapter 9, or
  67 // https://www.cs.princeton.edu/courses/archive/spr09/cos333/beautiful.html
  68 
  69 int matchhere(char*, char*);
  70 int matchstar(int, char*, char*);
  71 
  72 int
  73 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 }
  83 
  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 }
  97 
  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 }
 107