A small grep Tool

Yesterday, I was reading about the regular expressions design that how first developer wrote the complex code with minimal complexity, Since now I know the design of the minimal version of regular expression code I decided to share it. code in itself is quite simple and self explanatory..although I marked the complex part with appropriate comments.

 
int grep (char *regexp, char *text)
{
if (*regexp == '$' ){                       /*End of regular expression*/
if ( *(regexp+1) == '\0' ){
return 1;
} else {
return 0;
}
} else if ( *regexp == '.' ) {              /*Single character presence*/
return grep ( regexp + 1, text + 1 );
} else if ( *regexp == '*') {               /*multi character presence*/
return matchstar(regexp+1, text);
} else if ( *regexp == *text  ) {        
return grep ( regexp + 1, text + 1 );
} else {
return 0;
}
}

The above code has the four important regions and will detect the expressions for *,.,$ code will return 1 on success.

 
int matchstar(char *regexp, char *text)
{
if ( grep (regexp, text ) ){                  /*end of multi character match*/
return 1;
} else {
matchstar(regexp, text+1);
}
}

The code is matching multi character presence and checking it's end recursively.

 
int main(int argc, char **argv)
{
 FILE *file;
 char text[200];     
 char c;
 size_t size = 0;

 if (argc >= 3) {
  file = fopen(argv[1], "r");
  
  while(EOF != (c = getc(file))){
   if (c=='\n'){
    text[size] = '\0';
    int i = 0;
    for ( i = 0; i < size; i++){
     if (grep(argv[2], text + i)) {
      printf("%s\n", text);
      break;
     }
    }
    size = 0;
   } else {
    text[size] = c;
    size++;
   }
  }
 }
 return 0;
}

Compilation Instruction

[~/CODE]
13:04:24 $ gcc -o grep grep.c

[~/CODE]
13:04:36 $ gedit grep.c


./grep file_name regular_expression$

[~/CODE]
13:37:32 $ cat test
my
myname is
my name is khan
dog
cat is my pet
peterson is an awesome player

[~/CODE]
13:37:35 $ ./grep test m*k$
my
myname is
my name is khan
cat is my pet
peterson is an awesome player

[~/CODE]
13:37:39 $ ./grep test dog$
dog

[~/CODE]
13:37:43 $ ./grep test awsome$

[~/CODE]
13:37:49 $ ./grep test aw.some$
peterson is an awesome player



:-fat0ss

nmap scans : some basics over LAN

nmap : nmap(network mapper) is a network scanning tool written by Gordon Lyon and used to scan port, IP, operating systems,services on computer network

Here, I List some useful set of nmap scans over LAN :

1)ping a range of IP addresses
nmap -sP 192.168.1.100-254

-sP: Ping Scan

2)list all opened ports on host
nmap -p 1-65535 --open IP

-p : port range
--open: Only show open
Where IP may be a single IP, a hostname or a subnet

3)Get info about remote host ports and OS detection
nmap -sS -P0 -sV -O IP

Where IP may be a single IP, a hostname or a subnet
-sS TCP SYN scanning (also known as half-open, or stealth scanning)
-P0 option allows you to switch off ICMP pings.
-sV option enables version detection
-O flag attempt to identify the remote operating system
Other option:
-A option enables both OS fingerprinting and version detection
-v use -v twice for more verbosity.

Useful Links:

* Installation Guide
* Document of Nmap
* Books on Nmap


:-fat0ss

freopen() : change Mode back

Yesterday, I found a particular problem with the C function freopen(), so I decide to share it with you

Problem is with the function freopen(); the function change the role of the usual stdin, stdout for the file operation.

Problem : if I use freopen, how can I get the original stdout (or stdin) back?
FOF, this is not a good way to doing the file handling and my suggestion is always use your own explicit output (or input) stream variable, for the file operation.

Solution : solution of this particular problem is given by using dup, dup2 , dup and dup2 are the system call for duplicating underling file descriptor, which are present in UNIX/LINUX but not necessary any where else.

Reference:
http://c-faq.com/stdio/undofreopen.html
http://c-faq.com/stdio/rd.kirby.c

:-fat0ss