Intermediate C : String search in a large file

Question

I'm writing a 'C' code that stores the TCP payload of captured packets in a file (payload of each packet is separated by multiple "\n" characters). Using C, is it possible to search for a particular string in the file after all the packets are captured?

P.S : The file can be very large, depending upon the number of captured packets.

Why you want to search the file after being written, the incoming packets are being written into a buffer, I guess, can't you just search that? — Sourav Ghosh, Dec 14 '17 at 09:36
Aren't your TCP packets allowed to contain multiple `\n` bytes in a row as payload? — Gerhardh, Dec 14 '17 at 09:42
@SouravGhosh The buffer contains the entire packet (with the ethernet, ip and tcp headers) I intend to extract the URL and print it. — Nnn, Dec 14 '17 at 09:51
@nivedithanatarajan I won't write your code for you. Look at the answer below which might be what you want. — Jabberwocky, Dec 14 '17 at 09:56
I am not sure if that is your requirement to write the data to file , you can buffer it and the buffer itself you could check if the required string available using memcmp ? — cslrnr, Dec 14 '17 at 14:35
@nivedithanatarajan Then your idea separating packets with line breaks wont't work very well.. — Gerhardh, Dec 15 '17 at 08:27

score 2 · Accepted Answer · edited Dec 14 '17 at 10:14

Read the file line by line and search using strstr.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
FILE * fp;
char * line = NULL;
size_t len = 0;
ssize_t read;
char * pos;
int found = -1;

fp = fopen("filename", "r");
if (fp == NULL)
    exit(EXIT_FAILURE);

while ((read = getline(&line, &len, fp)) != -1) 
   {
      pos = strstr(line,"search_string");
      if(pos != NULL)
      {
          found = 1;
          break;
      }
   }

if(found==1)
    printf("Found");
else
    printf("Not Found");

fclose(fp);

if (line)
    free(line);

exit(EXIT_SUCCESS);
}

Intermediate C : String search in a large file

1 Answers1