#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(int argc, char *argv[]) {
   char buf[10000];
   FILE *fh;

   if(2 != argc) {
      fputs("Usage: filter <log>\n", stderr);
      exit(1);
   }
   if(!(fh = fopen(argv[1], "r"))) {
      perror("Cannot open log");
      exit(1);
   }
   
   while(!fgets(buf, sizeof(buf), fh)) {
      static const size_t START_OFFSET = 50;
      size_t len = strlen(buf);
      char *endp;

      if('\n' != buf[len-1]) {
         fputs("WARNING: line did not fit in buffer, skipped\n", stderr);
         continue;
      }
      
      endp = buf + START_OFFSET;
      len  = 20;
      // To search for a blank after the field instead of using a fixed width
      // endp = strchr(buf + START_OFFSET, ' ');
      // len  = endp ? endp - (buf + START_OFFSET) : len - START_OFFSET; // careful with strchr()==NULL
      fwrite(buf + START_OFFSET, 1, len, stdout);
   }
}