Question

I have a programme which is writing results to a file and I would like to read in real-time from that file. It is a normal text file and external programme always write a whole line. I need to run it just on a Linux system.

int last_read = 0;
int res;
FILE *file;
char *buf;
char *end = buf + MAXLINE - 1;
int c;
int fileSize;
char *dst;
while (external_programme_is_running()) {
   file = fopen(logfile, "r"); //without opening and closing it's not working
   fseek(file, 0, SEEK_END);
   fileSize = ftell(file);
   if (fileSize > last_read) {
      fseek(file, last_read, SEEK_SET);
      while (!feof(file)) {
        dst = buf;
        while ((c = fgetc(file)) != EOF && c != '\n' && dst < end)
            *dst++ = c;
        *dst = '\0';
        res = ((c == EOF && dst == buf) ? EOF : dst - buf);

         if (res != -1) {
            last_read = ftell(file);
            parse_result(buf)
         }
      }
  }
  fclose(file);
}

Is this a correct approach? Or would it be better to check the modification time and then open the file? Is is possible that reading would crash in case that the file would be modified at the very same time?

Answer 1

To avoid the need to close, re-open, and re-seek for every loop iteration, call clearerr on the stream after reading EOF.