OpenWrt 系统日志之logread - 谷动谷力

/* Suppose atomic memory read */
/* Max possible value for tail is shbuf->size - 1 */
cur = shbuf->tail;

/* Loop for logread -f, one pass if there was no -f */
do {
      unsigned shbuf_size;
      unsigned shbuf_tail;
      const char *shbuf_data;
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
      int i;
      int len_first_part;
      int len_total = len_total; /* for gcc */
      char *copy = copy; /* for gcc */
#endif
      if (semop(log_semid, SMrdn, 2) == -1)
         error_exit("semop[SMrdn]");

      /* Copy the info, helps gcc to realize that it doesn't change */
      shbuf_size = shbuf->size;
      shbuf_tail = shbuf->tail;
      shbuf_data = shbuf->data; /* pointer! */

      if (DEBUG)
         printf("cur:%u tail:%u size:%u\n",
                  cur, shbuf_tail, shbuf_size);

      if (!follow) {
         /* advance to oldest complete message */
         /* find NUL */
         cur += strlen(shbuf_data + cur);
         if (cur >= shbuf_size) { /* last byte in buffer? */
            cur = strnlen(shbuf_data, shbuf_tail);
            if (cur == shbuf_tail)
                  goto unlock; /* no complete messages */
         }
         /* advance to first byte of the message */
         cur++;
         if (cur >= shbuf_size) /* last byte in buffer? */
            cur = 0;
      } else { /* logread -f */
         if (cur == shbuf_tail) {
            sem_up(log_semid);
            fflush_all();
            sleep(1); /* TODO: replace me with a sleep_on */
            continue;
         }
      }

      /* Read from cur to tail */
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
      len_first_part = len_total = shbuf_tail - cur;
      if (len_total < 0) {
         /* message wraps: */
         /* [SECOND PART.........FIRST PART] */
         /*  ^data    ^tail ^cur    ^size */
         len_total += shbuf_size;
      }
      copy = xmalloc(len_total + 1);
      if (len_first_part < 0) {
         /* message wraps (see above) */
         len_first_part = shbuf_size - cur;
         memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
      }
      memcpy(copy, shbuf_data + cur, len_first_part);
      copy[len_total] = '\0';
      cur = shbuf_tail;
#else
      while (cur != shbuf_tail) {
         fputs(shbuf_data + cur, stdout);
         cur += strlen(shbuf_data + cur) + 1;
         if (cur >= shbuf_size)
            cur = 0;
      }
#endif