如果为负数，即size < 0，就表示出错了，退出程序。
接着验证日志缓冲区可读内容的大小，即调用Android::getLogReadableSize函数：
/* returns the readable size of the log's ring buffer (that is, amount of the log consumed) */ static int getLogReadableSize(int logfd) { return ioctl(logfd, LOGGER_GET_LOG_LEN); }
如果返回负数，即readable < 0，也表示出错了，退出程序。接下去的printf语句，就是输出日志缓冲区的大小以及可读日志的大小到控制台去了。
继续看下看代码，如果执行logcat命令的目的是清空日志或者获取日志的大小信息，则现在就完成使命了，可以退出程序了：
if (getLogSize) { return 0; } if (clearLog) { return 0; }
否则，就要开始读取设备文件的日志记录了：
android::readLogLines(devices); 至此日志设备文件就打开并且初始化好了，下面，我们继续分析从日志设备文件读取日志记录的操作，即readLogLines函数。
三. 读取日志设备文件。
读取日志设备文件内容的函数是readLogLines函数：
static void readLogLines(log_device_t* devices) { log_device_t* dev; int max = 0; int ret; int queued_lines = 0; bool sleep = true; int result; fd_set readset; for (dev=devices; dev; dev = dev->next) { if (dev->fd > max) { max = dev->fd; } } while (1) { do { timeval timeout = { 0, 5000 /* 5ms */ }; // If we oversleep it's ok, i.e. ignore EINTR. FD_ZERO(&readset); for (dev=devices; dev; dev = dev->next) { FD_SET(dev->fd, &readset); } result = select(max + 1, &readset, NULL, NULL, sleep ? NULL : &timeout); } while (result == -1 && errno == EINTR); if (result >= 0) { for (dev=devices; dev; dev = dev->next) { if (FD_ISSET(dev->fd, &readset)) { queued_entry_t* entry = new queued_entry_t(); /* NOTE: driver guarantees we read exactly one full entry */ ret = read(dev->fd, entry->buf, LOGGER_ENTRY_MAX_LEN); if (ret < 0) { if (errno == EINTR) { delete entry; goto next; } if (errno == EAGAIN) { delete entry; break; } perror("logcat read"); exit(EXIT_FAILURE); } else if (!ret) { fprintf(stderr, "read: Unexpected EOF!\n"); exit(EXIT_FAILURE); } entry->entry.msg[entry->entry.len] = '\0'; dev->enqueue(entry); ++queued_lines; } } if (result == 0) { // we did our short timeout trick and there's nothing new // print everything we have and wait for more data sleep = true; while (true) { chooseFirst(devices, &dev); if (dev == NULL) { break; } if (g_tail_lines == 0 || queued_lines <= g_tail_lines) { printNextEntry(dev); } else { skipNextEntry(dev); } --queued_lines; } // the caller requested to just dump the log and exit if (g_nonblock) { exit(0); } } else { // print all that aren't the last in their list sleep = false; while (g_tail_lines == 0 || queued_lines > g_tail_lines) { chooseFirst(devices, &dev); if (dev == NULL || dev->queue->next == NULL) { break; } if (g_tail_lines == 0) { printNextEntry(dev); } else { skipNextEntry(dev); } --queued_lines; } } } next: ; } } 由于可能同时打开了多个日志设备文件，这里使用select函数来同时监控哪个文件当前可读：
do { timeval timeout = { 0, 5000 /* 5ms */ }; // If we oversleep it's ok, i.e. ignore EINTR. FD_ZERO(&readset); for (dev=devices; dev; dev = dev->next) { FD_SET(dev->fd, &readset); } result = select(max + 1, &readset, NULL, NULL, sleep ? NULL : &timeout); } while (result == -1 && errno == EINTR); 如果result >= 0，就表示有日志设备文件可读或者超时。接着，用一个for语句检查哪个设备文件可读，即FD_ISSET(dev->fd, &readset)是否为true，如果为true，表明可读，就要进一步通过read函数将日志读出，注意，每次只读出一条日志记录：
for (dev=devices; dev; dev = dev->next) { if (FD_ISSET(dev->fd, &readset)) { queued_entry_t* entry = new queued_entry_t(); /* NOTE: driver guarantees we read exactly one full entry */ ret = read(dev->fd, entry->buf, LOGGER_ENTRY_MAX_LEN); if (ret < 0) { if (errno == EINTR) { delete entry; goto next; } if (errno == EAGAIN) { delete entry; break; } perror("logcat read"); exit(EXIT_FAILURE); } else if (!ret) { fprintf(stderr, "read: Unexpected EOF!\n"); exit(EXIT_FAILURE); } entry->entry.msg[entry->entry.len] = '\0'; dev->enqueue(entry); ++queued_lines; } }