7. Flush policy

###Automatic flush spdlog by default lets the the underlying libc flush whenever it sees fit to achieve good performance.

Use the boolean argument force_flush to indicate the flush to disk policy:

• false (default): let the underlying libc flush whenever it sees fit - recommended for maximum performance at the cost of delayed writes to the disk, and hence the risk of losing log entries if the application crashes.

• true: spdlog will flush to disk on every log call - recommended if losing log entries upon a crash is not tolerable, even at the cost of performance.

The following example sets the file logger to automatic flushing by setting the last force_flush argument to true:

auto my_logger = 
    spdlog::rotating_logger_st("my_logger", "filename", 1024*1024*10, 5, true);

###Manual flush You can use the logger->flush() function to instruct a logger to flush its contents. The logger will in turn call the flush() function on each of the underlying sinks.

Important: logger->flush() is always synchronous opertion, even if the logger is in async mode! In fact, in async mode it might take long time since spdlog will first wait for the queue to drain before flushing.

Severity based flush

You can set the minimum log level that will trigger automatic flush

For example, this will trigger flush whenever errors or more severe messages are logged:

my_logger->flush_on(spdlog::level::err); 

Interval based flush (async mode only)

In asynchronous logging mode only, spdlog supports setting the flush interval in milliseconds.

Set the flush_interval_ms in the set_asynch_mode(..) call, to turn on periodic flush.

For example, turn on periodic flush with interval of 2 seconds:

spdlog::set_async_mode(q_size, spdlog::async_overflow_policy::block_retry,
                       nullptr,
                       std::chrono::seconds(2));