Multiple loggers from different instances of the same application writing to the same file

[varkey98]

We have a library which uses spdlog. Inside the library, we only have a single logger writing to the file. Now there can be multiple instances of our library loaded into the memory by the application, all of them would be writing logs to the same file. How to program it?
This is what we do now, which results in logs from a single instance being written to the file:

  auto sink = std::make_shared<spdlog::sinks::rotating_file_sink_mt>(
      log_file, max_file_size, max_files);
  spdlog::init_thread_pool(spdlog::details::default_async_q_size, 1U);
  auto tp = spdlog::thread_pool();
  logger = std::make_shared<spdlog::async_logger>(
      service_name, sink.value(), tp,
      spdlog::async_overflow_policy::overrun_oldest);

  spdlog::initialize_logger(logger);
  logger->set_pattern("[%Y-%m-%d %T.%e %z] [%^%l%$] [%n] %v");
  logger->set_level(spdlog_level);
  spdlog::flush_every(flush_timeout);

This is the error getting output from the other instance which is not writing the logs:

[*** LOG ERROR #0103 ***] [2024-09-11 15:12:15] [random] async log: thread pool doesn't exist anymore

[tt4g]

[*** LOG ERROR #0103 ***] [2024-09-11 15:12:15] [random] async log: thread pool doesn't exist anymore

This error message reports that the thread pool used by the async logger has been deleted.
Please redesign your application so that the thread pool is not deleted.

spdlog/include/spdlog/async_logger-inl.h

Lines 33 to 49 in 2169a6f

	// send the log message to the thread pool
	SPDLOG_INLINE void spdlog::async_logger::sink_it_(const details::log_msg &msg){
	SPDLOG_TRY{if (auto pool_ptr = thread_pool_.lock()){
	pool_ptr->post_log(shared_from_this(), msg, overflow_policy_);
	}
	else {
	throw_spdlog_ex("async log: thread pool doesn't exist anymore");
	}
	}
	SPDLOG_LOGGER_CATCH(msg.source)
	}
	// send flush request to the thread pool
	SPDLOG_INLINE void spdlog::async_logger::flush_(){SPDLOG_TRY{auto pool_ptr = thread_pool_.lock();
	if (!pool_ptr) {
	throw_spdlog_ex("async flush: thread pool doesn't exist anymore");
	}

And writing to a single file from multiple processes is not safe: #937, #2951, #2633

[varkey98]

got it, thanks