stderrsink for error-level, stdout sink for other levels

[raphaelzulliger]

It seems to me like a standard use case to use stdout and stderr sinks in an application and wanting that all error logs are going into the stderr sink, while all other go into the stdout sink.

I was not able to find any docs explaining that SpdLog supports "sinks per level" or the like. As a workaround, I can create two loggers in my code, an "error only"-logger and an "all other messages"-logger configure them with stderr and stdout sinks respectively and use the former for all error messages and the latter for the rest.

But since it looks to me like the "most standard use case", I feel like I'm doing something wrong here... Any hints?

Thanks!

[gabime]

spdlog supports setting level per sink

[raphaelzulliger]

Either there is a misunderstanding or I don't understand the proposed solution. Let me show my "problem" by an example. Assume we have:

    auto stdout_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
    stdout_sink->set_level(spdlog::level::trace);

    auto stderr_sink = std::make_shared<spdlog::sinks::stderr_color_sink_mt>();
    stderr_sink->set_level(spdlog::level::err);

    spdlog::logger logger("multi_sink", {stdout_sink, stderr_sink});
    // enable all logs
    logger.set_level(spdlog::level::trace);

    // NOK unfortunately, appears on stderr AND stdout:
    logger.error("ERROR");
    // OK: Only on stdout
    logger.warn("WARNING");
    // OK: Only on stdout
    logger.trace("TRACE");

The following happens:

xxx:~/spdlog_minimal$ ./build/bin/SpdLog_Minimal > /dev/null
[2021-03-04 10:18:40.020] [multi_sink] [error] ERROR

That's cool.

xxx:~/spdlog_minimal$ ./build/bin/SpdLog_Minimal 2> /dev/null
[2021-03-04 10:18:41.584] [multi_sink] [error] ERROR
[2021-03-04 10:18:41.584] [multi_sink] [warning] WARNING
[2021-03-04 10:18:41.584] [multi_sink] [trace] TRACE

This case is less cool. I'd say we rather want that "errors" are only on stderr, not on stdout. From a logical point of view, what I'd need is something like this:

// whatever the `set_level`, do *ignore* `spdlog::level::err`
stdout_sink->set_level_ignore(spdlog::level::err);

Do I have wrong expectations? Is it "normal" to output errors on stdout AND stderr?

[tt4g]

The spdlog level filter will pass all log messages above the configured level.
If you want to record only a specific log level, create a custom sync.

The following issues for your help.

• Multiple loggers, sharing the same output sinks but using individual log levels for each sink #1357
• Using spdlog::details::log_msg in spdlog::sinks::sink interface. #1408

[raphaelzulliger]

I see, that may help with my use case. Thx!