write error and critical logs to stderr

[tmplt]

In a project I work on I use spdlog to report errors to the user. To match the behavior of other cli-programs, I want that output written to stderr. Could level::{err,critical} logging be changed to write to stderr, or could a define perhaps be exposed in tweakme.h?

Cheers.

[gabime]

You can add an additional stderr sink with its level to level::error.
This way your logger would have 2 sinks with different levels.

[tmplt]

The wiki seems to lack the level-setting part, unless I missed it completely. Could you show how this would be done?

[gabime]

Just call set_level(..) on any sink:

sink->set_level(spdlog::level::err);

[tmplt]

Ah, thanks.

I've come thus far:

auto setup_logger()
{
    std::vector<spdlog::sink_ptr> sinks;

    auto stdout_sink = spdlog::sinks::stdout_sink_mt::instance();
    stdout_sink->set_level(spdlog::level::info);

    auto stderr_sink = spdlog::sinks::stderr_sink_mt::instance();
    auto color_stderr_sink = std::make_shared<spdlog::sinks::ansicolor_sink>(stderr_sink);
    color_stderr_sink->set_level(spdlog::level::err);

    sinks.push_back(stdout_sink);
    sinks.push_back(color_stderr_sink);

    auto logger = std::make_shared<spdlog::logger>("main", std::begin(sinks), std::end(sinks));
    logger->set_pattern("%l: %v");

    return logger;
}

As wanted, error logs are now printed to stderr, but they are also printed to stdout. Can I assign stdout_sink to print everything except for error and critical logs, which color_stderr_sink handles?

On a side note: if I want stdout_sink to be quiet as a default (level::off, I presume), would logger->set_level(spdlog::level::info) affect both sinks?

[tmplt]

To answer my own questions:

logger->set_level(spdlog::level::info); does affect both sinks.

And I gather from details/logger_impl.h that it is not possible to assign a singular level or range of levels to a sink: stdout_sink prints err-logs since level::err>level::info. I'm quite tired, so I might have missed someting going through the library. But if I got it all right, would something like sink::limit_level() be a good addition?

[gabime]

Yes, you got it right, but note that if for example the logger has "info" level then debug messages won't get forwarded by the logger to the underlying sinks, regardless of their own level.

I am not sure that limit_level() would worth the extra cost just for this rare use case though.. Maybe you could implement a custon sink that wraps those 2 sinks by forwarding the messages according to the level. Should be easy I think.. here is an example of such sink.

[tmplt]

On 7 January 2017 10:51:25 CET, Gabi Melman ***@***.***> wrote: Yes, you got it right, but note that if for example the logger has "info" level then debug messages won't get forwarded by the logger to the underlying sinks, regardless of their own level. I am not sure that ```limit_level()``` would worth the extra cost just for this rare use case though.. Maybe you could implement a custon sink that wraps those 2 sinks by forwarding the messages accoriding to the level. should be easy I think. -- You are receiving this because you authored the thread. Reply to this email directly or view it on GitHub: #345 (comment)

Alright, makes sense. A custom sink does sound like a better idea. A singular if-statement should probably suffice. I'll see about writing and posting the solution here.

[tmplt]

The log message level is available in spdlog::details::log_msg, so:

class split_sink : public spdlog::sinks::sink {
    void log(const details::log_msg &msg) override
    {
        if (msg.level <= spdlog::level::warn)
            std::cout << msg.formatted.str();
        else
            std::cerr << msg.formatted.str();
    }

    void flush()
    {
        std::cout << std::flush;
    }
};

is what I came up with.
I now set the logger up with:

auto setup_logger()
{
    auto sink = std::make_shared<split_sink>();
    auto logger = std::make_shared<spdlog::logger>("main", sink);

    logger->set_pattern("%l: %v");
    logger->set_level(spdlog::level::err); // as default

    return logger;
}

Thanks for your help!

[gabime]

Looks good. Dont forget protect with mutex if you need thread safety.