Boost.Log-如何配置文本接收器后端以附加到旋转文件

Question

I have a sinks::text_file_backend sink. Say I already have a few rotated log files:

myLog001.log, myLog002.log and so on

I want the sink to keep writing to the last rotated file - myLog002.log, append to its contents and continue rotation from there on.

I have only managed to find keywords::open_mode = append but this only appends on top of the existing myLogX files, making them larger and of course very hard to read.

Can this be done in Boost.Log?

Answer 1

That functionality is built in to the text sink, and the documentation includes an example to set the file-name pattern and rules for rotating at certain sizes and times:

// The function registers file sink in the logging library
void init_logging()
{
    boost::shared_ptr< logging::core > core = logging::core::get();

    boost::shared_ptr< sinks::text_file_backend > backend =
        boost::make_shared< sinks::text_file_backend >(
            // file name pattern
            keywords::file_name = "file_%5N.log",
            // rotate the file upon reaching 5 MiB size...
            keywords::rotation_size = 5 * 1024 * 1024,
            // ...or at noon, whichever comes first
            keywords::time_based_rotation = sinks::file::rotation_at_time_point(12, 0, 0)
        );

    // Wrap it into the frontend and register in the core.
    // The backend requires synchronization in the frontend.
    typedef sinks::synchronous_sink< sinks::text_file_backend > sink_t;
    boost::shared_ptr< sink_t > sink(new sink_t(backend));

    core->add_sink(sink);
}

There is apparently no way to make the library append to existing files with this setup. You should call backend->scan_for_files(); prior to constructing sink , as shown under the "Managing rotated files" heading in the documentation, but that only prevents the library from overwriting previous logs before they're due for cleanup.

When this topic arose on a development mailing list in February 2013, the library's author explained that adding support for appending would be a nontrivial change that couldn't be made under the current design.

Answer 2

You have to specify the open_mode before use the text file. By default Boost.Log will use std::ios_base::trunc|std::ios_base::out as the open mode which obviously will truncate the old log file.

You can create text_file_backend instance with the following parameters:

    {
        boost::shared_ptr<sinks::text_file_backend> backend =
            boost::make_shared<sinks::text_file_backend>(
                keywords::file_name = logger_file_path,
                keywords::open_mode = std::ios_base::app|std::ios_base::out,
                keywords::rotation_size = 5 * 1024 * 1024,
                keywords::time_based_rotation = sinks::file::rotation_at_time_point(12, 0, 0));
        // Wrap it into the frontend and register in the core.
        // The backend requires synchronization in the frontend.
        typedef sinks::synchronous_sink<sinks::text_file_backend> sink_t;
        boost::shared_ptr<sink_t> sink(new sink_t(backend));
        sink->set_formatter(logFmt);
        core->add_sink(sink);
    }