How to use Boost::log not to rewrite the log file?

Question

Below is the simple example of using boost::log to write log,

#include <boost/log/trivial.hpp>

namespace logging = boost::log;

logging::add_file_log("sample.log")->set_filter(
    logging::trivial::severity >= logging::trivial::info
);

BOOST_LOG_TRIVIAL(info) << "log content";

Every time run logging::add_file_log("sample.log") would rewrite the log file -- erase the original stuff and write new log. So it can't be used for a multi-process-one-log-file system. How do I set not to rewrite the file?

Edit:

I wrap this boost::log in a dll and attempt to let other exe files to call it.

Answer 1

You can pass an openmode to the setup function:

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#include <boost/log/trivial.hpp>
#include <boost/log/utility/setup.hpp>
#include <random>

namespace logging = boost::log;
namespace logkw = logging::keywords;

int main()
{
    logging::add_file_log("sample.log", logkw::open_mode = std::ios::app)
        ->set_filter(                                            //
            logging::trivial::severity >= logging::trivial::info //
        );

    std::mt19937 mt(std::random_device{}());

    BOOST_LOG_TRIVIAL(info) << "log content " << std::uniform_int_distribution(5,50)(mt);
}

Prints e.g.

log content 33
log content 14
log content 39
log content 39
log content 46