erlang - 为什么erlang io是:format output being lost and what do I need to do to restore it?

Question

我正在编写一个 gen_server，我们将其称为 gen_server_db，没有什么特别之处。它使用的库 (emysql) 在尝试查找数据库服务器(在 gen_server_db:init() 中)时可能会遇到连接失败。当我捕获异常并尝试将某些内容打印到控制台时，我得到了 zip.下面的代码示例中的 io:format 消息都无法到达控制台。我似乎记得很久以前听过一个原因，但我不记得为什么。

init(Args) ->
    % I've condensed the way I actually get this stuff to one line, but if I have a
    % database online I connect properly, so I know that I'm getting the Host, User, etc.  
    {Host, User, Password, DB, PoolSize} = application:get_env(gen_server_db, config),

    init_mysql_connection(gen_server_db_pool, PoolSize, User, Password, Host, DB),

    % When the net connection to the db is down, I never get here.
    ok.

init_mysql_connection(bgo_db_pool, PoolSize, User, Password, Host, DB) ->
    try
        emysql:add_pool(bgo_db_pool, PoolSize, User, Password, Host, 3306, DB, utf8)
    catch
        exit:failed_to_connect_to_database ->
            io:format("Cannot connect to the mysql database server.  Retrying in 1 sec.~n"),
            timer:sleep(1000),
            init_mysql_connection(bgo_db_pool, PoolSize, User, Password, Host, DB);
        Error:Reason ->
            io:format("Database connection error: ~p:~p~n", [Error, Reason]),
            1/0
    end.

Answer 1

所有通过io:format发送的I/O都会发送到当前group_leader正在运行的进程。然后，该进程负责使用 io:fwrite 打印消息。

如果您只想在编码时转储一些调试信息，可以使用 erlang:display/1