什么时候需要使用stderr？ 重要错误还是所有错误？

Question

It's a basic question, but i can't find a good answer.

For example, if i use malloc , i check the malloc and if i saw an error i can do :

fprintf(stderr, "Message");

It's an important error message, the output is on stderr .

Now if i have a program who ask numbers between 1 an 5. The program ask to the user until he enters a good number.

If the user enter 10 for example, is it good things to redirect to error message to stderr ?

fprintf(stderr, "Message, wrong number, try again");

Or it's a normal error, and the message should go on stdout ?

Answer 1

If you'd ever want to redirect certain types of errors independently of all other output, send them to stderr .

Otherwise, I think that it's important to be consistent throughout your program, so decide on what goes where and stick to it.

Answer 2

It is up to you how and when you decide to use it.

A common application is to separate the program output from any sort of notification that is not part of the output, be it errors or just progress so the user knows the program didn't hang or anything.

I have, for instance, a code that fetches several tables of data in XML format from a web server, converts and generates a SQL dump as output, so I can pass it directly to the command mysql , as in my_importer | mysql -options my_importer | mysql -options . While the operation running, the program outputs feedback to stderr , just so I know its working and what it has imported so far.

Answer 3

Remember that stderr is not really about errors, but about diagnostics. Any diagnostic messages (including debug mode notifications and fancy progress bars) should go to stderr, while all the interactions, be it user or another program, should go on stdout. Also stderr is normally connected to a terminal, while stdout may be part of a shell pipe (thus invisible). And there is no way to connect stderr to next program separately from stdout.

Regarding your case, the message about wrong number is obviously the part of interaction, so use stdout. It is bad idea to move active conversation to other stream. But if it is not intended that user/peer process should read that and react, but is just info for someone diagnosing/debugging the program, then use stderr.

Answer 4

For an interactive program like the one you are discussing, I don't think it makes a lot of difference. If the program cannot be used with standard output and/or standard error redirected, you might as well simplify and use standard output for prompts and diagnostics as well.

Having said that, there are many scenarios where redirection makes sense, or even is imperative for some use cases to work correctly. If there is any imaginable way your program could be used that way, it is extremely important to make sure that standard output only contains actual production output, and everything else goes to standard error.