stderr vs stdout

[JoshG]

What is the different between the two?

fprintf(stdout, "Hello there\n");

Produces the same output as

fprintf(stderr, "Hello there\n");

Is there a practical use for stderr? Is it logged or something? Or I am thinking maybe it is suppose to be different but depends on the OS, does Linux use stderr the same way as DOS? Or does it log it or something?

[Prelude]

Within the confines of the C standard, stdout is fully buffered and stderr is not buffered. In other words, stdout will flush the buffer when the programmer explicitly asks for it or when it is most convenient, stderr writes the message immediately. Basically, stdout should be used for regular messages and stderr for diagnostic (error) messages.

-Prelude

[JoshG]

Why should stderr be used for error messages? I still do not understand.

[moi]

Originally posted by JoshG
Why should stderr be used for error messages? I still do not understand.

many users (smarter ones, not windoze idiots) will redirect stdout and stderr to different places at some times, and it doesn't help them if your error messages are not logically organized as such.

[Prelude]

>Why should stderr be used for error messages?
Diagnostic messages are commonly used for debugging, when debugging you want to change the effect of the program as little as possible from the production version. By using unbuffered output, you don't effect the flow of data nearly as much as you would by using buffered output, and thus gain a more effective debug version of the program. Of course, the obvious benefits of the error message being printed right away are understandable.

-Prelude

[JoshG]

Thanks, I was just wondering.

[master5001]

You can pipe stderr output to a file. Also you can redirect both stdout and stderr.

FILE *myerr = freopen("error.log", "wb", stderr);