c/c fprintf function with param stdout or stderr , why the output content sort is different the ca-CodePudding

My environment is Debian GNU/Linux 11 .

fprintf function with param stdout or stderr , output content order is not expected .

int main() {
    std::cout << "Hello, World!" << std::endl;
    fprintf(stderr, "22222\n");
    fprintf(stdout, "111\n");
    printf("3333 \n");
    printf("44444 \n");
    return 0;
}

Run many times , and got many result :

//①
22222
Hello, World!
111
3333 
44444

//②
Hello, World!
111
3333 
44444 
22222

What's the reason ? Or , I waht to understand the phenomenon , what knowledge i need ?

On my understand , the output log should like this:

//③
Hello, World!
22222
111
3333 
44444

About the two output log that ①/② , I don't understand .

I think log ③ is right , but it not appear ,that make me wonder .

I what to understand the phenomenon , what knowledge i need ?

CodePudding user response：

The output is never like ② because output to stderr isn't buffered so 22222 will be flushed immediately and would be before any other numbers. Output to stdout may be line-buffered (default on Linux) or full-buffered (default on Windows)

The output shouldn't be like ① either because you're already flushing with std::endl in the first line, hence Hello, World! would be printed first. That's why never use std::endl unless you really know what it's doing. Always use std::cout << "Hello, World!\n";. See "std::endl" vs "\n"

③ should be the correct output because on Linux it'll flush on every statement, and on Windows the final 3 writes to stdout are flushed at once at the end

Hello, World!
22222
111
3333 
44444

If there are any differences then there are some issues with your stdlib

By default the legacy and C streams are also synchronized so it's possible to mix both types of streams with some performance penalty. You can disable it like this

std::ios::sync_with_stdio(false);

to get better performance but now the Hello, World! string can be printed anywhere