Stdout buffer in c flush itself unexpectedly-CodePudding

I have just learned about stream buffering in C. As I understand, printf() in C is buffered and it only prints when it hits a new line, the buffer is full or when we flush stdout manually. Then I try these lines of code:

#include <stdio.h>

int main() {
    printf("Hello world");
    while(1);
}

Theoretically, the console will be received nothing, since Hello world is still in the buffer. But for some reasons, my console still show this string. Why is it like that?

EDIT: I am using the command prompt in Windows 10.

CodePudding user response：

printf() in C is buffered and it only prints when it hits a new line

Not quite.

Streams are commonly 1 of 3 modes: unbuffered, fully buffered, line buffered. Often stdout is line buffered, flushing data when printing a '\n', its internal buffer is full or due to explicit commands like fflush(stdout).

Yet C specifies about these 3 modes: Support for these characteristics is implementation-defined. C17dr § 7.21.3 3.

Thus both seeing output and not seeing output are compliant for OP's code.

To ensure output is seen, flush. Otherwise, live with implementation defined behavior.

printf("Hello world");
fflush(stdout); // add
while(1);

CodePudding user response：

I'm not sure what part of the C standard you were reading, but the behavior you are seeing in Windows is correct according to Microsoft's documentation of their C runtime library:

Files opened using the stream routines are buffered by default. The stdout and stderr functions are flushed whenever they are full or, if you are writing to a character device, after each library call.

That documentation also links to a Microsoft-specific setvbuf function you can use to control the buffering if you want. I generally just let the runtime library use its default settings and then I call fflush(stdout) explicitly at every place where I want to ensure the output is flushed.