setvbuf using IOFBF max buffer size

[tomas@tuxteam.de]

On Sat, May 03, 2025 at 11:11:48PM -0400, violette via Libc-help wrote:
> Hello mailing list people!

Hi, Violette

> I am trying to print a whole screen worth of text using `printf`.
> I have set my terminal as full line buffer using
> `setvbuf(stdout, buf, _IOFBF, sizeof(buf)` (with a buffer size of
> `UINT_16_MAX`). I am using glibc 2.41 and gcc.
> 
> Apparently, it buffers and has to print a few times times before
> `fflush` is even called. It also seems that it writes to screen
> the same number of time as when the buffer size is set to `BUFSIZ`.

If I'm reading the man page correctly, this is expected: you set
the mode to `_IOFBUF', i.e. "fully buffered": therefore, the buffer
will only get flushed once it gets full.

This matches the behaviourI think you are describing above: you
have to call printf a couple of times and then, all the stuff
collected in the buffer is sent out.

Or, perhaps I'm misunderstanding you? Could you explain a bit more
what behaviour you expect?

Cheers
-- 
tomás
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