setvbuf(stdout, NULL, _IONBF, 0) を試す . stdout を変更します アンバッファリング (_IONBF ) モード。
私はこれを行っていませんが、_IOLBF が正しい答えです。
$ man setvbuf
NAME
setvbuf — assign buffering to a stream
SYNOPSIS
#include <stdio.h>
int setvbuf(FILE *restrict stream, char *restrict buf, int type,
size_t size);
DESCRIPTION
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements described
here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 defers to the ISO C standard.
The setvbuf() function may be used after the stream pointed to by
stream is associated with an open file but before any other operation
(other than an unsuccessful call to setvbuf()) is performed on the
stream. The argument type determines how stream shall be buffered, as
follows:
* {_IOFBF} shall cause input/output to be fully buffered.
* {_IOLBF} shall cause input/output to be line buffered.
* {_IONBF} shall cause input/output to be unbuffered.
setbuf() と setvbuf() を見てください。