C equivalent of autoflush (flush stdout after each write)?

I haven't done this, but _IOLBF would be the right answer.

$ 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.

Try setvbuf(stdout, NULL, _IONBF, 0). It changes stdout to unbuffered (_IONBF) mode.