I don't believe a process reading from a pipe/socketpair/whatever can enforce such constraints on a writing process (except using heavy hackery like ptrace()). While it might be possible to adjust the pipe buffer size, I'm not aware of any convention requiring standard C I/O to respect this.

In any case, stdbuf doesn't seem to help with this:

  $ ./a | stdbuf -i0 -- cat

  #include <stdio.h>
  #include <unistd.h>
  int main(void) {
   for (;;) {
    printf("n");
    usleep(100000);
   }
  }

I'm sorry, but I don't understand what you're meaning. The issue in your example is the output buffering of a, not the input buffering of cat. You'd need `stdbuf -o0 ./a | cat` there.