int
_IO_setvbuf (FILE *fp, char *buf, int mode, size_t size)
{
  int result;
  CHECK_FILE (fp, EOF);
  _IO_acquire_lock (fp);
  switch (mode)
    {
    .
    .
    .
    case _IONBF:
      fp->_flags &= ~_IO_LINE_BUF;
      fp->_flags |= _IO_UNBUFFERED;
      buf = NULL;
      size = 0;
      break;
    default:
      result = EOF;
      goto unlock_return;
    }
  result = _IO_SETBUF (fp, buf, size) == NULL ? EOF : 0;

unlock_return:
  _IO_release_lock (fp);
  return result;
}
libc_hidden_def (_IO_setvbuf)

weak_alias (_IO_setvbuf, setvbuf)








## Summary

setvbuf(stdout, 0, 2, 0)
|_ _IO_setvbuf(stdout, 0, 2, 0) 
   |_ _IO_new_file_setbuf(stdout, NULL, 0)
      |_ _IO_default_setbuf(stdout, NULL, 0)
         |_ _IO_new_file_sync(stdout) (1)
            |_ _IO_do_flush(stdout) (2)
               |_ _IO_do_write(stdout, stdout->_IO_write_base, stdout->_IO_write_ptr - stdout->_IO_write_base)
                  |_ new_do_write(stdout, stdout->_IO_write_base, stdout->_IO_write_ptr - stdout->_IO_write_base) (3)
                     |_ _IO_new_file_write(stdout, stdout->_IO_write_base, stdout->_IO_write_ptr - stdout->_IO_write_base) (4)
                        |_ write(stdout, stdout->_IO_write_base, stdout->_IO_write_ptr - stdout->_IO_write_base)

INIT: _flags = "input" _flags & ~_IO_LINE_BUF (_IO_LINE_BUF=0x0200)
      _flags = _flags | _IO_UNBUFFERED (_IO_UNBUFFERED=0x0002)

Conditions:
FROM (1) -> (2): stdout->_IO_write_ptr > stdout-> _IO_write_base
FROM (2) -> (3): stdout->_mode <= 0
FROM (3) -> (4): stdout->_flags & _IO_IS_APPENDING != 0 (_IO_IS_APPENDING=0x1000)

_IO_buf_base = NULL

## Test

#include <stdio.h>


int main(){
    // libc 2.38
    long long new_IO_write_base;

    setvbuf(stdin, 0, 2, 0);
    setvbuf(stdout, 0, 2, 0);
    setvbuf(stderr, 0, 2, 0);

    new_IO_write_base = (long long)stdout->_IO_write_base ^ 0x43; //set the last byte of _IO_write_base to 0

    stdout->_flags = 0xfbad1000;
    stdout->_IO_write_base = (char *)new_IO_write_base;
    stdout->_IO_buf_base = NULL;
    stdout->_mode = 0;

    // trigger
    setvbuf(stdout, 0, 2, 0);    
    return 0;
}