1. libio/sebuf.c定義了sebbuf函式:
   

   void setbuf (FILE *fp, char *buf)
   {
     _IO_setbuffer (fp, buf, BUFSIZ);
   }

2. libio/iosetbuffer.c定義了_IO_setbuffer函式:

   void _IO_setbuffer (FILE *fp, char *buf, size_t size)
   {
     CHECK_FILE (fp, );
     _IO_acquire_lock (fp);
     fp->_flags &= ~_IO_LINE_BUF;
     if (!buf)
       size = 0;
     (void) _IO_SETBUF (fp, buf, size);
     if (_IO_vtable_offset (fp) == 0 && fp->_mode == 0 && _IO_CHECK_WIDE (fp))
       /* We also have to set the buffer using the wide char function.  */
       (void) _IO_WSETBUF (fp, buf, size);
     _IO_release_lock (fp);
   }

   除了鎖定FILE指標外，核心程式碼就是_IO_SETBUF和用於多位元組環境的_IO_WSETBUF,最後解析，重點介紹下_IO_SETBUF

3. libio/libioP.h定義了_IO_SETBUF和_IO_WSETBUF:

   typedef FILE* (*_IO_setbuf_t) (FILE *, char *, ssize_t);
   #define _IO_SETBUF(FP, BUFFER, LENGTH) JUMP2 (__setbuf, FP, BUFFER, LENGTH)
   #define _IO_WSETBUF(FP, BUFFER, LENGTH) WJUMP2 (__setbuf, FP, BUFFER, LENGTH)

       #define JUMP2(FUNC, THIS, X1, X2) (_IO_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)

       #define WJUMP2(FUNC, THIS, X1, X2) (_IO_WIDE_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)

       struct _IO_FILE_plus
       {
          FILE file;
          const struct _IO_jump_t *vtable;
        };

       1）_IO_JUMPS_FUNC(THIS)是通過_IO_CAST_FIELD_ACCESS將setbuf函式的引數fp轉成struct _IO_FILE_plus型別，

並返回struct _IO_FILE_plus的vtable變數,通過vtable呼叫__setbuf函式(JUMP2的引數),

      2）通過JUMP_INIT (setbuf, _IO_default_setbuf)定義了vtable預設的__setbuf函式

   4. libio/genops.c定義了_IO_default_setbuf及其呼叫的_IO_setb,_IO_setb設定了setbuf函式的引數fp的_IO_buf_base等buf變數:

        FILE * _IO_default_setbuf (FILE *fp, char *p, ssize_t len)
        {
            if (_IO_SYNC (fp) == EOF)
                   return NULL;
            if (p == NULL || len == 0)
              {
                    fp->_flags |= _IO_UNBUFFERED;
                    _IO_setb (fp, fp->_shortbuf, fp->_shortbuf+1, 0);
             }
            else
              {
                    fp->_flags &= ~_IO_UNBUFFERED;
                    _IO_setb (fp, p, p+len, 0);
              }
               fp->_IO_write_base = fp->_IO_write_ptr = fp->_IO_write_end = 0;
               fp->_IO_read_base = fp->_IO_read_ptr = fp->_IO_read_end = 0;
              return fp;
        }

       void _IO_setb (FILE *f, char *b, char *eb, int a)
      {
          if (f->_IO_buf_base && !(f->_flags & _IO_USER_BUF))
            free (f->_IO_buf_base);
          f->_IO_buf_base = b;
          f->_IO_buf_end = eb;
          if (a)
            f->_flags &= ~_IO_USER_BUF;
          else
            f->_flags |= _IO_USER_BUF;
    }