glibc 2.17的fread代码路径

 先看一下fread()吧

不玩儿虚的直接glibc搜代码，在libio/iofread.c里：

_IO_size_t
_IO_fread (buf, size, count, fp)
     void *buf;
     _IO_size_t size;
     _IO_size_t count;
     _IO_FILE *fp;
{
  _IO_size_t bytes_requested = size * count;
  _IO_size_t bytes_read;
  CHECK_FILE (fp, 0);
  if (bytes_requested == 0)
    return 0;
  _IO_acquire_lock (fp);
  bytes_read = _IO_sgetn (fp, (char *) buf, bytes_requested);
  _IO_release_lock (fp);
  return bytes_requested == bytes_read ? count : bytes_read / size;
}
libc_hidden_def (_IO_fread)

#ifdef weak_alias
weak_alias (_IO_fread, fread)

所以是那个_IO_sgetn()了。

这个_IO_sgetn在libio/genops.c里。

_IO_size_t
_IO_sgetn (fp, data, n)
     _IO_FILE *fp;
     void *data;
     _IO_size_t n;
{
  /* FIXME handle putback buffer here! */
  return _IO_XSGETN (fp, data, n);
}

_IO_XSGETN是个宏，见libioP.h

#define _IO_XSGETN(FP, DATA, N) JUMP2 (__xsgetn, FP, DATA, N)
#define JUMP2(FUNC, THIS, X1, X2) (_IO_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)

注意到这里面跳转到了一个叫做__xsgetn的符号那里。

在libioP.h定义有一个结构体，注意里面出现了__xsgetn：

struct _IO_jump_t
{
    JUMP_FIELD(size_t, __dummy);
    JUMP_FIELD(size_t, __dummy2);
    JUMP_FIELD(_IO_finish_t, __finish);
    JUMP_FIELD(_IO_overflow_t, __overflow);
    JUMP_FIELD(_IO_underflow_t, __underflow);
    JUMP_FIELD(_IO_underflow_t, __uflow);
    JUMP_FIELD(_IO_pbackfail_t, __pbackfail);
    /* showmany */
    JUMP_FIELD(_IO_xsputn_t, __xsputn);
    JUMP_FIELD(_IO_xsgetn_t, __xsgetn);
    JUMP_FIELD(_IO_seekoff_t, __seekoff);
    JUMP_FIELD(_IO_seekpos_t, __seekpos);
    JUMP_FIELD(_IO_setbuf_t, __setbuf);
    JUMP_FIELD(_IO_sync_t, __sync);
    JUMP_FIELD(_IO_doallocate_t, __doallocate);
    JUMP_FIELD(_IO_read_t, __read);
    JUMP_FIELD(_IO_write_t, __write);
    JUMP_FIELD(_IO_seek_t, __seek);
    JUMP_FIELD(_IO_close_t, __close);
    JUMP_FIELD(_IO_stat_t, __stat);
    JUMP_FIELD(_IO_showmanyc_t, __showmanyc);
    JUMP_FIELD(_IO_imbue_t, __imbue);
#if 0
    get_column;
    set_column;
#endif
};

这个JUMP_FILED就不展开了，其实就是定义了一堆函数指针，_IO_xsgetn_t是函数类型，__xsgetn是指针变量成员名。那glibc里搜一下_IO_jump_t实例就好了。在libio/fileops.c里面

const struct _IO_jump_t _IO_file_jumps =
{
  JUMP_INIT_DUMMY,
  JUMP_INIT(finish, _IO_file_finish),
  JUMP_INIT(overflow, _IO_file_overflow),
  JUMP_INIT(underflow, _IO_file_underflow),
  JUMP_INIT(uflow, _IO_default_uflow),
  JUMP_INIT(pbackfail, _IO_default_pbackfail),
  JUMP_INIT(xsputn, _IO_file_xsputn),
  JUMP_INIT(xsgetn, _IO_file_xsgetn),
  JUMP_INIT(seekoff, _IO_new_file_seekoff),
  JUMP_INIT(seekpos, _IO_default_seekpos),
  JUMP_INIT(setbuf, _IO_new_file_setbuf),
  JUMP_INIT(sync, _IO_new_file_sync),
  JUMP_INIT(doallocate, _IO_file_doallocate),
  JUMP_INIT(read, _IO_file_read),
  JUMP_INIT(write, _IO_new_file_write),
  JUMP_INIT(seek, _IO_file_seek),
  JUMP_INIT(close, _IO_file_close),
  JUMP_INIT(stat, _IO_file_stat),
  JUMP_INIT(showmanyc, _IO_default_showmanyc),
  JUMP_INIT(imbue, _IO_default_imbue)
};
libc_hidden_data_def (_IO_file_jumps)

所以实际调用_IO_file_xsgetn，位于fileops.c中。

_IO_size_t
_IO_file_xsgetn (fp, data, n)
     _IO_FILE *fp;
     void *data;
     _IO_size_t n;
{
  register _IO_size_t want, have;
  register _IO_ssize_t count;
  register char *s = data;

  want = n;

  if (fp->_IO_buf_base == NULL)
    {
      /* Maybe we already have a push back pointer.  */
      if (fp->_IO_save_base != NULL)
	{
	  free (fp->_IO_save_base);
	  fp->_flags &= ~_IO_IN_BACKUP;
	}
      _IO_doallocbuf (fp);
    }

  while (want > 0)
    {
      have = fp->_IO_read_end - fp->_IO_read_ptr;
      if (want <= have)
	{
	  memcpy (s, fp->_IO_read_ptr, want);
	  fp->_IO_read_ptr += want;
	  want = 0;
	}
      else
	{
	  if (have > 0)
	    {
#ifdef _LIBC
	      s = __mempcpy (s, fp->_IO_read_ptr, have);
#else
	      memcpy (s, fp->_IO_read_ptr, have);
	      s += have;
#endif
	      want -= have;
	      fp->_IO_read_ptr += have;
	    }

	  /* Check for backup and repeat */
	  if (_IO_in_backup (fp))
	    {
	      _IO_switch_to_main_get_area (fp);
	      continue;
	    }

	  /* If we now want less than a buffer, underflow and repeat
	     the copy.  Otherwise, _IO_SYSREAD directly to
	     the user buffer. */
	  if (fp->_IO_buf_base
	      && want < (size_t) (fp->_IO_buf_end - fp->_IO_buf_base))
	    {
	      if (__underflow (fp) == EOF)
		break;

	      continue;
	    }

	  /* These must be set before the sysread as we might longjmp out
	     waiting for input. */
	  _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);
	  _IO_setp (fp, fp->_IO_buf_base, fp->_IO_buf_base);

	  /* Try to maintain alignment: read a whole number of blocks.  */
	  count = want;
	  if (fp->_IO_buf_base)
	    {
	      _IO_size_t block_size = fp->_IO_buf_end - fp->_IO_buf_base;
	      if (block_size >= 128)
		count -= want % block_size;
	    }

	  count = _IO_SYSREAD (fp, s, count);
	  if (count <= 0)
	    {
	      if (count == 0)
		fp->_flags |= _IO_EOF_SEEN;
	      else
		fp->_flags |= _IO_ERR_SEEN;

	      break;
	    }

	  s += count;
	  want -= count;
	  if (fp->_offset != _IO_pos_BAD)
	    _IO_pos_adjust (fp->_offset, count);
	}
    }

  return n - want;
}

好了，最后贴个图

然后来看看fread_unlocked

在libio/iofread_u.c里

_IO_size_t
fread_unlocked (buf, size, count, fp)
     void *buf;
     _IO_size_t size;
     _IO_size_t count;
     _IO_FILE *fp;
{
  _IO_size_t bytes_requested = size * count;
  _IO_size_t bytes_read;
  CHECK_FILE (fp, 0);
  if (bytes_requested == 0)
    return 0;
  bytes_read = _IO_sgetn (fp, (char *) buf, bytes_requested);
  return bytes_requested == bytes_read ? count : bytes_read / size;
}

嗯，不用看了，相比fread()就是少了_IO_acquire_lock (fp)和_IO_release_lock (fp)

这里_IO_acquire_lock是glibc自己实现的一个递归锁

其实就是flockfile(stream) 在nptl/sysdeps/pthread/flockfile.c中

void
__flockfile (stream)
     FILE *stream;
{
  _IO_lock_lock (*stream->_lock);
}
strong_alias (__flockfile, _IO_flockfile)
weak_alias (__flockfile, flockfile)

在bits/stdio-lock.h中

#define _IO_lock_init(_name)	__libc_lock_init_recursive (_name)
#define _IO_lock_fini(_name)	__libc_lock_fini_recursive (_name)
#define _IO_lock_lock(_name)	__libc_lock_lock_recursive (_name)
#define _IO_lock_trylock(_name)	__libc_lock_trylock_recursive (_name)
#define _IO_lock_unlock(_name)	__libc_lock_unlock_recursive (_name)

在 nptl/sysdeps/pthread/bits/libc-lock.h中

/* Lock the recursive named lock variable.  */
#if defined _LIBC && (!defined NOT_IN_libc || defined IS_IN_libpthread)
# define __libc_lock_lock_recursive(NAME) \
  do {									      \
    void *self = THREAD_SELF;						      \
    if ((NAME).owner != self)						      \
      {									      \
	lll_lock ((NAME).lock, LLL_PRIVATE);				      \
	(NAME).owner = self;						      \
      }									      \
    ++(NAME).cnt;							      \
  } while (0)
#else
# define __libc_lock_lock_recursive(NAME) \
  __libc_maybe_call (__pthread_mutex_lock, (&(NAME).mutex), 0)
#endif

这里如果是nptl的实现，lll_lock网上的资料很多，不搬运了。

另外关于fopen，这篇非常详细：https://paper.seebug.org/920/