fwrite 是否可以写入一些字节但 return 为零？

Question

根据 fwrite 的手册页：

fread() and fwrite() return the number of items successfully read or written (i.e., not the number of characters). If an error occurs, or the end-of-file is reached, the return value is a short item count (or zero).

我想知道 fwrite(ptr, 65537, 1, fp) 是否可以只写 512 个字节而 return 为零？

我试图快速跟踪 glibc-2.12.2 源代码。

malloc/malloc.c:#define fwrite(buf, size, count, fp) _IO_fwrite (buf, size, count, fp)

我想真正的实现是在 iofwrite.c.

_IO_size_t
_IO_fwrite (buf, size, count, fp)
     const void *buf;
     _IO_size_t size;
     _IO_size_t count;
     _IO_FILE *fp;
{
  _IO_size_t request = size * count;
  _IO_size_t written = 0;
  CHECK_FILE (fp, 0);
  if (request == 0)
    return 0;
  _IO_acquire_lock (fp);     
  if (_IO_vtable_offset (fp) != 0 || _IO_fwide (fp, -1) == -1)
    written = _IO_sputn (fp, (const char *) buf, request);
  _IO_release_lock (fp);
  /* We have written all of the input in case the return value indicates
     this or EOF is returned.  The latter is a special case where we
     simply did not manage to flush the buffer.  But the data is in the
     buffer and therefore written as far as fwrite is concerned.  */
  if (written == request || written == EOF)
    return count;
  else
    return written / size;
}

所以，我想 fwrite 的 return 代码中有可能 return 0，即使它确实向文件中写入了一些字节。

Answer 1

是的 — fwrite(ptr, 65537, 1, fp) 到 return 0 作为 return 代码是可能的，即使它确实向文件中写入了一些字节。

例如，如果磁盘上只剩下 space 512 字节，那么你有一个短写，但它写了 0 个完整的 65537 字节单元（这是一个奇怪的数字；通常它会是 65536，不是吗？），因此 return 值必须为 0。

如果你想知道到底写了多少，你应该使用：

 nbytes = fwrite(ptr, 1, 65537, fp);

获取准确的字节数；现在你会从你假设的短文中得到 512。

fwrite 是否可以写入一些字节但 return 为零？

Is it possible for fwrite to write some bytes but return zero?

c

glibc