c - fwrite 是否可以写入一些字节但返回零？

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) 是否可以仅写入 512 字节并返回零？

我尝试快速跟踪 glibc-2.12.2 源代码。

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

我猜真正的实现是在 iofwrite.c 中。

_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，我猜即使 fwrite 确实已经向文件写入了一些字节，它的返回码也可能返回 0。

Answer 1

是的 - fwrite(ptr, 65537, 1, fp) 可能会返回 0 作为返回码，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 512 字节的空间，那么您的写入很短，但它写入了 0 个完整的 65537 字节单位(这是一个奇怪的数字；通常会是 65536，不会吗？)，所以返回值一定是0。

如果你想知道实际写了多少，你应该使用:

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

获取准确的字节数；现在您将从假设的简短写入中得到 512。