将文本添加到 C 中的文件中 [英] Prepend text to a file in C

问题描述

我想使用 C 向文件(或文件组)添加标准头.这些文件可能非常大，因此将它们加载到内存中或将它们复制到临时文件(我想想).

有没有办法简单地将标题直接添加到每个文件中?

标头本身很小，不超过 1 KB

解决方案

没有临时文件应该是可能的 - 你可以从文件的末尾，一个块一个块地读取文件，将每个块写回 (original_position +header_size).第一个块将以 header_size 写回，为标题留出空间.

但是，您并不真的想这样做.如果中止，它会损坏文件(想想:磁盘空间不足、其他 I/O 错误、断电等等).

因此，您实际上应该使用临时文件 - 将您需要的所有内容写入其中，然后将其重命名为原始文件的名称(假设您在同一文件系统上创建临时文件，否则您需要复制).

澄清我的意思，当整个文件适合RAM时的简化解决方案:

• 分配与文件大小相同的缓冲区
• 打开文件，并将其读入缓冲区
• seek(file, header_size) 并在此处写入缓冲区
• seek(file, 0) 写入头部

如果文件太大，您可以分配较小的缓冲区并重复读取/写入，从 file_size - buffer_size 开始读取，并在 file_size - buffer_size + header_size 写入.然后重复以 file_size - 2 * buffer_size 读取下一个块，以 file_size - 2 * buffer_size + header_size 写入，依此类推.

但让我再说一遍:如果文件失败，您可能会损坏文件！

I want to add a standard header to a file (or group of files) using C. These files could be quite large, so it would be a bad idea to load them into memory, or copy them into temporary files (I think).

Is there a way to simply prepend the header directly to each file?

The header itself is quite small, not more than 1 KB

解决方案

It should be possible without a temporary file - you can read the file from the end, block by block, writing each block back at (original_position + header_size). The first block would be written back at header_size, leaving room for the header.

However, you don't really want to do this. It would corrupt the file if aborted (think: out of disk space, other I/O error, power down, whatever).

Thus, you should actually use temporary file - write to it everything you need, then rename it to the original file's name (assuming you create temporary file on the same file system, otherwise you'd need to copy).

Edit: to clarify what I mean, simplified solution when the whole file fits in RAM:

• allocate buffer same size as the file
• open the file, and read it into the buffer
• seek(file, header_size) and write the buffer here
• seek(file, 0) write the header

If the file is to big, you can allocate smaller buffer and repeat reads/writes starting with read at file_size - buffer_size and write at file_size - buffer_size + header_size. Then repeat with next chunk read at file_size - 2 * buffer_size, write at file_size - 2 * buffer_size + header_size, and so on.

But let me repeat: you risk corrupting your file if it fails!

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