Edit the first line of a huge file in c++

Question

Is there any "fast" way to edit the first line of a big file(~100Mg) in C++?

I know we can read the file line by line, make changes, write it to a temporary file, and rename the temporary file. But, I am wondering if there is a faster way of doing this (something like in-place modification)?

Answer 1

You can probably use the fwrite / fprintf file manipulation methods to be able to write to the file depending on the file pointer's position.

You open the file with fopen for appending, use fseek to the beginning and write what you need. However, you should be careful with the length of the first line. If you write less than the original line you will still have that extra content left over. If you write more you will overwrite your other content.

Answer 2

100MB is not that big on modern computers. If this is a one time deal and you're not working on a really slow device, you can simply read the whole file, split it into lines, make your edit and write it all back in a moment.

If this is something that's going to happen more often, you could benefit from simply adding some whitespace padding to the first line (if possible) to create a "buffer" for things that you can put there the next time. Then you can use fwrite to overwrite just that first line, without touching the rest of the file.

There may be OS and filesystem specific ways to allocate additional space inside an existing file without moving the data. For example on Linux with XFS/ext4 you can use fallocate :

int fallocate(int fd, int mode, off_t offset, off_t len);

fallocate() allows the caller to directly manipulate the allocated disk space for the file referred to by fd for the byte range starting at offset and continuing for len bytes.

Answer 3

I believe the fastest way to accomplish your task is to create a new file that contains the first line value. Whenever you take a request to read the file, you read the first line value file first, then read the larger file, skipping over the first line that is actually stored with the larger file. Whenever you want to change the first line, just change the first line file.

Answer 4

You're thinking of a memory-mapped file , in which the entire file is "mapped" into memory but not actually loaded or rewritten until you attempt to access or modify a part of it. On POSIX systems, you can mmap() a part of a file (say, the first kilobyte), modify it as necessary, then use msync() to write just that chunk of memory back to the disk.