Seeking to a line in a file in g++

Question

Is there a way that I can seek to a certain line in a file to read or write data?

Let's say I want to write some data starting on the 10th line in a text file. There might be some data already in the first few lines, or the file could even be empty. Is there a way I can seek directly to the line I want without having to worry about what's already in the file?

Answer 1

You can seek to a position in a file, but that position must be a character offset from the start, end or current position - see for example fseek(). There is no way of seeking to a particular line, unless all the lines are exactly the same length.

Answer 2

Only if the lines are all the same length (seek to 9 * bytes_per_line). Otherwise, you'll just have to scan your way to the appropriate spot in the file.

Also be wary of writing into the middle of a file. It may not do what you expect (insert new lines). It will simply overwrite whatever content is already there, and won't respect existing line boundaries.

Answer 3

No, you have to process the data to find the line delimiters (unless you have fixed length lines). Have a look at getline(), ftell() and fseek(). http://www.pixelbeat.org/programming/readline/cpp.cpp

Answer 4

The easy best way is to read the file in memory inserting for instance each line in a vector of strings, then modifying/adding whatever you want, and re-write each line in a new file.

(supposing the file fits in memory)