How to replace a line on the middle of a txt file in C?

I am reading info (numbers) from a txt file开发者_运维技巧 and after that I am adding to those numbers, others I had in another file, with the same structure.

At the start of each line in the file is a number, that identifies a specific product. That code will allow me to search for the same product in the other file. In my program I have to add the other "variables" from one file to the other, and then replace it, in the same place in one of those files.

I didn't open any of those files with a or a+, I did it with r and r+ because i want to replace the information in the lines that may be in the middle of the file, and not in the end of it.

The program compiles, and runs, but when it comes to replace the info in the file, it just doesn't do anything.

How should I resolve the problem?

---

A program can replace (overwrite) text in the middle of the file. But the question is whether or not this should be performed.

In order to insert larger text or smaller text (and close up the gap), a new text file must be written. This is assuming the file is not fixed width. The fundamental rule is to copy all original text before the insertion to a new file. Write the new text. Finally write the remaining original text. This is a lot of work and will slow down even the simplest programs.

I suggest you design your data layout before you go any further. Also consider using a database, see my post: At what point is it worth using a database?

Your objective is to design the data to minimize duplication and data fetching.