如何写入空文件中的特定偏移量

I have a file in which I read in data. Suppose the file has the string "abcdefghij". Now, I'm going to be reading from the file at random times from different processes and they store that byte and offset somewhere. For instance, I save 'c' as my character with an offset of '3' because that is its location. For reference, I've been using lseek to get the offset in my files.I have a file in which I read in data. Suppose