### Question 12.30

I'm trying to update a file in place, by using `fopen` mode `"r+"`, reading a certain string, and writing back a modified string, but it's not working.

Be sure to call `fseek` before you write, both to seek back to the beginning of the string you're trying to overwrite, and because an `fseek` or `fflush` is always required between reading and writing in the read/write `"+"` modes. Also, remember that you can only overwrite characters with the same number of replacement characters; see also question 19.14.

References: ANSI Sec. 4.9.5.3
ISO Sec. 7.9.5.3

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