How to elegantly open or create new file for reading and writing but not discard existing contents?

If the file exists, open it for reading and writing, but don't discard existing content; else, create it.

Isn't it a very common use case? Why does fopen not seemingly support this?

Note that I don't want to append. I want to read and overwrite some part of the existing content while keep other parts.

Answer 1

[Walter Roberson]

fopen with 'a' and close immediately. Then fopen 'r+'

This does rely on no process deleting the file between the time it is fopen 'a' and it is fopen again.

[埃博拉酱]

But I don't want to append!

[Walter Roberson]

Notice that I said to close immediately after you open with 'a'. As you are not writing any data between the fopen 'a' and the fclose right after, nothing will be appended to the file. The effect of the fopen 'a' fclose() pair is to leave the file untouched if it existed. If it did not exist then the fopen 'a' would create it. Then you fclose.

After the above fopen / fclose you can be certain that the file now exists provided there is no race condition to delete it. And since it exists you can fopen with "r+" to read and write it (it just might be empty, if it did not exist before)

If the race condition is a significant concern then there is a modification that can reduce the risk.

However if the race condition of someone substituting a different file is a serious risk then you need to take steps in C or C++ code and rely on POSIX security guarantees (but LINUX is not certified POSIX, and Windows isn't POSIX anymore...)

[埃博拉酱]

Though I'm not satisfied, your answer seems to be the best possible

[Walter Roberson]

The underlying languages of implementation, C and C++, do not support the mode you are requesting.

The calls that do support that mode, open(), is not part of C, and is instead a POSIX call https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html . That is a problem on Windows systems as Windows no longer supports POSIX.

I have not examined to find out if Windows supports the mode. MacOS and Linux potentially could, but Mathworks would have to specifically rewrite the code to use non-portable functions because it is not an available mode using portable functions.

Answer 2

[KSSV]

Read the documentation of fopen, there are multiple options present.

https://in.mathworks.com/help/matlab/ref/fopen.html .

look for fopen with 'a+'. Also have a look on the function exist.

[埃博拉酱]

But I don't want to append!

[KSSV]

There are other options present....choose the one which fits you. Or tell us what exactly you are expecting.

[埃博拉酱]

I have read the documentation and found no options that exactly match. That's why I ask here. What I'm expecting is clearly written in the question: If the file exists, open it for reading and writing, but don't discard existing content or append; else, create it.

[Walter Roberson]

MATLAB does not provide that combination of open permissions.

C's fopen() does not support that combination https://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html and it has to instead be done through a C open() call followed by a call to upgrade the resulting file descriptor into a file*