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

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埃博拉酱 2021-8-4

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评论： Walter Roberson 2021-8-6

采纳的回答： Walter Roberson

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.

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Walter Roberson 2021-8-4

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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.

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埃博拉酱 2021-8-6

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

Walter Roberson 2021-8-6

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.

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KSSV 2021-8-4

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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.

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埃博拉酱 2021-8-4

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 2021-8-4

编辑：Walter Roberson 2021-8-6

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*

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