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How to write to a fd replacing the original data of the same length?

I have a file descriptor, now I want to write n bytes to that file replacing n bytes of original data (so the total file size won't change), like in replace mode of a word processor. Any convenient way to achieve this?

Solution

  • Just write to the fd. You might need to read up to the right position before or lseek or lseek64 directly to the right position.

    You must be sure that the fd is writable. You need to check open for that. open must have been called with O_WRONLY or O_RDWR but without O_TRUNC which would have truncated the file to zero length.

    Note: write to a file only changes (increases) the file size if you are writing over the end of the file. If you are writing right into the file it's size does not change.