How does emacs write files?

Question

When Emacs saves a file (save-buffer, write-file etc.), does it overwrite files directly or does it create a temporary file first?

More specifically, if I edit /path-to/file.txt, will Emacs directly open that file and overwrite the contents in-place, or will it write to a temporary file and replace the original file by it after successfully closing the file handle?

The latter convention is seen in a lot of other software. If Emacs has this behavior, or can be configured to use it, for my purposes it would be safe to disable backup files (M-x customize-variable make-backup-files).

Answer 1

As an answer I cite here mostly the comments of the normal execution path in basic-save-buffer-2 which is the working horse of save-buffer:

      ;; Write temp name, then rename it.
      ;; This requires write access to the containing dir,
      ;; which is why we don't try it if we don't have that access.
        ;; Create temp files with strict access rights.  It's easy to
        ;; loosen them later, whereas it's impossible to close the
        ;; time-window of loose permissions otherwise.
          ;; Call write-region in the appropriate way
          ;; for saving the buffer.
          (write-region nil nil tempname nil realname
                buffer-file-truename)
          ;; If we failed, restore the buffer's modtime.
        ;; Since we have created an entirely new file,
        ;; make sure it gets the right permission bits set.
        ;; We succeeded in writing the temp file,
        ;; so rename it.
        (rename-file tempname buffer-file-name t)

So in the normal case you are right that Emacs writes to a temp file and changes its name later to the value of buffer-file-name which is the name of the original file.

But, note that there are bypasses that circumvent the normal execution. See, e.g., the doc strings of the variables write-file-functions and write-contents-functions.