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### 24.4 Writing to Files

You can write the contents of a buffer, or part of a buffer, directly to a file on disk using the `append-to-file` and `write-region` functions. Don’t use these functions to write to files that are being visited; that could cause confusion in the mechanisms for visiting.

Command: append-to-file start end filename

  This function appends the contents of the region delimited by start and end in the current buffer to the end of file filename. If that file does not exist, it is created. This function returns `nil`.

  An error is signaled if filename specifies a nonwritable file, or a nonexistent file in a directory where files cannot be created.

  When called from Lisp, this function is completely equivalent to:

      (write-region start end filename t)

Command: write-region start end filename &optional append visit lockname mustbenew

  This function writes the region delimited by start and end in the current buffer into the file specified by filename.

  If start is `nil`, then the command writes the entire buffer contents (not just the accessible portion) to the file and ignores end.

  If start is a string, then `write-region` writes or appends that string, rather than text from the buffer. end is ignored in this case.

  If append is non-`nil`, then the specified text is appended to the existing file contents (if any). If append is a number, `write-region` seeks to that byte offset from the start of the file and writes the data from there.

  If mustbenew is non-`nil`, then `write-region` asks for confirmation if filename names an existing file. If mustbenew is the symbol `excl`, then `write-region` does not ask for confirmation, but instead it signals an error `file-already-exists` if the file already exists.

  The test for an existing file, when mustbenew is `excl`, uses a special system feature. At least for files on a local disk, there is no chance that some other program could create a file of the same name before Emacs does, without Emacs’s noticing.

  If visit is `t`, then Emacs establishes an association between the buffer and the file: the buffer is then visiting that file. It also sets the last file modification time for the current buffer to filename’s modtime, and marks the buffer as not modified. This feature is used by `save-buffer`, but you probably should not use it yourself.

  If visit is a string, it specifies the file name to visit. This way, you can write the data to one file (filename) while recording the buffer as visiting another file (visit). The argument visit is used in the echo area message and also for file locking; visit is stored in `buffer-file-name`. This feature is used to implement `file-precious-flag`; don’t use it yourself unless you really know what you’re doing.

  The optional argument lockname, if non-`nil`, specifies the file name to use for purposes of locking and unlocking, overriding filename and visit for that purpose.

  The function `write-region` converts the data which it writes to the appropriate file formats specified by `buffer-file-format` and also calls the functions in the list `write-region-annotate-functions`. See Format Conversion.

  Normally, `write-region` displays the message ‘`Wrote filename`’ in the echo area. This message is inhibited if visit is neither `t` nor `nil` nor a string, or if Emacs is operating in batch mode (see Batch Mode). This feature is useful for programs that use files for internal purposes, files that the user does not need to know about.

Macro: with-temp-file file body…

  The `with-temp-file` macro evaluates the body forms with a temporary buffer as the current buffer; then, at the end, it writes the buffer contents into file file. It kills the temporary buffer when finished, restoring the buffer that was current before the `with-temp-file` form. Then it returns the value of the last form in body.

  The current buffer is restored even in case of an abnormal exit via `throw` or error (see Nonlocal Exits).

  See also `with-temp-buffer` in The Current Buffer.

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