Creates, opens, and returns a file stream that is connected to a specified file.




open filespec &key direction element-type external-format if-exists if-does-not-exist => stream



A file designator.


If direction is :probe , external-format is ignored. The element type and external format of the returned stream are undefined.


By default, the value of *default-character-element-type* (the ANSI standard default is character ).


An external file format designator. By default, this is :default .


What to do if the file stream already exists. The possible values for this are as in the ANSI standard.


What to do if the file stream does not already exist. The possible values for this are as in the ANSI standard.



A file stream, or nil .


If external-format has a name which is not :default and the parameters include :eol-style , it is used as is.

Otherwise, the system decides which external format to use via guess-external-format. By default, this finds a match based on the filename; or (if that fails), looks in the EMACS-style ( -*- ) attribute line for an option called encoding or external-format ; or (if that fails), chooses from among likely encodings by analyzing the bytes near the start of the file. By default, it then also analyses the start of the file for byte patterns indicating the end-of-line style, and uses a default end-of-line style if no such pattern is found. This behavior is configurable.

After the external-format has been determined, it is verified using valid-external-format-p; and an error is signalled if this check fails.

If open gets :default as its element-type arg, it chooses the type on the basis of the external format. If open gets an element-type other than :default and the direction is :input or :io , the argument must be a supertype of the type of characters produced by the external format; if the direction is :output or :io , it must be a subtype of the type of characters accepted by the external format; if it does not satisfy these requirements, an error is signalled.

Standard stream input and output functions for character and binary data generally work in the obvious way on a file-stream with element-type base-char , (unsigned-byte 8) or (signed-byte 8) . For example, read-sequence can be called with a string buffer and a binary file-stream : the character data is constructed from the input as if by code-char . Similarly write-sequence can be called with a string buffer and a binary file-stream : the output is converted from the character data as if by char-code . Also, 8-bit binary data can be read from and written to a base-char file-stream .

All standard stream I/O functions except for write-byte and read-byte have this flexibility.

See also

LispWorks Reference Manual - 6 Apr 2005