《通链参考手册》


Function OPEN

●语法设定:

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

=> stream

●参数和值:

filespec---a pathname designator.

direction---one of :input, :output, :io, or :probe. The default is :input.

element-type---a type specifier for recognizable subtype of character; or a type specifier for a finite recognizable subtype of integer; or one of the symbols signed-byte, unsigned-byte, or :default. The default is character.

if-exists---one of :error, :new-version, :rename, :rename-and-delete, :overwrite, :append, :supersede, or nil. The default is :new-version if the version component of filespec is :newest, or :error otherwise.

if-does-not-exist---one of :error, :create, or nil. The default is :error if direction is :input or if-exists is :overwrite or :append; :create if direction is :output or :io, and if-exists is neither :overwrite nor :append; or nil when direction is :probe.

external-format---an external file format designator. The default is :default.

stream---a file stream or nil.

●详情:

open creates, opens, and returns a file stream that is connected to the file specified by filespec. Filespec is the name of the file to be opened. If the filespec designator is a stream, that stream is not closed first or otherwise affected.

The keyword arguments to open specify the characteristics of the file stream that is returned, and how to handle errors.

If direction is :input or :probe, or if if-exists is not :new-version and the version component of the filespec is :newest, then the file opened is that file already existing in the file system that has a version greater than that of any other file in the file system whose other pathname components are the same as those of filespec.

An implementation is required to recognize all of the open keyword options and to do something reasonable in the context of the host operating system. For example, if a file system does not support distinct file versions and does not distinguish the notions of deletion and expunging, :new-version might be treated the same as :rename or :supersede, and :rename-and-delete might be treated the same as :supersede.

When a file is opened, a file stream is constructed to serve as the file system's ambassador to the Lisp environment; operations on the file stream are reflected by operations on the file in the file system.

A file can be deleted, renamed, or destructively modified by open.

For information about opening relative pathnames, see Section 19.2.3 (Merging Pathnames).

●例子:

 (open filespec :direction :probe)  =>  #<Closed Probe File Stream...>
 (setq q (merge-pathnames (user-homedir-pathname) "test"))
=>  #<PATHNAME :HOST NIL :DEVICE device-name :DIRECTORY directory-name
    :NAME "test" :TYPE NIL :VERSION :NEWEST>
 (open filespec :if-does-not-exist :create) =>  #<Input File Stream...>
 (setq s (open filespec :direction :probe)) =>  #<Closed Probe File Stream...>
 (truename s) =>  #<PATHNAME :HOST NIL :DEVICE device-name :DIRECTORY
    directory-name :NAME filespec :TYPE extension :VERSION 1>
 (open s :direction :output :if-exists nil) =>  NIL 

●受制于:

The nature and state of the host computer's file system.

●例外情况:

If if-exists is :error, (subject to the constraints on the meaning of if-exists listed above), an error of type file-error is signaled.

If if-does-not-exist is :error (subject to the constraints on the meaning of if-does-not-exist listed above), an error of type file-error is signaled.

If it is impossible for an implementation to handle some option in a manner close to what is specified here, an error of type error might be signaled.

An error of type file-error is signaled if (wild-pathname-p filespec) returns true.

An error of type error is signaled if the external-format is not understood by the implementation.

The various file systems in existence today have widely differing capabilities, and some aspects of the file system are beyond the scope of this specification to define. A given implementation might not be able to support all of these options in exactly the manner stated. An implementation is required to recognize all of these option keywords and to try to do something ``reasonable'' in the context of the host file system. Where necessary to accomodate the file system, an implementation deviate slightly from the semantics specified here without being disqualified for consideration as a conforming implementation. If it is utterly impossible for an implementation to handle some option in a manner similar to what is specified here, it may simply signal an error.

With regard to the :element-type option, if a type is requested that is not supported by the file system, a substitution of types such as that which goes on in upgrading is permissible. As a minimum requirement, it should be the case that opening an output stream to a file in a given element type and later opening an input stream to the same file in the same element type should work compatibly.

●更多信息:

with-open-file, close, pathname, logical-pathname, Section 19.2.3 (Merging Pathnames), Section 19.1.2 (Pathnames as Filenames)

●说明:

open does not automatically close the file when an abnormal exit occurs.

When element-type is a subtype of character, read-char and/or write-char can be used on the resulting file stream.

When element-type is a subtype of integer, read-byte and/or write-byte can be used on the resulting file stream.

When element-type is :default, the type can be determined by using stream-element-type.


X3J13设计清单非标准部分,可查下面章节:


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