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Before Scheme can access a file for reading or writing, it is necessary
to open a port to the file. This section describes procedures used to
open ports to files. Such ports are closed (like any other port) by
close-port
. File ports are automatically closed if and when they
are reclaimed by the garbage collector.
Before opening a file for input or output, by whatever method, the
filename argument is converted to canonical form by calling the
procedure merge-pathnames
with filename as its sole
argument. Thus, filename can be either a string or a pathname,
and it is merged with the current pathname defaults to produce the
pathname that is then opened.
Any file can be opened in one of two modes, normal or
binary. Normal mode is for accessing text files, and binary
mode is for accessing other files. Unix does not distinguish these
modes, but Windows do: in normal mode, their file ports perform
newline translation, mapping between the
carriage-return/linefeed sequence that terminates text lines in files,
and the #\newline
that terminates lines in Scheme. In binary
mode, such ports do not perform newline translation. Unless otherwise
mentioned, the procedures in this section open files in normal mode.
Takes a filename referring to an existing file and returns an input port
capable of delivering characters from the file. If the file cannot be
opened, an error of type condition-type:file-operation-error
is
signalled.
Takes a filename referring to an output file to be created and returns
an output port capable of writing characters to a new file by that name.
If the file cannot be opened, an error of type
condition-type:file-operation-error
is signalled.
The optional argument append? is an MIT/GNU Scheme extension. If
append? is given and not #f
, the file is opened in
append mode. In this mode, the contents of the file are not
overwritten; instead any characters written to the file are appended to
the end of the existing contents. If the file does not exist, append
mode creates the file and writes to it in the normal way.
Takes a filename referring to an existing file and returns an
I/O port capable of both reading and writing the file. If
the file cannot be opened, an error of type
condition-type:file-operation-error
is signalled.
This procedure is often used to open special files. For example, under unix this procedure can be used to open terminal device files, PTY device files, and named pipes.
These procedures open files in binary mode. In all other respects they
are identical to open-input-file
, open-output-file
, and
open-i/o-file
, respectively.
This procedure closes all file ports that are open at the time that it is called, and returns an unspecified value.
These procedures call procedure with one argument: the port
obtained by opening the named file for input or output, respectively.
If the file cannot be opened, an error of type
condition-type:file-operation-error
is signalled. If
procedure returns, then the port is closed automatically and the
value yielded by procedure is returned. If procedure does
not return, then the port will not be closed automatically unless it is
reclaimed by the garbage collector.15
These procedures open files in binary mode. In all other respects they
are identical to call-with-input-file
and
call-with-output-file
, respectively.
Thunk must be a procedure of no arguments.
The file is opened for input or output, an input or output port
connected to it is made the default value returned by
current-input-port
or current-output-port
, and the
thunk is called with no arguments. When the thunk returns,
the port is closed and the previous default is restored.
with-input-from-file
and with-output-to-file
return the
value yielded by thunk. If an escape procedure is used to escape
from the continuation of these procedures, their behavior is
implementation-dependent; in that situation MIT/GNU Scheme leaves the files
open.
These procedures open files in binary mode. In all other respects they
are identical to with-input-from-file
and
with-output-to-file
, respectively.
Because Scheme’s escape
procedures have unlimited extent, it is possible to escape from the
current continuation but later to escape back in. If implementations
were permitted to close the port on any escape from the current
continuation, then it would be impossible to write portable code using
both call-with-current-continuation
and
call-with-input-file
or call-with-output-file
.
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