archie/tcl7.6/doc/open.n

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'\" Copyright (c) 1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\" SCCS: @(#) open.n 1.15 96/09/18 15:05:40
'\" 
.so man.macros
.TH open n 7.6 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
open \- Open a file-based or command pipeline channel
.SH SYNOPSIS
.sp
\fBopen \fIfileName\fR
.br
\fBopen \fIfileName access\fR
.br
\fBopen \fIfileName access permissions\fR
.BE

.SH DESCRIPTION
.PP
This command opens a file or command pipeline and returns a channel
identifier that may be used in future invocations of commands like
\fBread\fR, \fBputs\fR, and \fBclose\fR.
If the first character of \fIfileName\fR is not \fB|\fR then
the command opens a file:
\fIfileName\fR gives the name of the file to open, and it must conform to the
conventions described in the \fBfilename\fR manual entry.
.PP
The \fIaccess\fR argument, if present, indicates the way in which the file
(or command pipeline) is to be accessed.
In the first form \fIaccess\fR may have any of the following values:
.TP 15
\fBr\fR
Open the file for reading only; the file must already exist. This is the
default value if \fIaccess\fR is not specified.
.TP 15
\fBr+\fR
Open the file for both reading and writing; the file must
already exist.
.TP 15
\fBw\fR
Open the file for writing only.  Truncate it if it exists.  If it doesn't
exist, create a new file.
.TP 15
\fBw+\fR
Open the file for reading and writing.  Truncate it if it exists.
If it doesn't exist, create a new file.
.TP 15
\fBa\fR
Open the file for writing only.  The file must already exist, and the file
is positioned so that new data is appended to the file.
.TP 15
\fBa+\fR
Open the file for reading and writing.  If the file doesn't exist,
create a new empty file.
Set the initial access position  to the end of the file.
.PP
In the second form, \fIaccess\fR consists of a list of any of the
following flags, all of which have the standard POSIX meanings.
One of the flags must be either \fBRDONLY\fR, \fBWRONLY\fR or \fBRDWR\fR.
.TP 15
\fBRDONLY\fR
Open the file for reading only.
.TP 15
\fBWRONLY\fR
Open the file for writing only.
.TP 15
\fBRDWR\fR
Open the file for both reading and writing.
.TP 15
\fBAPPEND\fR
Set the file pointer to the end of the file prior to each write.
.TP 15
\fBCREAT\fR
Create the file if it doesn't already exist (without this flag it
is an error for the file not to exist).
.TP 15
\fBEXCL\fR
If \fBCREAT\fR is also specified, an error is returned if the
file already exists.
.TP 15
\fBNOCTTY\fR
If the file is a terminal device, this flag prevents the file from
becoming the controlling terminal of the process.
.TP 15
\fBNONBLOCK\fR
Prevents the process from blocking while opening the file, and
possibly in subsequent I/O operations.  The exact behavior of
this flag is system- and device-dependent;  its use is discouraged
(it is better to use the \fBfconfigure\fR command to put a file
in nonblocking mode).
For details refer to your system documentation on the \fBopen\fR system
call's \fBO_NONBLOCK\fR flag.
.TP 15
\fBTRUNC\fR
If the file exists it is truncated to zero length.
.PP
If a new file is created as part of opening it, \fIpermissions\fR
(an integer) is used to set the permissions for the new file in
conjunction with the process's file mode creation mask.
\fIPermissions\fR defaults to 0666.
.SH "COMMAND PIPELINES"
.PP
If the first character of \fIfileName\fR is ``|'' then the
remaining characters of \fIfileName\fR are treated as a list of arguments
that describe a command pipeline to invoke, in the same style as the
arguments for \fBexec\fR.
In this case, the channel identifier returned by \fBopen\fR may be used
to write to the command's input pipe or read from its output pipe,
depending on the value of \fIaccess\fR.
If write-only access is used (e.g. \fIaccess\fR is \fBw\fR), then
standard output for the pipeline is directed to the current standard
output unless overridden by the command.
If read-only access is used (e.g. \fIaccess\fR is \fBr\fR),
standard input for the pipeline is taken from the current standard
input unless overridden by the command.

.VS
.SH "PORTABILITY ISSUES"
.sp
.TP
\fBWindows NT\fR
.
When running Tcl interactively, there may be some strange interactions
between the real console, if one is present, and a command pipeline that uses
standard input or output.  If a command pipeline is opened for reading, some
of the lines entered at the console will be sent to the command pipeline and
some will be sent to the Tcl evaluator.  If a command pipeline is opened for
writing, keystrokes entered into the console are not visible until the the
pipe is closed.  This behavior occurs whether the command pipeline is
executing 16-bit or 32-bit applications.  These problems only occur because
both Tcl and the child application are competing for the console at
the same time.  If the command pipeline is started from a script, so that Tcl
is not accessing the console, or if the command pipeline does not use
standard input or output, but is redirected from or to a file, then the
above problems do not occur.  
.TP
\fBWindows 95\fR 
.
A command pipeline that executes a 16-bit DOS application cannot be opened
for both reading and writing, since 16-bit DOS applications that receive
standard input from a pipe and send standard output to a pipe run
synchronously.  Command pipelines that do not execute 16-bit DOS
applications run asynchronously and can be opened for both reading and
writing.  
.sp
When running Tcl interactively, there may be some strange interactions
between the real console, if one is present, and a command pipeline that uses
standard input or output.  If a command pipeline is opened for reading from
a 32-bit application, some of the keystrokes entered at the console will be
sent to the command pipeline and some will be sent to the Tcl evaluator.  If
a command pipeline is opened for writing to a 32-bit application, no output
is visible on the console until the the pipe is closed.  These problems only
occur because both Tcl and the child application are competing for the
console at the same time.  If the command pipeline is started from a script,
so that Tcl is not accessing the console, or if the command pipeline does
not use standard input or output, but is redirected from or to a file, then
the above problems do not occur.  
.sp
Whether or not Tcl is running interactively, if a command pipeline is opened
for reading from a 16-bit DOS application, the call to \fBopen\fR will not
return until end-of-file has been received from the command pipeline's
standard output.  If a command pipeline is opened for writing to a 16-bit DOS
application, no data will be sent to the command pipeline's standard output
until the pipe is actually closed.  This problem occurs because 16-bit DOS
applications are run synchronously, as described above.  
.TP
\fBWindows 3.X\fR 
.
A command pipeline can execute 16-bit or 32-bit DOS or Windows
applications, but the call to \fBopen\fR will not return until the last
program in the pipeline has finished executing; command pipelines run
synchronously.  If the pipeline is opened with write access (either just
writing or both reading and writing) the first application in the
pipeline will instead see an immediate end-of-file; any data the caller
writes to the open pipe will instead be discarded.
.sp
Since Tcl cannot be run with a real console under Windows 3.X, there are
no interactions between command pipelines and the console.
.TP
\fBMacintosh\fR
Opening a command pipeline is not supported under Macintosh, since 
applications do not support the concept of standard input or output.
.TP
\fBUnix\fR\0\0\0\0\0\0\0
When running Tcl interactively, there may be some strange interactions
between the console, if one is present, and a command pipeline that uses
standard input.  If a command pipeline is opened for reading, some
of the lines entered at the console will be sent to the command pipeline and
some will be sent to the Tcl evaluator.  This problem only occurs because
both Tcl and the child application are competing for the console at the
same time.  If the command pipeline is started from a script, so that Tcl is
not accessing the console, or if the command pipeline does not use standard
input, but is redirected from a file, then the above problem does not occur.  
.LP
See the PORTABILITY ISSUES section of the \fBexec\fR command for additional
information not specific to command pipelines about executing
applications on the various platforms
.SH "SEE ALSO"
close(n), filename(n), gets(n), read(n), puts(n), exec(n)
.VE
.SH KEYWORDS
access mode, append, create, file, non-blocking, open, permissions,
pipeline, process