Tcl Built-In Commands - 7.6 - open manual page

This command opens a file or command pipeline and returns a channel identifier that may be used in future invocations of commands like read, puts, and close. If the first character of fileName is not | then the command opens a file: fileName gives the name of the file to open, and it must conform to the conventions described in the filename manual entry.

The access argument, if present, indicates the way in which the file (or command pipeline) is to be accessed. In the first form access may have any of the following values:

r: Open the file for reading only; the file must already exist. This is the default value if access is not specified.
r+: Open the file for both reading and writing; the file must already exist.
w: Open the file for writing only. Truncate it if it exists. If it doesn't exist, create a new file.
w+: Open the file for reading and writing. Truncate it if it exists. If it doesn't exist, create a new file.
a: 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.
a+: 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.

In the second form, access 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 RDONLY, WRONLY or RDWR.

RDONLY: Open the file for reading only.
WRONLY: Open the file for writing only.
RDWR: Open the file for both reading and writing.
APPEND: Set the file pointer to the end of the file prior to each write.
CREAT: Create the file if it doesn't already exist (without this flag it is an error for the file not to exist).
EXCL: If CREAT is also specified, an error is returned if the file already exists.
NOCTTY: If the file is a terminal device, this flag prevents the file from becoming the controlling terminal of the process.
NONBLOCK: 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 fconfigure command to put a file in nonblocking mode). For details refer to your system documentation on the open system call's O_NONBLOCK flag.
TRUNC: If the file exists it is truncated to zero length.

If a new file is created as part of opening it, permissions (an integer) is used to set the permissions for the new file in conjunction with the process's file mode creation mask. Permissions defaults to 0666.

COMMAND PIPELINES

If the first character of fileName is ``|'' then the remaining characters of fileName are treated as a list of arguments that describe a command pipeline to invoke, in the same style as the arguments for exec. In this case, the channel identifier returned by open may be used to write to the command's input pipe or read from its output pipe, depending on the value of access. If write-only access is used (e.g. access is w), 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. access is r), standard input for the pipeline is taken from the current standard input unless overridden by the command.

PORTABILITY ISSUES

Windows NT

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.

Windows 95

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.

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.

Whether or not Tcl is running interactively, if a command pipeline is opened for reading from a 16-bit DOS application, the call to open 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.

Windows 3.X

A command pipeline can execute 16-bit or 32-bit DOS or Windows applications, but the call to open 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.

Since Tcl cannot be run with a real console under Windows 3.X, there are no interactions between command pipelines and the console.

Macintosh

Opening a command pipeline is not supported under Macintosh, since applications do not support the concept of standard input or output.

Unix

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.

See the PORTABILITY ISSUES section of the exec command for additional information not specific to command pipelines about executing applications on the various platforms

KEYWORDS

access mode, append, create, file, non-blocking, open, permissions, pipeline, process

Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995, 1996 Roger E. Critchlow Jr.

NAME

SYNOPSIS

DESCRIPTION

COMMAND PIPELINES

PORTABILITY ISSUES

SEE ALSO

KEYWORDS