open(3Tcl) open(3Tcl)
open - Open a file
open fileName ?access? ?permissions? |
This command opens a file and returns an identifier that may be used in
future invocations of commands like read, puts, and close. FileName
gives the name of the file to open; if it starts with a tilde then tilde
substitution is performed as described for Tcl_TildeSubst. If the first
character of fileName is ``|'' then the remaining characters of fileName
are treated as a command pipeline to invoke, in the same style as for
exec. In this case, the identifier returned by open may be used to write
to the command's input pipe or read from its output pipe.
The access argument indicates the way in which the file (or command
pipeline) is to be accessed. It may take two forms, either a string in |
the form that would be passed to the fopen library procedure or a list of|
POSIX access flags. It defaults to ``r''. In the first form access may |
have any of the following values:
r Open the file for reading only; the file must already
exist.
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. |
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open(3Tcl) open(3Tcl)
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.|
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.
If a file is opened for both reading and writing then seek must be
invoked between a read and a write, or vice versa (this restriction does
not apply to command pipelines opened with open). When fileName
specifies a command pipeline and a write-only access is used, then
standard output from the pipeline is directed to the current standard
output unless overridden by the command. When fileName specifies a
command pipeline and a read-only access is used, then standard input from
the pipeline is taken from the current standard input unless overridden
by the command.
access mode, append, controlling terminal, create, file, non-blocking,
open, permissions, pipeline, process
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