fcntl is a command available within the TclX extension. Here is the appropriate part of its man page.
fcntl fileId attribute ?value?
This command either sets or clears a file option or returns
its current value. If value is not specified, then the
current value of attribute is returned. All values are
boolean. Some attributes maybe only be gotten, not modified.
The following attributes may be specified:
RDONLY
The file is opened for reading only. (Get only)
WRONLY
The file is opened for writing only. (Get only)
RDWR The file is opened for reading and writing. (Get only)
READ If the file is readable. (Get only).
WRITE
If the file is writable. (Get only).
APPEND
The file is opened for append-only writes. All writes will
be forced to the end of the file. (Get or set).
NONBLOCK
The file is to be accessed with non-blocking I/O. See the
read system call for a description of how it affects the
behavior of file reads.
CLOEXEC
Close the file on an process exec. If the execl command or
some other mechanism causes the process to do an exec, the
file will be closed if this option is set.
NOBUF
The file is not buffered. If set, then there no buffering
for the file.
LINEBUF
Output the file will be line buffered. The buffer will be
flushed when a newline is written, when the buffer is full,
or when input is requested.
KEEPALIVE
Keep a socket connection alive. If SIGPIPE is enabled, then
it is sent if connection is broken and data is written to
the socket. If SIGPIPE is ignored, an error is returned on
the write. This attribute is valid only on sockets. By
default, SIGPIPE is ignored in Tcl.
The NONBLOCK, NOBUF and LINEBUF are provided for compatibil-
ity with older scripts. Thefconfigure command is preferred
method of getting and setting these attributes.
The APPEND and CLOEXEC options are not available on Windows
95/NT.